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| author | obrien <obrien@FreeBSD.org> | 1999-04-09 18:51:21 +0000 |
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| committer | obrien <obrien@FreeBSD.org> | 1999-04-09 18:51:21 +0000 |
| commit | 8f6b9f8b2f25ec9892a5277179b79f02d8750d3e (patch) | |
| tree | e22d81a82759cc908def36f7c936cbe7ec0a32b0 /contrib | |
| parent | 20977f06d87667b8a7067e1e6100789b4c0dc22f (diff) | |
| download | FreeBSD-src-8f6b9f8b2f25ec9892a5277179b79f02d8750d3e.zip FreeBSD-src-8f6b9f8b2f25ec9892a5277179b79f02d8750d3e.tar.gz | |
Did not mean to import these with 2.0b1pl17
(was the docs/ dir in previous ISC distfiles)
Diffstat (limited to 'contrib')
| -rw-r--r-- | contrib/isc-dhcp/doc/rfc2131.txt | 2523 | ||||
| -rw-r--r-- | contrib/isc-dhcp/doc/rfc2132.txt | 1907 | ||||
| -rw-r--r-- | contrib/isc-dhcp/doc/rfc951.txt | 684 |
3 files changed, 0 insertions, 5114 deletions
diff --git a/contrib/isc-dhcp/doc/rfc2131.txt b/contrib/isc-dhcp/doc/rfc2131.txt deleted file mode 100644 index f45d9b8..0000000 --- a/contrib/isc-dhcp/doc/rfc2131.txt +++ /dev/null @@ -1,2523 +0,0 @@ - - - - - - -Network Working Group R. Droms -Request for Comments: 2131 Bucknell University -Obsoletes: 1541 March 1997 -Category: Standards Track - - Dynamic Host Configuration Protocol - -Status of this memo - - This document specifies an Internet standards track protocol for the - Internet community, and requests discussion and suggestions for - improvements. Please refer to the current edition of the "Internet - Official Protocol Standards" (STD 1) for the standardization state - and status of this protocol. Distribution of this memo is unlimited. - -Abstract - - The Dynamic Host Configuration Protocol (DHCP) provides a framework - for passing configuration information to hosts on a TCPIP network. - DHCP is based on the Bootstrap Protocol (BOOTP) [7], adding the - capability of automatic allocation of reusable network addresses and - additional configuration options [19]. DHCP captures the behavior of - BOOTP relay agents [7, 21], and DHCP participants can interoperate - with BOOTP participants [9]. - -Table of Contents - - 1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 2 - 1.1 Changes to RFC1541. . . . . . . . . . . . . . . . . . . . . . 3 - 1.2 Related Work. . . . . . . . . . . . . . . . . . . . . . . . . 4 - 1.3 Problem definition and issues . . . . . . . . . . . . . . . . 4 - 1.4 Requirements. . . . . . . . . . . . . . . . . . . . . . . . . 5 - 1.5 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 - 1.6 Design goals. . . . . . . . . . . . . . . . . . . . . . . . . 6 - 2. Protocol Summary. . . . . . . . . . . . . . . . . . . . . . . 8 - 2.1 Configuration parameters repository . . . . . . . . . . . . . 11 - 2.2 Dynamic allocation of network addresses . . . . . . . . . . . 12 - 3. The Client-Server Protocol. . . . . . . . . . . . . . . . . . 13 - 3.1 Client-server interaction - allocating a network address. . . 13 - 3.2 Client-server interaction - reusing a previously allocated - network address . . . . . . . . . . . . . . . . . . . . . . . 17 - 3.3 Interpretation and representation of time values. . . . . . . 20 - 3.4 Obtaining parameters with externally configured network - address . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 - 3.5 Client parameters in DHCP . . . . . . . . . . . . . . . . . . 21 - 3.6 Use of DHCP in clients with multiple interfaces . . . . . . . 22 - 3.7 When clients should use DHCP. . . . . . . . . . . . . . . . . 22 - 4. Specification of the DHCP client-server protocol. . . . . . . 22 - - - -Droms Standards Track [Page 1] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - 4.1 Constructing and sending DHCP messages. . . . . . . . . . . . 22 - 4.2 DHCP server administrative controls . . . . . . . . . . . . . 25 - 4.3 DHCP server behavior. . . . . . . . . . . . . . . . . . . . . 26 - 4.4 DHCP client behavior. . . . . . . . . . . . . . . . . . . . . 34 - 5. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . .42 - 6. References . . . . . . . . . . . . . . . . . . . . . . . . . .42 - 7. Security Considerations. . . . . . . . . . . . . . . . . . . .43 - 8. Author's Address . . . . . . . . . . . . . . . . . . . . . . .44 - A. Host Configuration Parameters . . . . . . . . . . . . . . . .45 -List of Figures - 1. Format of a DHCP message . . . . . . . . . . . . . . . . . . . 9 - 2. Format of the 'flags' field. . . . . . . . . . . . . . . . . . 11 - 3. Timeline diagram of messages exchanged between DHCP client and - servers when allocating a new network address. . . . . . . . . 15 - 4. Timeline diagram of messages exchanged between DHCP client and - servers when reusing a previously allocated network address. . 18 - 5. State-transition diagram for DHCP clients. . . . . . . . . . . 34 -List of Tables - 1. Description of fields in a DHCP message. . . . . . . . . . . . 10 - 2. DHCP messages. . . . . . . . . . . . . . . . . . . . . . . . . 14 - 3. Fields and options used by DHCP servers. . . . . . . . . . . . 28 - 4. Client messages from various states. . . . . . . . . . . . . . 33 - 5. Fields and options used by DHCP clients. . . . . . . . . . . . 37 - -1. Introduction - - The Dynamic Host Configuration Protocol (DHCP) provides configuration - parameters to Internet hosts. DHCP consists of two components: a - protocol for delivering host-specific configuration parameters from a - DHCP server to a host and a mechanism for allocation of network - addresses to hosts. - - DHCP is built on a client-server model, where designated DHCP server - hosts allocate network addresses and deliver configuration parameters - to dynamically configured hosts. Throughout the remainder of this - document, the term "server" refers to a host providing initialization - parameters through DHCP, and the term "client" refers to a host - requesting initialization parameters from a DHCP server. - - A host should not act as a DHCP server unless explicitly configured - to do so by a system administrator. The diversity of hardware and - protocol implementations in the Internet would preclude reliable - operation if random hosts were allowed to respond to DHCP requests. - For example, IP requires the setting of many parameters within the - protocol implementation software. Because IP can be used on many - dissimilar kinds of network hardware, values for those parameters - cannot be guessed or assumed to have correct defaults. Also, - distributed address allocation schemes depend on a polling/defense - - - -Droms Standards Track [Page 2] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - mechanism for discovery of addresses that are already in use. IP - hosts may not always be able to defend their network addresses, so - that such a distributed address allocation scheme cannot be - guaranteed to avoid allocation of duplicate network addresses. - - DHCP supports three mechanisms for IP address allocation. In - "automatic allocation", DHCP assigns a permanent IP address to a - client. In "dynamic allocation", DHCP assigns an IP address to a - client for a limited period of time (or until the client explicitly - relinquishes the address). In "manual allocation", a client's IP - address is assigned by the network administrator, and DHCP is used - simply to convey the assigned address to the client. A particular - network will use one or more of these mechanisms, depending on the - policies of the network administrator. - - Dynamic allocation is the only one of the three mechanisms that - allows automatic reuse of an address that is no longer needed by the - client to which it was assigned. Thus, dynamic allocation is - particularly useful for assigning an address to a client that will be - connected to the network only temporarily or for sharing a limited - pool of IP addresses among a group of clients that do not need - permanent IP addresses. Dynamic allocation may also be a good choice - for assigning an IP address to a new client being permanently - connected to a network where IP addresses are sufficiently scarce - that it is important to reclaim them when old clients are retired. - Manual allocation allows DHCP to be used to eliminate the error-prone - process of manually configuring hosts with IP addresses in - environments where (for whatever reasons) it is desirable to manage - IP address assignment outside of the DHCP mechanisms. - - The format of DHCP messages is based on the format of BOOTP messages, - to capture the BOOTP relay agent behavior described as part of the - BOOTP specification [7, 21] and to allow interoperability of existing - BOOTP clients with DHCP servers. Using BOOTP relay agents eliminates - the necessity of having a DHCP server on each physical network - segment. - -1.1 Changes to RFC 1541 - - This document updates the DHCP protocol specification that appears in - RFC1541. A new DHCP message type, DHCPINFORM, has been added; see - section 3.4, 4.3 and 4.4 for details. The classing mechanism for - identifying DHCP clients to DHCP servers has been extended to include - "vendor" classes as defined in sections 4.2 and 4.3. The minimum - lease time restriction has been removed. Finally, many editorial - changes have been made to clarify the text as a result of experience - gained in DHCP interoperability tests. - - - - -Droms Standards Track [Page 3] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - -1.2 Related Work - - There are several Internet protocols and related mechanisms that - address some parts of the dynamic host configuration problem. The - Reverse Address Resolution Protocol (RARP) [10] (through the - extensions defined in the Dynamic RARP (DRARP) [5]) explicitly - addresses the problem of network address discovery, and includes an - automatic IP address assignment mechanism. The Trivial File Transfer - Protocol (TFTP) [20] provides for transport of a boot image from a - boot server. The Internet Control Message Protocol (ICMP) [16] - provides for informing hosts of additional routers via "ICMP - redirect" messages. ICMP also can provide subnet mask information - through the "ICMP mask request" message and other information through - the (obsolete) "ICMP information request" message. Hosts can locate - routers through the ICMP router discovery mechanism [8]. - - BOOTP is a transport mechanism for a collection of configuration - information. BOOTP is also extensible, and official extensions [17] - have been defined for several configuration parameters. Morgan has - proposed extensions to BOOTP for dynamic IP address assignment [15]. - The Network Information Protocol (NIP), used by the Athena project at - MIT, is a distributed mechanism for dynamic IP address assignment - [19]. The Resource Location Protocol RLP [1] provides for location - of higher level services. Sun Microsystems diskless workstations use - a boot procedure that employs RARP, TFTP and an RPC mechanism called - "bootparams" to deliver configuration information and operating - system code to diskless hosts. (Sun Microsystems, Sun Workstation - and SunOS are trademarks of Sun Microsystems, Inc.) Some Sun - networks also use DRARP and an auto-installation mechanism to - automate the configuration of new hosts in an existing network. - - In other related work, the path minimum transmission unit (MTU) - discovery algorithm can determine the MTU of an arbitrary internet - path [14]. The Address Resolution Protocol (ARP) has been proposed - as a transport protocol for resource location and selection [6]. - Finally, the Host Requirements RFCs [3, 4] mention specific - requirements for host reconfiguration and suggest a scenario for - initial configuration of diskless hosts. - -1.3 Problem definition and issues - - DHCP is designed to supply DHCP clients with the configuration - parameters defined in the Host Requirements RFCs. After obtaining - parameters via DHCP, a DHCP client should be able to exchange packets - with any other host in the Internet. The TCP/IP stack parameters - supplied by DHCP are listed in Appendix A. - - - - - -Droms Standards Track [Page 4] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - Not all of these parameters are required for a newly initialized - client. A client and server may negotiate for the transmission of - only those parameters required by the client or specific to a - particular subnet. - - DHCP allows but does not require the configuration of client - parameters not directly related to the IP protocol. DHCP also does - not address registration of newly configured clients with the Domain - Name System (DNS) [12, 13]. - - DHCP is not intended for use in configuring routers. - -1.4 Requirements - - Throughout this document, the words that are used to define the - significance of particular requirements are capitalized. These words - are: - - o "MUST" - - This word or the adjective "REQUIRED" means that the - item is an absolute requirement of this specification. - - o "MUST NOT" - - This phrase means that the item is an absolute prohibition - of this specification. - - o "SHOULD" - - This word or the adjective "RECOMMENDED" means that there - may exist valid reasons in particular circumstances to ignore - this item, but the full implications should be understood and - the case carefully weighed before choosing a different course. - - o "SHOULD NOT" - - This phrase means that there may exist valid reasons in - particular circumstances when the listed behavior is acceptable - or even useful, but the full implications should be understood - and the case carefully weighed before implementing any behavior - described with this label. - - - - - - - - - -Droms Standards Track [Page 5] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - o "MAY" - - This word or the adjective "OPTIONAL" means that this item is - truly optional. One vendor may choose to include the item - because a particular marketplace requires it or because it - enhances the product, for example; another vendor may omit the - same item. - -1.5 Terminology - - This document uses the following terms: - - o "DHCP client" - - A DHCP client is an Internet host using DHCP to obtain - configuration parameters such as a network address. - - o "DHCP server" - - A DHCP server is an Internet host that returns configuration - parameters to DHCP clients. - - o "BOOTP relay agent" - - A BOOTP relay agent or relay agent is an Internet host or router - that passes DHCP messages between DHCP clients and DHCP servers. - DHCP is designed to use the same relay agent behavior as specified - in the BOOTP protocol specification. - - o "binding" - - A binding is a collection of configuration parameters, including - at least an IP address, associated with or "bound to" a DHCP - client. Bindings are managed by DHCP servers. - -1.6 Design goals - - The following list gives general design goals for DHCP. - - o DHCP should be a mechanism rather than a policy. DHCP must - allow local system administrators control over configuration - parameters where desired; e.g., local system administrators - should be able to enforce local policies concerning allocation - and access to local resources where desired. - - - - - - - -Droms Standards Track [Page 6] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - o Clients should require no manual configuration. Each client - should be able to discover appropriate local configuration - parameters without user intervention and incorporate those - parameters into its own configuration. - - o Networks should require no manual configuration for individual - clients. Under normal circumstances, the network manager - should not have to enter any per-client configuration - parameters. - - o DHCP should not require a server on each subnet. To allow for - scale and economy, DHCP must work across routers or through the - intervention of BOOTP relay agents. - - o A DHCP client must be prepared to receive multiple responses - to a request for configuration parameters. Some installations - may include multiple, overlapping DHCP servers to enhance - reliability and increase performance. - - o DHCP must coexist with statically configured, non-participating - hosts and with existing network protocol implementations. - - o DHCP must interoperate with the BOOTP relay agent behavior as - described by RFC 951 and by RFC 1542 [21]. - - o DHCP must provide service to existing BOOTP clients. - - The following list gives design goals specific to the transmission of - the network layer parameters. DHCP must: - - o Guarantee that any specific network address will not be in - use by more than one DHCP client at a time, - - o Retain DHCP client configuration across DHCP client reboot. A - DHCP client should, whenever possible, be assigned the same - configuration parameters (e.g., network address) in response - to each request, - - o Retain DHCP client configuration across server reboots, and, - whenever possible, a DHCP client should be assigned the same - configuration parameters despite restarts of the DHCP mechanism, - - o Allow automated assignment of configuration parameters to new - clients to avoid hand configuration for new clients, - - o Support fixed or permanent allocation of configuration - parameters to specific clients. - - - - -Droms Standards Track [Page 7] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - -2. Protocol Summary - - From the client's point of view, DHCP is an extension of the BOOTP - mechanism. This behavior allows existing BOOTP clients to - interoperate with DHCP servers without requiring any change to the - clients' initialization software. RFC 1542 [2] details the - interactions between BOOTP and DHCP clients and servers [9]. There - are some new, optional transactions that optimize the interaction - between DHCP clients and servers that are described in sections 3 and - 4. - - Figure 1 gives the format of a DHCP message and table 1 describes - each of the fields in the DHCP message. The numbers in parentheses - indicate the size of each field in octets. The names for the fields - given in the figure will be used throughout this document to refer to - the fields in DHCP messages. - - There are two primary differences between DHCP and BOOTP. First, - DHCP defines mechanisms through which clients can be assigned a - network address for a finite lease, allowing for serial reassignment - of network addresses to different clients. Second, DHCP provides the - mechanism for a client to acquire all of the IP configuration - parameters that it needs in order to operate. - - DHCP introduces a small change in terminology intended to clarify the - meaning of one of the fields. What was the "vendor extensions" field - in BOOTP has been re-named the "options" field in DHCP. Similarly, - the tagged data items that were used inside the BOOTP "vendor - extensions" field, which were formerly referred to as "vendor - extensions," are now termed simply "options." - - - - - - - - - - - - - - - - - - - - - -Droms Standards Track [Page 8] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - 0 1 2 3 - 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - | op (1) | htype (1) | hlen (1) | hops (1) | - +---------------+---------------+---------------+---------------+ - | xid (4) | - +-------------------------------+-------------------------------+ - | secs (2) | flags (2) | - +-------------------------------+-------------------------------+ - | ciaddr (4) | - +---------------------------------------------------------------+ - | yiaddr (4) | - +---------------------------------------------------------------+ - | siaddr (4) | - +---------------------------------------------------------------+ - | giaddr (4) | - +---------------------------------------------------------------+ - | | - | chaddr (16) | - | | - | | - +---------------------------------------------------------------+ - | | - | sname (64) | - +---------------------------------------------------------------+ - | | - | file (128) | - +---------------------------------------------------------------+ - | | - | options (variable) | - +---------------------------------------------------------------+ - - Figure 1: Format of a DHCP message - - DHCP defines a new 'client identifier' option that is used to pass an - explicit client identifier to a DHCP server. This change eliminates - the overloading of the 'chaddr' field in BOOTP messages, where - 'chaddr' is used both as a hardware address for transmission of BOOTP - reply messages and as a client identifier. The 'client identifier' - is an opaque key, not to be interpreted by the server; for example, - the 'client identifier' may contain a hardware address, identical to - the contents of the 'chaddr' field, or it may contain another type of - identifier, such as a DNS name. The 'client identifier' chosen by a - DHCP client MUST be unique to that client within the subnet to which - the client is attached. If the client uses a 'client identifier' in - one message, it MUST use that same identifier in all subsequent - messages, to ensure that all servers correctly identify the client. - - - - -Droms Standards Track [Page 9] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - DHCP clarifies the interpretation of the 'siaddr' field as the - address of the server to use in the next step of the client's - bootstrap process. A DHCP server may return its own address in the - 'siaddr' field, if the server is prepared to supply the next - bootstrap service (e.g., delivery of an operating system executable - image). A DHCP server always returns its own address in the 'server - identifier' option. - - FIELD OCTETS DESCRIPTION - ----- ------ ----------- - - op 1 Message op code / message type. - 1 = BOOTREQUEST, 2 = BOOTREPLY - htype 1 Hardware address type, see ARP section in "Assigned - Numbers" RFC; e.g., '1' = 10mb ethernet. - hlen 1 Hardware address length (e.g. '6' for 10mb - ethernet). - hops 1 Client sets to zero, optionally used by relay agents - when booting via a relay agent. - xid 4 Transaction ID, a random number chosen by the - client, used by the client and server to associate - messages and responses between a client and a - server. - secs 2 Filled in by client, seconds elapsed since client - began address acquisition or renewal process. - flags 2 Flags (see figure 2). - ciaddr 4 Client IP address; only filled in if client is in - BOUND, RENEW or REBINDING state and can respond - to ARP requests. - yiaddr 4 'your' (client) IP address. - siaddr 4 IP address of next server to use in bootstrap; - returned in DHCPOFFER, DHCPACK by server. - giaddr 4 Relay agent IP address, used in booting via a - relay agent. - chaddr 16 Client hardware address. - sname 64 Optional server host name, null terminated string. - file 128 Boot file name, null terminated string; "generic" - name or null in DHCPDISCOVER, fully qualified - directory-path name in DHCPOFFER. - options var Optional parameters field. See the options - documents for a list of defined options. - - Table 1: Description of fields in a DHCP message - - The 'options' field is now variable length. A DHCP client must be - prepared to receive DHCP messages with an 'options' field of at least - length 312 octets. This requirement implies that a DHCP client must - be prepared to receive a message of up to 576 octets, the minimum IP - - - -Droms Standards Track [Page 10] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - datagram size an IP host must be prepared to accept [3]. DHCP - clients may negotiate the use of larger DHCP messages through the - 'maximum DHCP message size' option. The options field may be further - extended into the 'file' and 'sname' fields. - - In the case of a client using DHCP for initial configuration (before - the client's TCP/IP software has been completely configured), DHCP - requires creative use of the client's TCP/IP software and liberal - interpretation of RFC 1122. The TCP/IP software SHOULD accept and - forward to the IP layer any IP packets delivered to the client's - hardware address before the IP address is configured; DHCP servers - and BOOTP relay agents may not be able to deliver DHCP messages to - clients that cannot accept hardware unicast datagrams before the - TCP/IP software is configured. - - To work around some clients that cannot accept IP unicast datagrams - before the TCP/IP software is configured as discussed in the previous - paragraph, DHCP uses the 'flags' field [21]. The leftmost bit is - defined as the BROADCAST (B) flag. The semantics of this flag are - discussed in section 4.1 of this document. The remaining bits of the - flags field are reserved for future use. They MUST be set to zero by - clients and ignored by servers and relay agents. Figure 2 gives the - format of the 'flags' field. - - 1 1 1 1 1 1 - 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - |B| MBZ | - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - B: BROADCAST flag - - MBZ: MUST BE ZERO (reserved for future use) - - Figure 2: Format of the 'flags' field - -2.1 Configuration parameters repository - - The first service provided by DHCP is to provide persistent storage - of network parameters for network clients. The model of DHCP - persistent storage is that the DHCP service stores a key-value entry - for each client, where the key is some unique identifier (for - example, an IP subnet number and a unique identifier within the - subnet) and the value contains the configuration parameters for the - client. - - For example, the key might be the pair (IP-subnet-number, hardware- - address) (note that the "hardware-address" should be typed by the - - - -Droms Standards Track [Page 11] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - type of hardware to accommodate possible duplication of hardware - addresses resulting from bit-ordering problems in a mixed-media, - bridged network) allowing for serial or concurrent reuse of a - hardware address on different subnets, and for hardware addresses - that may not be globally unique. Alternately, the key might be the - pair (IP-subnet-number, hostname), allowing the server to assign - parameters intelligently to a DHCP client that has been moved to a - different subnet or has changed hardware addresses (perhaps because - the network interface failed and was replaced). The protocol defines - that the key will be (IP-subnet-number, hardware-address) unless the - client explicitly supplies an identifier using the 'client - identifier' option. A client can query the DHCP service to - retrieve its configuration parameters. The client interface to the - configuration parameters repository consists of protocol messages to - request configuration parameters and responses from the server - carrying the configuration parameters. - -2.2 Dynamic allocation of network addresses - - The second service provided by DHCP is the allocation of temporary or - permanent network (IP) addresses to clients. The basic mechanism for - the dynamic allocation of network addresses is simple: a client - requests the use of an address for some period of time. The - allocation mechanism (the collection of DHCP servers) guarantees not - to reallocate that address within the requested time and attempts to - return the same network address each time the client requests an - address. In this document, the period over which a network address - is allocated to a client is referred to as a "lease" [11]. The - client may extend its lease with subsequent requests. The client may - issue a message to release the address back to the server when the - client no longer needs the address. The client may ask for a - permanent assignment by asking for an infinite lease. Even when - assigning "permanent" addresses, a server may choose to give out - lengthy but non-infinite leases to allow detection of the fact that - the client has been retired. - - In some environments it will be necessary to reassign network - addresses due to exhaustion of available addresses. In such - environments, the allocation mechanism will reuse addresses whose - lease has expired. The server should use whatever information is - available in the configuration information repository to choose an - address to reuse. For example, the server may choose the least - recently assigned address. As a consistency check, the allocating - server SHOULD probe the reused address before allocating the address, - e.g., with an ICMP echo request, and the client SHOULD probe the - newly received address, e.g., with ARP. - - - - - -Droms Standards Track [Page 12] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - -3. The Client-Server Protocol - - DHCP uses the BOOTP message format defined in RFC 951 and given in - table 1 and figure 1. The 'op' field of each DHCP message sent from - a client to a server contains BOOTREQUEST. BOOTREPLY is used in the - 'op' field of each DHCP message sent from a server to a client. - - The first four octets of the 'options' field of the DHCP message - contain the (decimal) values 99, 130, 83 and 99, respectively (this - is the same magic cookie as is defined in RFC 1497 [17]). The - remainder of the 'options' field consists of a list of tagged - parameters that are called "options". All of the "vendor extensions" - listed in RFC 1497 are also DHCP options. RFC 1533 gives the - complete set of options defined for use with DHCP. - - Several options have been defined so far. One particular option - - the "DHCP message type" option - must be included in every DHCP - message. This option defines the "type" of the DHCP message. - Additional options may be allowed, required, or not allowed, - depending on the DHCP message type. - - Throughout this document, DHCP messages that include a 'DHCP message - type' option will be referred to by the type of the message; e.g., a - DHCP message with 'DHCP message type' option type 1 will be referred - to as a "DHCPDISCOVER" message. - -3.1 Client-server interaction - allocating a network address - - The following summary of the protocol exchanges between clients and - servers refers to the DHCP messages described in table 2. The - timeline diagram in figure 3 shows the timing relationships in a - typical client-server interaction. If the client already knows its - address, some steps may be omitted; this abbreviated interaction is - described in section 3.2. - - 1. The client broadcasts a DHCPDISCOVER message on its local physical - subnet. The DHCPDISCOVER message MAY include options that suggest - values for the network address and lease duration. BOOTP relay - agents may pass the message on to DHCP servers not on the same - physical subnet. - - 2. Each server may respond with a DHCPOFFER message that includes an - available network address in the 'yiaddr' field (and other - configuration parameters in DHCP options). Servers need not - reserve the offered network address, although the protocol will - work more efficiently if the server avoids allocating the offered - network address to another client. When allocating a new address, - servers SHOULD check that the offered network address is not - - - -Droms Standards Track [Page 13] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - already in use; e.g., the server may probe the offered address - with an ICMP Echo Request. Servers SHOULD be implemented so that - network administrators MAY choose to disable probes of newly - allocated addresses. The server transmits the DHCPOFFER message - to the client, using the BOOTP relay agent if necessary. - - Message Use - ------- --- - - DHCPDISCOVER - Client broadcast to locate available servers. - - DHCPOFFER - Server to client in response to DHCPDISCOVER with - offer of configuration parameters. - - DHCPREQUEST - Client message to servers either (a) requesting - offered parameters from one server and implicitly - declining offers from all others, (b) confirming - correctness of previously allocated address after, - e.g., system reboot, or (c) extending the lease on a - particular network address. - - DHCPACK - Server to client with configuration parameters, - including committed network address. - - DHCPNAK - Server to client indicating client's notion of network - address is incorrect (e.g., client has moved to new - subnet) or client's lease as expired - - DHCPDECLINE - Client to server indicating network address is already - in use. - - DHCPRELEASE - Client to server relinquishing network address and - cancelling remaining lease. - - DHCPINFORM - Client to server, asking only for local configuration - parameters; client already has externally configured - network address. - - Table 2: DHCP messages - - - - - - - - - - - - -Droms Standards Track [Page 14] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - Server Client Server - (not selected) (selected) - - v v v - | | | - | Begins initialization | - | | | - | _____________/|\____________ | - |/DHCPDISCOVER | DHCPDISCOVER \| - | | | - Determines | Determines - configuration | configuration - | | | - |\ | ____________/ | - | \________ | /DHCPOFFER | - | DHCPOFFER\ |/ | - | \ | | - | Collects replies | - | \| | - | Selects configuration | - | | | - | _____________/|\____________ | - |/ DHCPREQUEST | DHCPREQUEST\ | - | | | - | | Commits configuration - | | | - | | _____________/| - | |/ DHCPACK | - | | | - | Initialization complete | - | | | - . . . - . . . - | | | - | Graceful shutdown | - | | | - | |\ ____________ | - | | DHCPRELEASE \| - | | | - | | Discards lease - | | | - v v v - Figure 3: Timeline diagram of messages exchanged between DHCP - client and servers when allocating a new network address - - - - - - - -Droms Standards Track [Page 15] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - 3. The client receives one or more DHCPOFFER messages from one or more - servers. The client may choose to wait for multiple responses. - The client chooses one server from which to request configuration - parameters, based on the configuration parameters offered in the - DHCPOFFER messages. The client broadcasts a DHCPREQUEST message - that MUST include the 'server identifier' option to indicate which - server it has selected, and that MAY include other options - specifying desired configuration values. The 'requested IP - address' option MUST be set to the value of 'yiaddr' in the - DHCPOFFER message from the server. This DHCPREQUEST message is - broadcast and relayed through DHCP/BOOTP relay agents. To help - ensure that any BOOTP relay agents forward the DHCPREQUEST message - to the same set of DHCP servers that received the original - DHCPDISCOVER message, the DHCPREQUEST message MUST use the same - value in the DHCP message header's 'secs' field and be sent to the - same IP broadcast address as the original DHCPDISCOVER message. - The client times out and retransmits the DHCPDISCOVER message if - the client receives no DHCPOFFER messages. - - 4. The servers receive the DHCPREQUEST broadcast from the client. - Those servers not selected by the DHCPREQUEST message use the - message as notification that the client has declined that server's - offer. The server selected in the DHCPREQUEST message commits the - binding for the client to persistent storage and responds with a - DHCPACK message containing the configuration parameters for the - requesting client. The combination of 'client identifier' or - 'chaddr' and assigned network address constitute a unique - identifier for the client's lease and are used by both the client - and server to identify a lease referred to in any DHCP messages. - Any configuration parameters in the DHCPACK message SHOULD NOT - conflict with those in the earlier DHCPOFFER message to which the - client is responding. The server SHOULD NOT check the offered - network address at this point. The 'yiaddr' field in the DHCPACK - messages is filled in with the selected network address. - - If the selected server is unable to satisfy the DHCPREQUEST message - (e.g., the requested network address has been allocated), the - server SHOULD respond with a DHCPNAK message. - - A server MAY choose to mark addresses offered to clients in - DHCPOFFER messages as unavailable. The server SHOULD mark an - address offered to a client in a DHCPOFFER message as available if - the server receives no DHCPREQUEST message from that client. - - 5. The client receives the DHCPACK message with configuration - parameters. The client SHOULD perform a final check on the - parameters (e.g., ARP for allocated network address), and notes the - duration of the lease specified in the DHCPACK message. At this - - - -Droms Standards Track [Page 16] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - point, the client is configured. If the client detects that the - address is already in use (e.g., through the use of ARP), the - client MUST send a DHCPDECLINE message to the server and restarts - the configuration process. The client SHOULD wait a minimum of ten - seconds before restarting the configuration process to avoid - excessive network traffic in case of looping. - - If the client receives a DHCPNAK message, the client restarts the - configuration process. - - The client times out and retransmits the DHCPREQUEST message if the - client receives neither a DHCPACK or a DHCPNAK message. The client - retransmits the DHCPREQUEST according to the retransmission - algorithm in section 4.1. The client should choose to retransmit - the DHCPREQUEST enough times to give adequate probability of - contacting the server without causing the client (and the user of - that client) to wait overly long before giving up; e.g., a client - retransmitting as described in section 4.1 might retransmit the - DHCPREQUEST message four times, for a total delay of 60 seconds, - before restarting the initialization procedure. If the client - receives neither a DHCPACK or a DHCPNAK message after employing the - retransmission algorithm, the client reverts to INIT state and - restarts the initialization process. The client SHOULD notify the - user that the initialization process has failed and is restarting. - - 6. The client may choose to relinquish its lease on a network address - by sending a DHCPRELEASE message to the server. The client - identifies the lease to be released with its 'client identifier', - or 'chaddr' and network address in the DHCPRELEASE message. If the - client used a 'client identifier' when it obtained the lease, it - MUST use the same 'client identifier' in the DHCPRELEASE message. - -3.2 Client-server interaction - reusing a previously allocated network - address - - If a client remembers and wishes to reuse a previously allocated - network address, a client may choose to omit some of the steps - described in the previous section. The timeline diagram in figure 4 - shows the timing relationships in a typical client-server interaction - for a client reusing a previously allocated network address. - - - - - - - - - - - -Droms Standards Track [Page 17] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - 1. The client broadcasts a DHCPREQUEST message on its local subnet. - The message includes the client's network address in the - 'requested IP address' option. As the client has not received its - network address, it MUST NOT fill in the 'ciaddr' field. BOOTP - relay agents pass the message on to DHCP servers not on the same - subnet. If the client used a 'client identifier' to obtain its - address, the client MUST use the same 'client identifier' in the - DHCPREQUEST message. - - 2. Servers with knowledge of the client's configuration parameters - respond with a DHCPACK message to the client. Servers SHOULD NOT - check that the client's network address is already in use; the - client may respond to ICMP Echo Request messages at this point. - - Server Client Server - - v v v - | | | - | Begins | - | initialization | - | | | - | /|\ | - | _________ __/ | \__________ | - | /DHCPREQU EST | DHCPREQUEST\ | - |/ | \| - | | | - Locates | Locates - configuration | configuration - | | | - |\ | /| - | \ | ___________/ | - | \ | / DHCPACK | - | \ _______ |/ | - | DHCPACK\ | | - | Initialization | - | complete | - | \| | - | | | - | (Subsequent | - | DHCPACKS | - | ignored) | - | | | - | | | - v v v - - Figure 4: Timeline diagram of messages exchanged between DHCP - client and servers when reusing a previously allocated - network address - - - -Droms Standards Track [Page 18] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - If the client's request is invalid (e.g., the client has moved - to a new subnet), servers SHOULD respond with a DHCPNAK message to - the client. Servers SHOULD NOT respond if their information is not - guaranteed to be accurate. For example, a server that identifies a - request for an expired binding that is owned by another server SHOULD - NOT respond with a DHCPNAK unless the servers are using an explicit - mechanism to maintain coherency among the servers. - - If 'giaddr' is 0x0 in the DHCPREQUEST message, the client is on - the same subnet as the server. The server MUST - broadcast the DHCPNAK message to the 0xffffffff broadcast address - because the client may not have a correct network address or subnet - mask, and the client may not be answering ARP requests. - Otherwise, the server MUST send the DHCPNAK message to the IP - address of the BOOTP relay agent, as recorded in 'giaddr'. The - relay agent will, in turn, forward the message directly to the - client's hardware address, so that the DHCPNAK can be delivered even - if the client has moved to a new network. - - 3. The client receives the DHCPACK message with configuration - parameters. The client performs a final check on the parameters - (as in section 3.1), and notes the duration of the lease specified - in the DHCPACK message. The specific lease is implicitly identified - by the 'client identifier' or 'chaddr' and the network address. At - this point, the client is configured. - - If the client detects that the IP address in the DHCPACK message - is already in use, the client MUST send a DHCPDECLINE message to the - server and restarts the configuration process by requesting a - new network address. This action corresponds to the client - moving to the INIT state in the DHCP state diagram, which is - described in section 4.4. - - If the client receives a DHCPNAK message, it cannot reuse its - remembered network address. It must instead request a new - address by restarting the configuration process, this time - using the (non-abbreviated) procedure described in section - 3.1. This action also corresponds to the client moving to - the INIT state in the DHCP state diagram. - - The client times out and retransmits the DHCPREQUEST message if - the client receives neither a DHCPACK nor a DHCPNAK message. The - client retransmits the DHCPREQUEST according to the retransmission - algorithm in section 4.1. The client should choose to retransmit - the DHCPREQUEST enough times to give adequate probability of - contacting the server without causing the client (and the user of - that client) to wait overly long before giving up; e.g., a client - retransmitting as described in section 4.1 might retransmit the - - - -Droms Standards Track [Page 19] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - DHCPREQUEST message four times, for a total delay of 60 seconds, - before restarting the initialization procedure. If the client - receives neither a DHCPACK or a DHCPNAK message after employing - the retransmission algorithm, the client MAY choose to use the - previously allocated network address and configuration parameters - for the remainder of the unexpired lease. This corresponds to - moving to BOUND state in the client state transition diagram shown - in figure 5. - - 4. The client may choose to relinquish its lease on a network - address by sending a DHCPRELEASE message to the server. The - client identifies the lease to be released with its - 'client identifier', or 'chaddr' and network address in the - DHCPRELEASE message. - - Note that in this case, where the client retains its network - address locally, the client will not normally relinquish its - lease during a graceful shutdown. Only in the case where the - client explicitly needs to relinquish its lease, e.g., the client - is about to be moved to a different subnet, will the client send - a DHCPRELEASE message. - -3.3 Interpretation and representation of time values - - A client acquires a lease for a network address for a fixed period of - time (which may be infinite). Throughout the protocol, times are to - be represented in units of seconds. The time value of 0xffffffff is - reserved to represent "infinity". - - As clients and servers may not have synchronized clocks, times are - represented in DHCP messages as relative times, to be interpreted - with respect to the client's local clock. Representing relative - times in units of seconds in an unsigned 32 bit word gives a range of - relative times from 0 to approximately 100 years, which is sufficient - for the relative times to be measured using DHCP. - - The algorithm for lease duration interpretation given in the previous - paragraph assumes that client and server clocks are stable relative - to each other. If there is drift between the two clocks, the server - may consider the lease expired before the client does. To - compensate, the server may return a shorter lease duration to the - client than the server commits to its local database of client - information. - -3.4 Obtaining parameters with externally configured network address - - If a client has obtained a network address through some other means - (e.g., manual configuration), it may use a DHCPINFORM request message - - - -Droms Standards Track [Page 20] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - to obtain other local configuration parameters. Servers receiving a - DHCPINFORM message construct a DHCPACK message with any local - configuration parameters appropriate for the client without: - allocating a new address, checking for an existing binding, filling - in 'yiaddr' or including lease time parameters. The servers SHOULD - unicast the DHCPACK reply to the address given in the 'ciaddr' field - of the DHCPINFORM message. - - The server SHOULD check the network address in a DHCPINFORM message - for consistency, but MUST NOT check for an existing lease. The - server forms a DHCPACK message containing the configuration - parameters for the requesting client and sends the DHCPACK message - directly to the client. - -3.5 Client parameters in DHCP - - Not all clients require initialization of all parameters listed in - Appendix A. Two techniques are used to reduce the number of - parameters transmitted from the server to the client. First, most of - the parameters have defaults defined in the Host Requirements RFCs; - if the client receives no parameters from the server that override - the defaults, a client uses those default values. Second, in its - initial DHCPDISCOVER or DHCPREQUEST message, a client may provide the - server with a list of specific parameters the client is interested - in. If the client includes a list of parameters in a DHCPDISCOVER - message, it MUST include that list in any subsequent DHCPREQUEST - messages. - - The client SHOULD include the 'maximum DHCP message size' option to - let the server know how large the server may make its DHCP messages. - The parameters returned to a client may still exceed the space - allocated to options in a DHCP message. In this case, two additional - options flags (which must appear in the 'options' field of the - message) indicate that the 'file' and 'sname' fields are to be used - for options. - - The client can inform the server which configuration parameters the - client is interested in by including the 'parameter request list' - option. The data portion of this option explicitly lists the options - requested by tag number. - - In addition, the client may suggest values for the network address - and lease time in the DHCPDISCOVER message. The client may include - the 'requested IP address' option to suggest that a particular IP - address be assigned, and may include the 'IP address lease time' - option to suggest the lease time it would like. Other options - representing "hints" at configuration parameters are allowed in a - DHCPDISCOVER or DHCPREQUEST message. However, additional options may - - - -Droms Standards Track [Page 21] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - be ignored by servers, and multiple servers may, therefore, not - return identical values for some options. The 'requested IP address' - option is to be filled in only in a DHCPREQUEST message when the - client is verifying network parameters obtained previously. The - client fills in the 'ciaddr' field only when correctly configured - with an IP address in BOUND, RENEWING or REBINDING state. - - If a server receives a DHCPREQUEST message with an invalid 'requested - IP address', the server SHOULD respond to the client with a DHCPNAK - message and may choose to report the problem to the system - administrator. The server may include an error message in the - 'message' option. - -3.6 Use of DHCP in clients with multiple interfaces - - A client with multiple network interfaces must use DHCP through each - interface independently to obtain configuration information - parameters for those separate interfaces. - -3.7 When clients should use DHCP - - A client SHOULD use DHCP to reacquire or verify its IP address and - network parameters whenever the local network parameters may have - changed; e.g., at system boot time or after a disconnection from the - local network, as the local network configuration may change without - the client's or user's knowledge. - - If a client has knowledge of a previous network address and is unable - to contact a local DHCP server, the client may continue to use the - previous network address until the lease for that address expires. - If the lease expires before the client can contact a DHCP server, the - client must immediately discontinue use of the previous network - address and may inform local users of the problem. - -4. Specification of the DHCP client-server protocol - - In this section, we assume that a DHCP server has a block of network - addresses from which it can satisfy requests for new addresses. Each - server also maintains a database of allocated addresses and leases in - local permanent storage. - -4.1 Constructing and sending DHCP messages - - DHCP clients and servers both construct DHCP messages by filling in - fields in the fixed format section of the message and appending - tagged data items in the variable length option area. The options - area includes first a four-octet 'magic cookie' (which was described - in section 3), followed by the options. The last option must always - - - -Droms Standards Track [Page 22] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - be the 'end' option. - - DHCP uses UDP as its transport protocol. DHCP messages from a client - to a server are sent to the 'DHCP server' port (67), and DHCP - messages from a server to a client are sent to the 'DHCP client' port - (68). A server with multiple network address (e.g., a multi-homed - host) MAY use any of its network addresses in outgoing DHCP messages. - - The 'server identifier' field is used both to identify a DHCP server - in a DHCP message and as a destination address from clients to - servers. A server with multiple network addresses MUST be prepared - to to accept any of its network addresses as identifying that server - in a DHCP message. To accommodate potentially incomplete network - connectivity, a server MUST choose an address as a 'server - identifier' that, to the best of the server's knowledge, is reachable - from the client. For example, if the DHCP server and the DHCP client - are connected to the same subnet (i.e., the 'giaddr' field in the - message from the client is zero), the server SHOULD select the IP - address the server is using for communication on that subnet as the - 'server identifier'. If the server is using multiple IP addresses on - that subnet, any such address may be used. If the server has - received a message through a DHCP relay agent, the server SHOULD - choose an address from the interface on which the message was - recieved as the 'server identifier' (unless the server has other, - better information on which to make its choice). DHCP clients MUST - use the IP address provided in the 'server identifier' option for any - unicast requests to the DHCP server. - - DHCP messages broadcast by a client prior to that client obtaining - its IP address must have the source address field in the IP header - set to 0. - - If the 'giaddr' field in a DHCP message from a client is non-zero, - the server sends any return messages to the 'DHCP server' port on the - BOOTP relay agent whose address appears in 'giaddr'. If the 'giaddr' - field is zero and the 'ciaddr' field is nonzero, then the server - unicasts DHCPOFFER and DHCPACK messages to the address in 'ciaddr'. - If 'giaddr' is zero and 'ciaddr' is zero, and the broadcast bit is - set, then the server broadcasts DHCPOFFER and DHCPACK messages to - 0xffffffff. If the broadcast bit is not set and 'giaddr' is zero and - 'ciaddr' is zero, then the server unicasts DHCPOFFER and DHCPACK - messages to the client's hardware address and 'yiaddr' address. In - all cases, when 'giaddr' is zero, the server broadcasts any DHCPNAK - messages to 0xffffffff. - - If the options in a DHCP message extend into the 'sname' and 'file' - fields, the 'option overload' option MUST appear in the 'options' - field, with value 1, 2 or 3, as specified in RFC 1533. If the - - - -Droms Standards Track [Page 23] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - 'option overload' option is present in the 'options' field, the - options in the 'options' field MUST be terminated by an 'end' option, - and MAY contain one or more 'pad' options to fill the options field. - The options in the 'sname' and 'file' fields (if in use as indicated - by the 'options overload' option) MUST begin with the first octet of - the field, MUST be terminated by an 'end' option, and MUST be - followed by 'pad' options to fill the remainder of the field. Any - individual option in the 'options', 'sname' and 'file' fields MUST be - entirely contained in that field. The options in the 'options' field - MUST be interpreted first, so that any 'option overload' options may - be interpreted. The 'file' field MUST be interpreted next (if the - 'option overload' option indicates that the 'file' field contains - DHCP options), followed by the 'sname' field. - - The values to be passed in an 'option' tag may be too long to fit in - the 255 octets available to a single option (e.g., a list of routers - in a 'router' option [21]). Options may appear only once, unless - otherwise specified in the options document. The client concatenates - the values of multiple instances of the same option into a single - parameter list for configuration. - - DHCP clients are responsible for all message retransmission. The - client MUST adopt a retransmission strategy that incorporates a - randomized exponential backoff algorithm to determine the delay - between retransmissions. The delay between retransmissions SHOULD be - chosen to allow sufficient time for replies from the server to be - delivered based on the characteristics of the internetwork between - the client and the server. For example, in a 10Mb/sec Ethernet - internetwork, the delay before the first retransmission SHOULD be 4 - seconds randomized by the value of a uniform random number chosen - from the range -1 to +1. Clients with clocks that provide resolution - granularity of less than one second may choose a non-integer - randomization value. The delay before the next retransmission SHOULD - be 8 seconds randomized by the value of a uniform number chosen from - the range -1 to +1. The retransmission delay SHOULD be doubled with - subsequent retransmissions up to a maximum of 64 seconds. The client - MAY provide an indication of retransmission attempts to the user as - an indication of the progress of the configuration process. - - The 'xid' field is used by the client to match incoming DHCP messages - with pending requests. A DHCP client MUST choose 'xid's in such a - way as to minimize the chance of using an 'xid' identical to one used - by another client. For example, a client may choose a different, - random initial 'xid' each time the client is rebooted, and - subsequently use sequential 'xid's until the next reboot. Selecting - a new 'xid' for each retransmission is an implementation decision. A - client may choose to reuse the same 'xid' or select a new 'xid' for - each retransmitted message. - - - -Droms Standards Track [Page 24] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - Normally, DHCP servers and BOOTP relay agents attempt to deliver - DHCPOFFER, DHCPACK and DHCPNAK messages directly to the client using - uicast delivery. The IP destination address (in the IP header) is - set to the DHCP 'yiaddr' address and the link-layer destination - address is set to the DHCP 'chaddr' address. Unfortunately, some - client implementations are unable to receive such unicast IP - datagrams until the implementation has been configured with a valid - IP address (leading to a deadlock in which the client's IP address - cannot be delivered until the client has been configured with an IP - address). - - A client that cannot receive unicast IP datagrams until its protocol - software has been configured with an IP address SHOULD set the - BROADCAST bit in the 'flags' field to 1 in any DHCPDISCOVER or - DHCPREQUEST messages that client sends. The BROADCAST bit will - provide a hint to the DHCP server and BOOTP relay agent to broadcast - any messages to the client on the client's subnet. A client that can - receive unicast IP datagrams before its protocol software has been - configured SHOULD clear the BROADCAST bit to 0. The BOOTP - clarifications document discusses the ramifications of the use of the - BROADCAST bit [21]. - - A server or relay agent sending or relaying a DHCP message directly - to a DHCP client (i.e., not to a relay agent specified in the - 'giaddr' field) SHOULD examine the BROADCAST bit in the 'flags' - field. If this bit is set to 1, the DHCP message SHOULD be sent as - an IP broadcast using an IP broadcast address (preferably 0xffffffff) - as the IP destination address and the link-layer broadcast address as - the link-layer destination address. If the BROADCAST bit is cleared - to 0, the message SHOULD be sent as an IP unicast to the IP address - specified in the 'yiaddr' field and the link-layer address specified - in the 'chaddr' field. If unicasting is not possible, the message - MAY be sent as an IP broadcast using an IP broadcast address - (preferably 0xffffffff) as the IP destination address and the link- - layer broadcast address as the link-layer destination address. - -4.2 DHCP server administrative controls - - DHCP servers are not required to respond to every DHCPDISCOVER and - DHCPREQUEST message they receive. For example, a network - administrator, to retain stringent control over the clients attached - to the network, may choose to configure DHCP servers to respond only - to clients that have been previously registered through some external - mechanism. The DHCP specification describes only the interactions - between clients and servers when the clients and servers choose to - interact; it is beyond the scope of the DHCP specification to - describe all of the administrative controls that system - administrators might want to use. Specific DHCP server - - - -Droms Standards Track [Page 25] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - implementations may incorporate any controls or policies desired by a - network administrator. - - In some environments, a DHCP server will have to consider the values - of the vendor class options included in DHCPDISCOVER or DHCPREQUEST - messages when determining the correct parameters for a particular - client. - - A DHCP server needs to use some unique identifier to associate a - client with its lease. The client MAY choose to explicitly provide - the identifier through the 'client identifier' option. If the client - supplies a 'client identifier', the client MUST use the same 'client - identifier' in all subsequent messages, and the server MUST use that - identifier to identify the client. If the client does not provide a - 'client identifier' option, the server MUST use the contents of the - 'chaddr' field to identify the client. It is crucial for a DHCP - client to use an identifier unique within the subnet to which the - client is attached in the 'client identifier' option. Use of - 'chaddr' as the client's unique identifier may cause unexpected - results, as that identifier may be associated with a hardware - interface that could be moved to a new client. Some sites may choose - to use a manufacturer's serial number as the 'client identifier', to - avoid unexpected changes in a clients network address due to transfer - of hardware interfaces among computers. Sites may also choose to use - a DNS name as the 'client identifier', causing address leases to be - associated with the DNS name rather than a specific hardware box. - - DHCP clients are free to use any strategy in selecting a DHCP server - among those from which the client receives a DHCPOFFER message. The - client implementation of DHCP SHOULD provide a mechanism for the user - to select directly the 'vendor class identifier' values. - -4.3 DHCP server behavior - - A DHCP server processes incoming DHCP messages from a client based on - the current state of the binding for that client. A DHCP server can - receive the following messages from a client: - - o DHCPDISCOVER - - o DHCPREQUEST - - o DHCPDECLINE - - o DHCPRELEASE - - o DHCPINFORM - - - - -Droms Standards Track [Page 26] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - Table 3 gives the use of the fields and options in a DHCP message by - a server. The remainder of this section describes the action of the - DHCP server for each possible incoming message. - -4.3.1 DHCPDISCOVER message - - When a server receives a DHCPDISCOVER message from a client, the - server chooses a network address for the requesting client. If no - address is available, the server may choose to report the problem to - the system administrator. If an address is available, the new address - SHOULD be chosen as follows: - - o The client's current address as recorded in the client's current - binding, ELSE - - o The client's previous address as recorded in the client's (now - expired or released) binding, if that address is in the server's - pool of available addresses and not already allocated, ELSE - - o The address requested in the 'Requested IP Address' option, if that - address is valid and not already allocated, ELSE - - o A new address allocated from the server's pool of available - addresses; the address is selected based on the subnet from which - the message was received (if 'giaddr' is 0) or on the address of - the relay agent that forwarded the message ('giaddr' when not 0). - - As described in section 4.2, a server MAY, for administrative - reasons, assign an address other than the one requested, or may - refuse to allocate an address to a particular client even though free - addresses are available. - - Note that, in some network architectures (e.g., internets with more - than one IP subnet assigned to a physical network segment), it may be - the case that the DHCP client should be assigned an address from a - different subnet than the address recorded in 'giaddr'. Thus, DHCP - does not require that the client be assigned as address from the - subnet in 'giaddr'. A server is free to choose some other subnet, - and it is beyond the scope of the DHCP specification to describe ways - in which the assigned IP address might be chosen. - - While not required for correct operation of DHCP, the server SHOULD - NOT reuse the selected network address before the client responds to - the server's DHCPOFFER message. The server may choose to record the - address as offered to the client. - - The server must also choose an expiration time for the lease, as - follows: - - - -Droms Standards Track [Page 27] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - o IF the client has not requested a specific lease in the - DHCPDISCOVER message and the client already has an assigned network - address, the server returns the lease expiration time previously - assigned to that address (note that the client must explicitly - request a specific lease to extend the expiration time on a - previously assigned address), ELSE - - o IF the client has not requested a specific lease in the - DHCPDISCOVER message and the client does not have an assigned - network address, the server assigns a locally configured default - lease time, ELSE - - o IF the client has requested a specific lease in the DHCPDISCOVER - message (regardless of whether the client has an assigned network - address), the server may choose either to return the requested - lease (if the lease is acceptable to local policy) or select - another lease. - -Field DHCPOFFER DHCPACK DHCPNAK ------ --------- ------- ------- -'op' BOOTREPLY BOOTREPLY BOOTREPLY -'htype' (From "Assigned Numbers" RFC) -'hlen' (Hardware address length in octets) -'hops' 0 0 0 -'xid' 'xid' from client 'xid' from client 'xid' from client - DHCPDISCOVER DHCPREQUEST DHCPREQUEST - message message message -'secs' 0 0 0 -'ciaddr' 0 'ciaddr' from 0 - DHCPREQUEST or 0 -'yiaddr' IP address offered IP address 0 - to client assigned to client -'siaddr' IP address of next IP address of next 0 - bootstrap server bootstrap server -'flags' 'flags' from 'flags' from 'flags' from - client DHCPDISCOVER client DHCPREQUEST client DHCPREQUEST - message message message -'giaddr' 'giaddr' from 'giaddr' from 'giaddr' from - client DHCPDISCOVER client DHCPREQUEST client DHCPREQUEST - message message message -'chaddr' 'chaddr' from 'chaddr' from 'chaddr' from - client DHCPDISCOVER client DHCPREQUEST client DHCPREQUEST - message message message -'sname' Server host name Server host name (unused) - or options or options -'file' Client boot file Client boot file (unused) - name or options name or options -'options' options options - - - -Droms Standards Track [Page 28] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - -Option DHCPOFFER DHCPACK DHCPNAK ------- --------- ------- ------- -Requested IP address MUST NOT MUST NOT MUST NOT -IP address lease time MUST MUST (DHCPREQUEST) MUST NOT - MUST NOT (DHCPINFORM) -Use 'file'/'sname' fields MAY MAY MUST NOT -DHCP message type DHCPOFFER DHCPACK DHCPNAK -Parameter request list MUST NOT MUST NOT MUST NOT -Message SHOULD SHOULD SHOULD -Client identifier MUST NOT MUST NOT MAY -Vendor class identifier MAY MAY MAY -Server identifier MUST MUST MUST -Maximum message size MUST NOT MUST NOT MUST NOT -All others MAY MAY MUST NOT - - Table 3: Fields and options used by DHCP servers - - Once the network address and lease have been determined, the server - constructs a DHCPOFFER message with the offered configuration - parameters. It is important for all DHCP servers to return the same - parameters (with the possible exception of a newly allocated network - address) to ensure predictable client behavior regardless of which - server the client selects. The configuration parameters MUST be - selected by applying the following rules in the order given below. - The network administrator is responsible for configuring multiple - DHCP servers to ensure uniform responses from those servers. The - server MUST return to the client: - - o The client's network address, as determined by the rules given - earlier in this section, - - o The expiration time for the client's lease, as determined by the - rules given earlier in this section, - - o Parameters requested by the client, according to the following - rules: - - -- IF the server has been explicitly configured with a default - value for the parameter, the server MUST include that value - in an appropriate option in the 'option' field, ELSE - - -- IF the server recognizes the parameter as a parameter - defined in the Host Requirements Document, the server MUST - include the default value for that parameter as given in the - Host Requirements Document in an appropriate option in the - 'option' field, ELSE - - -- The server MUST NOT return a value for that parameter, - - - -Droms Standards Track [Page 29] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - The server MUST supply as many of the requested parameters as - possible and MUST omit any parameters it cannot provide. The - server MUST include each requested parameter only once unless - explicitly allowed in the DHCP Options and BOOTP Vendor - Extensions document. - - o Any parameters from the existing binding that differ from the Host - Requirements Document defaults, - - o Any parameters specific to this client (as identified by - the contents of 'chaddr' or 'client identifier' in the DHCPDISCOVER - or DHCPREQUEST message), e.g., as configured by the network - administrator, - - o Any parameters specific to this client's class (as identified - by the contents of the 'vendor class identifier' - option in the DHCPDISCOVER or DHCPREQUEST message), - e.g., as configured by the network administrator; the parameters - MUST be identified by an exact match between the client's vendor - class identifiers and the client's classes identified in the - server, - - o Parameters with non-default values on the client's subnet. - - The server MAY choose to return the 'vendor class identifier' used to - determine the parameters in the DHCPOFFER message to assist the - client in selecting which DHCPOFFER to accept. The server inserts - the 'xid' field from the DHCPDISCOVER message into the 'xid' field of - the DHCPOFFER message and sends the DHCPOFFER message to the - requesting client. - -4.3.2 DHCPREQUEST message - - A DHCPREQUEST message may come from a client responding to a - DHCPOFFER message from a server, from a client verifying a previously - allocated IP address or from a client extending the lease on a - network address. If the DHCPREQUEST message contains a 'server - identifier' option, the message is in response to a DHCPOFFER - message. Otherwise, the message is a request to verify or extend an - existing lease. If the client uses a 'client identifier' in a - DHCPREQUEST message, it MUST use that same 'client identifier' in all - subsequent messages. If the client included a list of requested - parameters in a DHCPDISCOVER message, it MUST include that list in - all subsequent messages. - - - - - - - -Droms Standards Track [Page 30] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - Any configuration parameters in the DHCPACK message SHOULD NOT - conflict with those in the earlier DHCPOFFER message to which the - client is responding. The client SHOULD use the parameters in the - DHCPACK message for configuration. - - Clients send DHCPREQUEST messages as follows: - - o DHCPREQUEST generated during SELECTING state: - - Client inserts the address of the selected server in 'server - identifier', 'ciaddr' MUST be zero, 'requested IP address' MUST be - filled in with the yiaddr value from the chosen DHCPOFFER. - - Note that the client may choose to collect several DHCPOFFER - messages and select the "best" offer. The client indicates its - selection by identifying the offering server in the DHCPREQUEST - message. If the client receives no acceptable offers, the client - may choose to try another DHCPDISCOVER message. Therefore, the - servers may not receive a specific DHCPREQUEST from which they can - decide whether or not the client has accepted the offer. Because - the servers have not committed any network address assignments on - the basis of a DHCPOFFER, servers are free to reuse offered - network addresses in response to subsequent requests. As an - implementation detail, servers SHOULD NOT reuse offered addresses - and may use an implementation-specific timeout mechanism to decide - when to reuse an offered address. - - o DHCPREQUEST generated during INIT-REBOOT state: - - 'server identifier' MUST NOT be filled in, 'requested IP address' - option MUST be filled in with client's notion of its previously - assigned address. 'ciaddr' MUST be zero. The client is seeking to - verify a previously allocated, cached configuration. Server SHOULD - send a DHCPNAK message to the client if the 'requested IP address' - is incorrect, or is on the wrong network. - - Determining whether a client in the INIT-REBOOT state is on the - correct network is done by examining the contents of 'giaddr', the - 'requested IP address' option, and a database lookup. If the DHCP - server detects that the client is on the wrong net (i.e., the - result of applying the local subnet mask or remote subnet mask (if - 'giaddr' is not zero) to 'requested IP address' option value - doesn't match reality), then the server SHOULD send a DHCPNAK - message to the client. - - - - - - - -Droms Standards Track [Page 31] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - If the network is correct, then the DHCP server should check if - the client's notion of its IP address is correct. If not, then the - server SHOULD send a DHCPNAK message to the client. If the DHCP - server has no record of this client, then it MUST remain silent, - and MAY output a warning to the network administrator. This - behavior is necessary for peaceful coexistence of non- - communicating DHCP servers on the same wire. - - If 'giaddr' is 0x0 in the DHCPREQUEST message, the client is on - the same subnet as the server. The server MUST broadcast the - DHCPNAK message to the 0xffffffff broadcast address because the - client may not have a correct network address or subnet mask, and - the client may not be answering ARP requests. - - If 'giaddr' is set in the DHCPREQUEST message, the client is on a - different subnet. The server MUST set the broadcast bit in the - DHCPNAK, so that the relay agent will broadcast the DHCPNAK to the - client, because the client may not have a correct network address - or subnet mask, and the client may not be answering ARP requests. - - o DHCPREQUEST generated during RENEWING state: - - 'server identifier' MUST NOT be filled in, 'requested IP address' - option MUST NOT be filled in, 'ciaddr' MUST be filled in with - client's IP address. In this situation, the client is completely - configured, and is trying to extend its lease. This message will - be unicast, so no relay agents will be involved in its - transmission. Because 'giaddr' is therefore not filled in, the - DHCP server will trust the value in 'ciaddr', and use it when - replying to the client. - - A client MAY choose to renew or extend its lease prior to T1. The - server may choose not to extend the lease (as a policy decision by - the network administrator), but should return a DHCPACK message - regardless. - - o DHCPREQUEST generated during REBINDING state: - - 'server identifier' MUST NOT be filled in, 'requested IP address' - option MUST NOT be filled in, 'ciaddr' MUST be filled in with - client's IP address. In this situation, the client is completely - configured, and is trying to extend its lease. This message MUST - be broadcast to the 0xffffffff IP broadcast address. The DHCP - server SHOULD check 'ciaddr' for correctness before replying to - the DHCPREQUEST. - - - - - - -Droms Standards Track [Page 32] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - The DHCPREQUEST from a REBINDING client is intended to accommodate - sites that have multiple DHCP servers and a mechanism for - maintaining consistency among leases managed by multiple servers. - A DHCP server MAY extend a client's lease only if it has local - administrative authority to do so. - -4.3.3 DHCPDECLINE message - - If the server receives a DHCPDECLINE message, the client has - discovered through some other means that the suggested network - address is already in use. The server MUST mark the network address - as not available and SHOULD notify the local system administrator of - a possible configuration problem. - -4.3.4 DHCPRELEASE message - - Upon receipt of a DHCPRELEASE message, the server marks the network - address as not allocated. The server SHOULD retain a record of the - client's initialization parameters for possible reuse in response to - subsequent requests from the client. - -4.3.5 DHCPINFORM message - - The server responds to a DHCPINFORM message by sending a DHCPACK - message directly to the address given in the 'ciaddr' field of the - DHCPINFORM message. The server MUST NOT send a lease expiration time - to the client and SHOULD NOT fill in 'yiaddr'. The server includes - other parameters in the DHCPACK message as defined in section 4.3.1. - -4.3.6 Client messages - - Table 4 details the differences between messages from clients in - various states. - - --------------------------------------------------------------------- - | |INIT-REBOOT |SELECTING |RENEWING |REBINDING | - --------------------------------------------------------------------- - |broad/unicast |broadcast |broadcast |unicast |broadcast | - |server-ip |MUST NOT |MUST |MUST NOT |MUST NOT | - |requested-ip |MUST |MUST |MUST NOT |MUST NOT | - |ciaddr |zero |zero |IP address |IP address| - --------------------------------------------------------------------- - - Table 4: Client messages from different states - - - - - - - -Droms Standards Track [Page 33] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - -4.4 DHCP client behavior - - Figure 5 gives a state-transition diagram for a DHCP client. A - client can receive the following messages from a server: - - o DHCPOFFER - - o DHCPACK - - o DHCPNAK - - The DHCPINFORM message is not shown in figure 5. A client simply - sends the DHCPINFORM and waits for DHCPACK messages. Once the client - has selected its parameters, it has completed the configuration - process. - - Table 5 gives the use of the fields and options in a DHCP message by - a client. The remainder of this section describes the action of the - DHCP client for each possible incoming message. The description in - the following section corresponds to the full configuration procedure - previously described in section 3.1, and the text in the subsequent - section corresponds to the abbreviated configuration procedure - described in section 3.2. - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Droms Standards Track [Page 34] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - -------- ------- -| | +-------------------------->| |<-------------------+ -| INIT- | | +-------------------->| INIT | | -| REBOOT |DHCPNAK/ +---------->| |<---+ | -| |Restart| | ------- | | - -------- | DHCPNAK/ | | | - | Discard offer | -/Send DHCPDISCOVER | --/Send DHCPREQUEST | | | - | | | DHCPACK v | | - ----------- | (not accept.)/ ----------- | | -| | | Send DHCPDECLINE | | | -| REBOOTING | | | | SELECTING |<----+ | -| | | / | | |DHCPOFFER/ | - ----------- | / ----------- | |Collect | - | | / | | | replies | -DHCPACK/ | / +----------------+ +-------+ | -Record lease, set| | v Select offer/ | -timers T1, T2 ------------ send DHCPREQUEST | | - | +----->| | DHCPNAK, Lease expired/ | - | | | REQUESTING | Halt network | - DHCPOFFER/ | | | | - Discard ------------ | | - | | | | ----------- | - | +--------+ DHCPACK/ | | | - | Record lease, set -----| REBINDING | | - | timers T1, T2 / | | | - | | DHCPACK/ ----------- | - | v Record lease, set ^ | - +----------------> ------- /timers T1,T2 | | - +----->| |<---+ | | - | | BOUND |<---+ | | - DHCPOFFER, DHCPACK, | | | T2 expires/ DHCPNAK/ - DHCPNAK/Discard ------- | Broadcast Halt network - | | | | DHCPREQUEST | - +-------+ | DHCPACK/ | | - T1 expires/ Record lease, set | | - Send DHCPREQUEST timers T1, T2 | | - to leasing server | | | - | ---------- | | - | | |------------+ | - +->| RENEWING | | - | |----------------------------+ - ---------- - Figure 5: State-transition diagram for DHCP clients - - - - - - - -Droms Standards Track [Page 35] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - -4.4.1 Initialization and allocation of network address - - The client begins in INIT state and forms a DHCPDISCOVER message. - The client SHOULD wait a random time between one and ten seconds to - desynchronize the use of DHCP at startup. The client sets 'ciaddr' - to 0x00000000. The client MAY request specific parameters by - including the 'parameter request list' option. The client MAY - suggest a network address and/or lease time by including the - 'requested IP address' and 'IP address lease time' options. The - client MUST include its hardware address in the 'chaddr' field, if - necessary for delivery of DHCP reply messages. The client MAY - include a different unique identifier in the 'client identifier' - option, as discussed in section 4.2. If the client included a list - of requested parameters in a DHCPDISCOVER message, it MUST include - that list in all subsequent messages. - - The client generates and records a random transaction identifier and - inserts that identifier into the 'xid' field. The client records its - own local time for later use in computing the lease expiration. The - client then broadcasts the DHCPDISCOVER on the local hardware - broadcast address to the 0xffffffff IP broadcast address and 'DHCP - server' UDP port. - - If the 'xid' of an arriving DHCPOFFER message does not match the - 'xid' of the most recent DHCPDISCOVER message, the DHCPOFFER message - must be silently discarded. Any arriving DHCPACK messages must be - silently discarded. - - The client collects DHCPOFFER messages over a period of time, selects - one DHCPOFFER message from the (possibly many) incoming DHCPOFFER - messages (e.g., the first DHCPOFFER message or the DHCPOFFER message - from the previously used server) and extracts the server address from - the 'server identifier' option in the DHCPOFFER message. The time - over which the client collects messages and the mechanism used to - select one DHCPOFFER are implementation dependent. - - - - - - - - - - - - - - - - -Droms Standards Track [Page 36] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - -Field DHCPDISCOVER DHCPREQUEST DHCPDECLINE, - DHCPINFORM DHCPRELEASE ------ ------------ ----------- ----------- -'op' BOOTREQUEST BOOTREQUEST BOOTREQUEST -'htype' (From "Assigned Numbers" RFC) -'hlen' (Hardware address length in octets) -'hops' 0 0 0 -'xid' selected by client 'xid' from server selected by - DHCPOFFER message client -'secs' 0 or seconds since 0 or seconds since 0 - DHCP process started DHCP process started -'flags' Set 'BROADCAST' Set 'BROADCAST' 0 - flag if client flag if client - requires broadcast requires broadcast - reply reply -'ciaddr' 0 (DHCPDISCOVER) 0 or client's 0 (DHCPDECLINE) - client's network address client's network - network address (BOUND/RENEW/REBIND) address - (DHCPINFORM) (DHCPRELEASE) -'yiaddr' 0 0 0 -'siaddr' 0 0 0 -'giaddr' 0 0 0 -'chaddr' client's hardware client's hardware client's hardware - address address address -'sname' options, if options, if (unused) - indicated in indicated in - 'sname/file' 'sname/file' - option; otherwise option; otherwise - unused unused -'file' options, if options, if (unused) - indicated in indicated in - 'sname/file' 'sname/file' - option; otherwise option; otherwise - unused unused -'options' options options (unused) - - - - - - - - - - - - - - - - -Droms Standards Track [Page 37] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - -Option DHCPDISCOVER DHCPREQUEST DHCPDECLINE, - DHCPINFORM DHCPRELEASE ------- ------------ ----------- ----------- -Requested IP address MAY MUST (in MUST - (DISCOVER) SELECTING or (DHCPDECLINE), - MUST NOT INIT-REBOOT) MUST NOT - (INFORM) MUST NOT (in (DHCPRELEASE) - BOUND or - RENEWING) -IP address lease time MAY MAY MUST NOT - (DISCOVER) - MUST NOT - (INFORM) -Use 'file'/'sname' fields MAY MAY MAY -DHCP message type DHCPDISCOVER/ DHCPREQUEST DHCPDECLINE/ - DHCPINFORM DHCPRELEASE -Client identifier MAY MAY MAY -Vendor class identifier MAY MAY MUST NOT -Server identifier MUST NOT MUST (after MUST - SELECTING) - MUST NOT (after - INIT-REBOOT, - BOUND, RENEWING - or REBINDING) -Parameter request list MAY MAY MUST NOT -Maximum message size MAY MAY MUST NOT -Message SHOULD NOT SHOULD NOT SHOULD -Site-specific MAY MAY MUST NOT -All others MAY MAY MUST NOT - - Table 5: Fields and options used by DHCP clients - - If the parameters are acceptable, the client records the address of - the server that supplied the parameters from the 'server identifier' - field and sends that address in the 'server identifier' field of a - DHCPREQUEST broadcast message. Once the DHCPACK message from the - server arrives, the client is initialized and moves to BOUND state. - The DHCPREQUEST message contains the same 'xid' as the DHCPOFFER - message. The client records the lease expiration time as the sum of - the time at which the original request was sent and the duration of - the lease from the DHCPACK message. The client SHOULD perform a - check on the suggested address to ensure that the address is not - already in use. For example, if the client is on a network that - supports ARP, the client may issue an ARP request for the suggested - request. When broadcasting an ARP request for the suggested address, - the client must fill in its own hardware address as the sender's - hardware address, and 0 as the sender's IP address, to avoid - confusing ARP caches in other hosts on the same subnet. If the - - - -Droms Standards Track [Page 38] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - network address appears to be in use, the client MUST send a - DHCPDECLINE message to the server. The client SHOULD broadcast an ARP - reply to announce the client's new IP address and clear any outdated - ARP cache entries in hosts on the client's subnet. - -4.4.2 Initialization with known network address - - The client begins in INIT-REBOOT state and sends a DHCPREQUEST - message. The client MUST insert its known network address as a - 'requested IP address' option in the DHCPREQUEST message. The client - may request specific configuration parameters by including the - 'parameter request list' option. The client generates and records a - random transaction identifier and inserts that identifier into the - 'xid' field. The client records its own local time for later use in - computing the lease expiration. The client MUST NOT include a - 'server identifier' in the DHCPREQUEST message. The client then - broadcasts the DHCPREQUEST on the local hardware broadcast address to - the 'DHCP server' UDP port. - - Once a DHCPACK message with an 'xid' field matching that in the - client's DHCPREQUEST message arrives from any server, the client is - initialized and moves to BOUND state. The client records the lease - expiration time as the sum of the time at which the DHCPREQUEST - message was sent and the duration of the lease from the DHCPACK - message. - -4.4.3 Initialization with an externally assigned network address - - The client sends a DHCPINFORM message. The client may request - specific configuration parameters by including the 'parameter request - list' option. The client generates and records a random transaction - identifier and inserts that identifier into the 'xid' field. The - client places its own network address in the 'ciaddr' field. The - client SHOULD NOT request lease time parameters. - - The client then unicasts the DHCPINFORM to the DHCP server if it - knows the server's address, otherwise it broadcasts the message to - the limited (all 1s) broadcast address. DHCPINFORM messages MUST be - directed to the 'DHCP server' UDP port. - - Once a DHCPACK message with an 'xid' field matching that in the - client's DHCPINFORM message arrives from any server, the client is - initialized. - - If the client does not receive a DHCPACK within a reasonable period - of time (60 seconds or 4 tries if using timeout suggested in section - 4.1), then it SHOULD display a message informing the user of the - problem, and then SHOULD begin network processing using suitable - - - -Droms Standards Track [Page 39] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - defaults as per Appendix A. - -4.4.4 Use of broadcast and unicast - - The DHCP client broadcasts DHCPDISCOVER, DHCPREQUEST and DHCPINFORM - messages, unless the client knows the address of a DHCP server. The - client unicasts DHCPRELEASE messages to the server. Because the - client is declining the use of the IP address supplied by the server, - the client broadcasts DHCPDECLINE messages. - - When the DHCP client knows the address of a DHCP server, in either - INIT or REBOOTING state, the client may use that address in the - DHCPDISCOVER or DHCPREQUEST rather than the IP broadcast address. - The client may also use unicast to send DHCPINFORM messages to a - known DHCP server. If the client receives no response to DHCP - messages sent to the IP address of a known DHCP server, the DHCP - client reverts to using the IP broadcast address. - -4.4.5 Reacquisition and expiration - - The client maintains two times, T1 and T2, that specify the times at - which the client tries to extend its lease on its network address. - T1 is the time at which the client enters the RENEWING state and - attempts to contact the server that originally issued the client's - network address. T2 is the time at which the client enters the - REBINDING state and attempts to contact any server. T1 MUST be - earlier than T2, which, in turn, MUST be earlier than the time at - which the client's lease will expire. - - To avoid the need for synchronized clocks, T1 and T2 are expressed in - options as relative times [2]. - - At time T1 the client moves to RENEWING state and sends (via unicast) - a DHCPREQUEST message to the server to extend its lease. The client - sets the 'ciaddr' field in the DHCPREQUEST to its current network - address. The client records the local time at which the DHCPREQUEST - message is sent for computation of the lease expiration time. The - client MUST NOT include a 'server identifier' in the DHCPREQUEST - message. - - Any DHCPACK messages that arrive with an 'xid' that does not match - the 'xid' of the client's DHCPREQUEST message are silently discarded. - When the client receives a DHCPACK from the server, the client - computes the lease expiration time as the sum of the time at which - the client sent the DHCPREQUEST message and the duration of the lease - in the DHCPACK message. The client has successfully reacquired its - network address, returns to BOUND state and may continue network - processing. - - - -Droms Standards Track [Page 40] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - If no DHCPACK arrives before time T2, the client moves to REBINDING - state and sends (via broadcast) a DHCPREQUEST message to extend its - lease. The client sets the 'ciaddr' field in the DHCPREQUEST to its - current network address. The client MUST NOT include a 'server - identifier' in the DHCPREQUEST message. - - Times T1 and T2 are configurable by the server through options. T1 - defaults to (0.5 * duration_of_lease). T2 defaults to (0.875 * - duration_of_lease). Times T1 and T2 SHOULD be chosen with some - random "fuzz" around a fixed value, to avoid synchronization of - client reacquisition. - - A client MAY choose to renew or extend its lease prior to T1. The - server MAY choose to extend the client's lease according to policy - set by the network administrator. The server SHOULD return T1 and - T2, and their values SHOULD be adjusted from their original values to - take account of the time remaining on the lease. - - In both RENEWING and REBINDING states, if the client receives no - response to its DHCPREQUEST message, the client SHOULD wait one-half - of the remaining time until T2 (in RENEWING state) and one-half of - the remaining lease time (in REBINDING state), down to a minimum of - 60 seconds, before retransmitting the DHCPREQUEST message. - - If the lease expires before the client receives a DHCPACK, the client - moves to INIT state, MUST immediately stop any other network - processing and requests network initialization parameters as if the - client were uninitialized. If the client then receives a DHCPACK - allocating that client its previous network address, the client - SHOULD continue network processing. If the client is given a new - network address, it MUST NOT continue using the previous network - address and SHOULD notify the local users of the problem. - -4.4.6 DHCPRELEASE - - If the client no longer requires use of its assigned network address - (e.g., the client is gracefully shut down), the client sends a - DHCPRELEASE message to the server. Note that the correct operation - of DHCP does not depend on the transmission of DHCPRELEASE messages. - - - - - - - - - - - - -Droms Standards Track [Page 41] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - -5. Acknowledgments - - The author thanks the many (and too numerous to mention!) members of - the DHC WG for their tireless and ongoing efforts in the development - of DHCP and this document. - - The efforts of J Allard, Mike Carney, Dave Lapp, Fred Lien and John - Mendonca in organizing DHCP interoperability testing sessions are - gratefully acknowledged. - - The development of this document was supported in part by grants from - the Corporation for National Research Initiatives (CNRI), Bucknell - University and Sun Microsystems. - -6. References - - [1] Acetta, M., "Resource Location Protocol", RFC 887, CMU, December - 1983. - - [2] Alexander, S., and R. Droms, "DHCP Options and BOOTP Vendor - Extensions", RFC 1533, Lachman Technology, Inc., Bucknell - University, October 1993. - - [3] Braden, R., Editor, "Requirements for Internet Hosts -- - Communication Layers", STD 3, RFC 1122, USC/Information Sciences - Institute, October 1989. - - [4] Braden, R., Editor, "Requirements for Internet Hosts -- - Application and Support, STD 3, RFC 1123, USC/Information - Sciences Institute, October 1989. - - [5] Brownell, D, "Dynamic Reverse Address Resolution Protocol - (DRARP)", Work in Progress. - - [6] Comer, D., and R. Droms, "Uniform Access to Internet Directory - Services", Proc. of ACM SIGCOMM '90 (Special issue of Computer - Communications Review), 20(4):50--59, 1990. - - [7] Croft, B., and J. Gilmore, "Bootstrap Protocol (BOOTP)", RFC 951, - Stanford and SUN Microsystems, September 1985. - - [8] Deering, S., "ICMP Router Discovery Messages", RFC 1256, Xerox - PARC, September 1991. - - [9] Droms, D., "Interoperation between DHCP and BOOTP", RFC 1534, - Bucknell University, October 1993. - - - - - -Droms Standards Track [Page 42] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - [10] Finlayson, R., Mann, T., Mogul, J., and M. Theimer, "A Reverse - Address Resolution Protocol", RFC 903, Stanford, June 1984. - - [11] Gray C., and D. Cheriton, "Leases: An Efficient Fault-Tolerant - Mechanism for Distributed File Cache Consistency", In Proc. of - the Twelfth ACM Symposium on Operating Systems Design, 1989. - - [12] Mockapetris, P., "Domain Names -- Concepts and Facilities", STD - 13, RFC 1034, USC/Information Sciences Institute, November 1987. - - [13] Mockapetris, P., "Domain Names -- Implementation and - Specification", STD 13, RFC 1035, USC/Information Sciences - Institute, November 1987. - - [14] Mogul J., and S. Deering, "Path MTU Discovery", RFC 1191, - November 1990. - - [15] Morgan, R., "Dynamic IP Address Assignment for Ethernet Attached - Hosts", Work in Progress. - - [16] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, - USC/Information Sciences Institute, September 1981. - - [17] Reynolds, J., "BOOTP Vendor Information Extensions", RFC 1497, - USC/Information Sciences Institute, August 1993. - - [18] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1700, - USC/Information Sciences Institute, October 1994. - - [19] Jeffrey Schiller and Mark Rosenstein. A Protocol for the Dynamic - Assignment of IP Addresses for use on an Ethernet. (Available - from the Athena Project, MIT), 1989. - - [20] Sollins, K., "The TFTP Protocol (Revision 2)", RFC 783, NIC, - June 1981. - - [21] Wimer, W., "Clarifications and Extensions for the Bootstrap - Protocol", RFC 1542, Carnegie Mellon University, October 1993. - -7. Security Considerations - - DHCP is built directly on UDP and IP which are as yet inherently - insecure. Furthermore, DHCP is generally intended to make - maintenance of remote and/or diskless hosts easier. While perhaps - not impossible, configuring such hosts with passwords or keys may be - difficult and inconvenient. Therefore, DHCP in its current form is - quite insecure. - - - - -Droms Standards Track [Page 43] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - - Unauthorized DHCP servers may be easily set up. Such servers can - then send false and potentially disruptive information to clients - such as incorrect or duplicate IP addresses, incorrect routing - information (including spoof routers, etc.), incorrect domain - nameserver addresses (such as spoof nameservers), and so on. - Clearly, once this seed information is in place, an attacker can - further compromise affected systems. - - Malicious DHCP clients could masquerade as legitimate clients and - retrieve information intended for those legitimate clients. Where - dynamic allocation of resources is used, a malicious client could - claim all resources for itself, thereby denying resources to - legitimate clients. - -8. Author's Address - - Ralph Droms - Computer Science Department - 323 Dana Engineering - Bucknell University - Lewisburg, PA 17837 - - Phone: (717) 524-1145 - EMail: droms@bucknell.edu - - - - - - - - - - - - - - - - - - - - - - - - - - - -Droms Standards Track [Page 44] - -RFC 2131 Dynamic Host Configuration Protocol March 1997 - - -A. Host Configuration Parameters - - IP-layer_parameters,_per_host:_ - - Be a router on/off HRC 3.1 - Non-local source routing on/off HRC 3.3.5 - Policy filters for - non-local source routing (list) HRC 3.3.5 - Maximum reassembly size integer HRC 3.3.2 - Default TTL integer HRC 3.2.1.7 - PMTU aging timeout integer MTU 6.6 - MTU plateau table (list) MTU 7 - IP-layer_parameters,_per_interface:_ - IP address (address) HRC 3.3.1.6 - Subnet mask (address mask) HRC 3.3.1.6 - MTU integer HRC 3.3.3 - All-subnets-MTU on/off HRC 3.3.3 - Broadcast address flavor 0x00000000/0xffffffff HRC 3.3.6 - Perform mask discovery on/off HRC 3.2.2.9 - Be a mask supplier on/off HRC 3.2.2.9 - Perform router discovery on/off RD 5.1 - Router solicitation address (address) RD 5.1 - Default routers, list of: - router address (address) HRC 3.3.1.6 - preference level integer HRC 3.3.1.6 - Static routes, list of: - destination (host/subnet/net) HRC 3.3.1.2 - destination mask (address mask) HRC 3.3.1.2 - type-of-service integer HRC 3.3.1.2 - first-hop router (address) HRC 3.3.1.2 - ignore redirects on/off HRC 3.3.1.2 - PMTU integer MTU 6.6 - perform PMTU discovery on/off MTU 6.6 - - Link-layer_parameters,_per_interface:_ - Trailers on/off HRC 2.3.1 - ARP cache timeout integer HRC 2.3.2.1 - Ethernet encapsulation (RFC 894/RFC 1042) HRC 2.3.3 - - TCP_parameters,_per_host:_ - TTL integer HRC 4.2.2.19 - Keep-alive interval integer HRC 4.2.3.6 - Keep-alive data size 0/1 HRC 4.2.3.6 - -Key: - - MTU = Path MTU Discovery (RFC 1191, Proposed Standard) - RD = Router Discovery (RFC 1256, Proposed Standard) - - - -Droms Standards Track [Page 45] - diff --git a/contrib/isc-dhcp/doc/rfc2132.txt b/contrib/isc-dhcp/doc/rfc2132.txt deleted file mode 100644 index e9c4f4b..0000000 --- a/contrib/isc-dhcp/doc/rfc2132.txt +++ /dev/null @@ -1,1907 +0,0 @@ - - - - - - -Network Working Group S. Alexander -Request for Comments: 2132 Silicon Graphics, Inc. -Obsoletes: 1533 R. Droms -Category: Standards Track Bucknell University - March 1997 - - DHCP Options and BOOTP Vendor Extensions - -Status of this memo - - This document specifies an Internet standards track protocol for the - Internet community, and requests discussion and suggestions for - improvements. Please refer to the current edition of the "Internet - Official Protocol Standards" (STD 1) for the standardization state - and status of this protocol. Distribution of this memo is unlimited. - -Abstract - - The Dynamic Host Configuration Protocol (DHCP) [1] provides a - framework for passing configuration information to hosts on a TCP/IP - network. Configuration parameters and other control information are - carried in tagged data items that are stored in the 'options' field - of the DHCP message. The data items themselves are also called - "options." - - This document specifies the current set of DHCP options. Future - options will be specified in separate RFCs. The current list of - valid options is also available in ftp://ftp.isi.edu/in- - notes/iana/assignments [22]. - - All of the vendor information extensions defined in RFC 1497 [2] may - be used as DHCP options. The definitions given in RFC 1497 are - included in this document, which supersedes RFC 1497. All of the - DHCP options defined in this document, except for those specific to - DHCP as defined in section 9, may be used as BOOTP vendor information - extensions. - -Table of Contents - - 1. Introduction .............................................. 2 - 2. BOOTP Extension/DHCP Option Field Format .................. 4 - 3. RFC 1497 Vendor Extensions ................................ 5 - 4. IP Layer Parameters per Host .............................. 11 - 5. IP Layer Parameters per Interface ........................ 13 - 6. Link Layer Parameters per Interface ....................... 16 - 7. TCP Parameters ............................................ 17 - 8. Application and Service Parameters ........................ 18 - 9. DHCP Extensions ........................................... 25 - - - -Alexander & Droms Standards Track [Page 1] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - 10. Defining new extensions ................................... 31 - 11. Acknowledgements .......................................... 31 - 12. References ................................................ 32 - 13. Security Considerations ................................... 33 - 14. Authors' Addresses ........................................ 34 - -1. Introduction - - This document specifies options for use with both the Dynamic Host - Configuration Protocol and the Bootstrap Protocol. - - The full description of DHCP packet formats may be found in the DHCP - specification document [1], and the full description of BOOTP packet - formats may be found in the BOOTP specification document [3]. This - document defines the format of information in the last field of DHCP - packets ('options') and of BOOTP packets ('vend'). The remainder of - this section defines a generalized use of this area for giving - information useful to a wide class of machines, operating systems and - configurations. Sites with a single DHCP or BOOTP server that is - shared among heterogeneous clients may choose to define other, site- - specific formats for the use of the 'options' field. - - Section 2 of this memo describes the formats of DHCP options and - BOOTP vendor extensions. Section 3 describes options defined in - previous documents for use with BOOTP (all may also be used with - DHCP). Sections 4-8 define new options intended for use with both - DHCP and BOOTP. Section 9 defines options used only in DHCP. - - References further describing most of the options defined in sections - 2-6 can be found in section 12. The use of the options defined in - section 9 is described in the DHCP specification [1]. - - Information on registering new options is contained in section 10. - - This document updates the definition of DHCP/BOOTP options that - appears in RFC1533. The classing mechanism has been extended to - include vendor classes as described in section 8.4 and 9.13. The new - procedure for defining new DHCP/BOOTP options in described in section - 10. Several new options, including NIS+ domain and servers, Mobile - IP home agent, SMTP server, TFTP server and Bootfile server, have - been added. Text giving definitions used throughout the document has - been added in section 1.1. Text emphasizing the need for uniqueness - of client-identifiers has been added to section 9.14. - - - - - - - - -Alexander & Droms Standards Track [Page 2] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - -1.1 Requirements - - Throughout this document, the words that are used to define the - significance of particular requirements are capitalized. These words - are: - - o "MUST" - - This word or the adjective "REQUIRED" means that the item is an - absolute requirement of this specification. - - o "MUST NOT" - - This phrase means that the item is an absolute prohibition of - this specification. - - o "SHOULD" - - This word or the adjective "RECOMMENDED" means that there may - exist valid reasons in particular circumstances to ignore this - item, but the full implications should be understood and the case - carefully weighed before choosing a different course. - - o "SHOULD NOT" - - This phrase means that there may exist valid reasons in - particular circumstances when the listed behavior is acceptable - or even useful, but the full implications should be understood - and the case carefully weighed before implementing any behavior - described with this label. - - o "MAY" - - This word or the adjective "OPTIONAL" means that this item is - truly optional. One vendor may choose to include the item - because a particular marketplace requires it or because it - enhances the product, for example; another vendor may omit the - same item. - -1.2 Terminology - - This document uses the following terms: - - o "DHCP client" - - A DHCP client or "client" is an Internet host using DHCP to - obtain configuration parameters such as a network address. - - - - -Alexander & Droms Standards Track [Page 3] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - o "DHCP server" - - A DHCP server of "server"is an Internet host that returns - configuration parameters to DHCP clients. - - o "binding" - - A binding is a collection of configuration parameters, including - at least an IP address, associated with or "bound to" a DHCP - client. Bindings are managed by DHCP servers. - -2. BOOTP Extension/DHCP Option Field Format - - - DHCP options have the same format as the BOOTP 'vendor extensions' - defined in RFC 1497 [2]. Options may be fixed length or variable - length. All options begin with a tag octet, which uniquely - identifies the option. Fixed-length options without data consist of - only a tag octet. Only options 0 and 255 are fixed length. All - other options are variable-length with a length octet following the - tag octet. The value of the length octet does not include the two - octets specifying the tag and length. The length octet is followed - by "length" octets of data. Options containing NVT ASCII data SHOULD - NOT include a trailing NULL; however, the receiver of such options - MUST be prepared to delete trailing nulls if they exist. The - receiver MUST NOT require that a trailing null be included in the - data. In the case of some variable-length options the length field - is a constant but must still be specified. - - Any options defined subsequent to this document MUST contain a length - octet even if the length is fixed or zero. - - All multi-octet quantities are in network byte-order. - - When used with BOOTP, the first four octets of the vendor information - field have been assigned to the "magic cookie" (as suggested in RFC - 951). This field identifies the mode in which the succeeding data is - to be interpreted. The value of the magic cookie is the 4 octet - dotted decimal 99.130.83.99 (or hexadecimal number 63.82.53.63) in - network byte order. - - All of the "vendor extensions" defined in RFC 1497 are also DHCP - options. - - Option codes 128 to 254 (decimal) are reserved for site-specific - options. - - - - - -Alexander & Droms Standards Track [Page 4] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - Except for the options in section 9, all options may be used with - either DHCP or BOOTP. - - Many of these options have their default values specified in other - documents. In particular, RFC 1122 [4] specifies default values for - most IP and TCP configuration parameters. - - Many options supply one or more 32-bit IP address. Use of IP - addresses rather than fully-qualified Domain Names (FQDNs) may make - future renumbering of IP hosts more difficult. Use of these - addresses is discouraged at sites that may require renumbering. - -3. RFC 1497 Vendor Extensions - - This section lists the vendor extensions as defined in RFC 1497. - They are defined here for completeness. - -3.1. Pad Option - - The pad option can be used to cause subsequent fields to align on - word boundaries. - - The code for the pad option is 0, and its length is 1 octet. - - Code - +-----+ - | 0 | - +-----+ - -3.2. End Option - - The end option marks the end of valid information in the vendor - field. Subsequent octets should be filled with pad options. - - The code for the end option is 255, and its length is 1 octet. - - Code - +-----+ - | 255 | - +-----+ - -3.3. Subnet Mask - - The subnet mask option specifies the client's subnet mask as per RFC - 950 [5]. - - If both the subnet mask and the router option are specified in a DHCP - reply, the subnet mask option MUST be first. - - - -Alexander & Droms Standards Track [Page 5] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - The code for the subnet mask option is 1, and its length is 4 octets. - - Code Len Subnet Mask - +-----+-----+-----+-----+-----+-----+ - | 1 | 4 | m1 | m2 | m3 | m4 | - +-----+-----+-----+-----+-----+-----+ - -3.4. Time Offset - - The time offset field specifies the offset of the client's subnet in - seconds from Coordinated Universal Time (UTC). The offset is - expressed as a two's complement 32-bit integer. A positive offset - indicates a location east of the zero meridian and a negative offset - indicates a location west of the zero meridian. - - The code for the time offset option is 2, and its length is 4 octets. - - Code Len Time Offset - +-----+-----+-----+-----+-----+-----+ - | 2 | 4 | n1 | n2 | n3 | n4 | - +-----+-----+-----+-----+-----+-----+ - -3.5. Router Option - - The router option specifies a list of IP addresses for routers on the - client's subnet. Routers SHOULD be listed in order of preference. - - The code for the router option is 3. The minimum length for the - router option is 4 octets, and the length MUST always be a multiple - of 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 3 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -3.6. Time Server Option - - The time server option specifies a list of RFC 868 [6] time servers - available to the client. Servers SHOULD be listed in order of - preference. - - The code for the time server option is 4. The minimum length for - this option is 4 octets, and the length MUST always be a multiple of - 4. - - - - - - -Alexander & Droms Standards Track [Page 6] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 4 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -3.7. Name Server Option - - The name server option specifies a list of IEN 116 [7] name servers - available to the client. Servers SHOULD be listed in order of - preference. - - The code for the name server option is 5. The minimum length for - this option is 4 octets, and the length MUST always be a multiple of - 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 5 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -3.8. Domain Name Server Option - - The domain name server option specifies a list of Domain Name System - (STD 13, RFC 1035 [8]) name servers available to the client. Servers - SHOULD be listed in order of preference. - - The code for the domain name server option is 6. The minimum length - for this option is 4 octets, and the length MUST always be a multiple - of 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 6 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -3.9. Log Server Option - - The log server option specifies a list of MIT-LCS UDP log servers - available to the client. Servers SHOULD be listed in order of - preference. - - The code for the log server option is 7. The minimum length for this - option is 4 octets, and the length MUST always be a multiple of 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 7 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - - - -Alexander & Droms Standards Track [Page 7] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - -3.10. Cookie Server Option - - The cookie server option specifies a list of RFC 865 [9] cookie - servers available to the client. Servers SHOULD be listed in order - of preference. - - The code for the log server option is 8. The minimum length for this - option is 4 octets, and the length MUST always be a multiple of 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 8 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -3.11. LPR Server Option - - The LPR server option specifies a list of RFC 1179 [10] line printer - servers available to the client. Servers SHOULD be listed in order - of preference. - - The code for the LPR server option is 9. The minimum length for this - option is 4 octets, and the length MUST always be a multiple of 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 9 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -3.12. Impress Server Option - - The Impress server option specifies a list of Imagen Impress servers - available to the client. Servers SHOULD be listed in order of - preference. - - The code for the Impress server option is 10. The minimum length for - this option is 4 octets, and the length MUST always be a multiple of - 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 10 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -3.13. Resource Location Server Option - - This option specifies a list of RFC 887 [11] Resource Location - servers available to the client. Servers SHOULD be listed in order - of preference. - - - -Alexander & Droms Standards Track [Page 8] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - The code for this option is 11. The minimum length for this option - is 4 octets, and the length MUST always be a multiple of 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 11 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -3.14. Host Name Option - - This option specifies the name of the client. The name may or may - not be qualified with the local domain name (see section 3.17 for the - preferred way to retrieve the domain name). See RFC 1035 for - character set restrictions. - - The code for this option is 12, and its minimum length is 1. - - Code Len Host Name - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 12 | n | h1 | h2 | h3 | h4 | h5 | h6 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -3.15. Boot File Size Option - - This option specifies the length in 512-octet blocks of the default - boot image for the client. The file length is specified as an - unsigned 16-bit integer. - - The code for this option is 13, and its length is 2. - - Code Len File Size - +-----+-----+-----+-----+ - | 13 | 2 | l1 | l2 | - +-----+-----+-----+-----+ - -3.16. Merit Dump File - - This option specifies the path-name of a file to which the client's - core image should be dumped in the event the client crashes. The - path is formatted as a character string consisting of characters from - the NVT ASCII character set. - - The code for this option is 14. Its minimum length is 1. - - Code Len Dump File Pathname - +-----+-----+-----+-----+-----+-----+--- - | 14 | n | n1 | n2 | n3 | n4 | ... - +-----+-----+-----+-----+-----+-----+--- - - - -Alexander & Droms Standards Track [Page 9] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - -3.17. Domain Name - - This option specifies the domain name that client should use when - resolving hostnames via the Domain Name System. - - The code for this option is 15. Its minimum length is 1. - - Code Len Domain Name - +-----+-----+-----+-----+-----+-----+-- - | 15 | n | d1 | d2 | d3 | d4 | ... - +-----+-----+-----+-----+-----+-----+-- - -3.18. Swap Server - - This specifies the IP address of the client's swap server. - - The code for this option is 16 and its length is 4. - - Code Len Swap Server Address - +-----+-----+-----+-----+-----+-----+ - | 16 | n | a1 | a2 | a3 | a4 | - +-----+-----+-----+-----+-----+-----+ - -3.19. Root Path - - This option specifies the path-name that contains the client's root - disk. The path is formatted as a character string consisting of - characters from the NVT ASCII character set. - - The code for this option is 17. Its minimum length is 1. - - Code Len Root Disk Pathname - +-----+-----+-----+-----+-----+-----+--- - | 17 | n | n1 | n2 | n3 | n4 | ... - +-----+-----+-----+-----+-----+-----+--- - -3.20. Extensions Path - - A string to specify a file, retrievable via TFTP, which contains - information which can be interpreted in the same way as the 64-octet - vendor-extension field within the BOOTP response, with the following - exceptions: - - - the length of the file is unconstrained; - - all references to Tag 18 (i.e., instances of the - BOOTP Extensions Path field) within the file are - ignored. - - - - -Alexander & Droms Standards Track [Page 10] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - The code for this option is 18. Its minimum length is 1. - - Code Len Extensions Pathname - +-----+-----+-----+-----+-----+-----+--- - | 18 | n | n1 | n2 | n3 | n4 | ... - +-----+-----+-----+-----+-----+-----+--- - -4. IP Layer Parameters per Host - - This section details the options that affect the operation of the IP - layer on a per-host basis. - -4.1. IP Forwarding Enable/Disable Option - - This option specifies whether the client should configure its IP - layer for packet forwarding. A value of 0 means disable IP - forwarding, and a value of 1 means enable IP forwarding. - - The code for this option is 19, and its length is 1. - - Code Len Value - +-----+-----+-----+ - | 19 | 1 | 0/1 | - +-----+-----+-----+ - -4.2. Non-Local Source Routing Enable/Disable Option - - This option specifies whether the client should configure its IP - layer to allow forwarding of datagrams with non-local source routes - (see Section 3.3.5 of [4] for a discussion of this topic). A value - of 0 means disallow forwarding of such datagrams, and a value of 1 - means allow forwarding. - - The code for this option is 20, and its length is 1. - - Code Len Value - +-----+-----+-----+ - | 20 | 1 | 0/1 | - +-----+-----+-----+ - -4.3. Policy Filter Option - - This option specifies policy filters for non-local source routing. - The filters consist of a list of IP addresses and masks which specify - destination/mask pairs with which to filter incoming source routes. - - Any source routed datagram whose next-hop address does not match one - of the filters should be discarded by the client. - - - -Alexander & Droms Standards Track [Page 11] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - See [4] for further information. - - The code for this option is 21. The minimum length of this option is - 8, and the length MUST be a multiple of 8. - - Code Len Address 1 Mask 1 - +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+ - | 21 | n | a1 | a2 | a3 | a4 | m1 | m2 | m3 | m4 | - +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+ - Address 2 Mask 2 - +-----+-----+-----+-----+-----+-----+-----+-----+--- - | a1 | a2 | a3 | a4 | m1 | m2 | m3 | m4 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+--- - -4.4. Maximum Datagram Reassembly Size - - This option specifies the maximum size datagram that the client - should be prepared to reassemble. The size is specified as a 16-bit - unsigned integer. The minimum value legal value is 576. - - The code for this option is 22, and its length is 2. - - Code Len Size - +-----+-----+-----+-----+ - | 22 | 2 | s1 | s2 | - +-----+-----+-----+-----+ - -4.5. Default IP Time-to-live - - This option specifies the default time-to-live that the client should - use on outgoing datagrams. The TTL is specified as an octet with a - value between 1 and 255. - - The code for this option is 23, and its length is 1. - - Code Len TTL - +-----+-----+-----+ - | 23 | 1 | ttl | - +-----+-----+-----+ - -4.6. Path MTU Aging Timeout Option - - This option specifies the timeout (in seconds) to use when aging Path - MTU values discovered by the mechanism defined in RFC 1191 [12]. The - timeout is specified as a 32-bit unsigned integer. - - The code for this option is 24, and its length is 4. - - - - -Alexander & Droms Standards Track [Page 12] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - Code Len Timeout - +-----+-----+-----+-----+-----+-----+ - | 24 | 4 | t1 | t2 | t3 | t4 | - +-----+-----+-----+-----+-----+-----+ - -4.7. Path MTU Plateau Table Option - - This option specifies a table of MTU sizes to use when performing - Path MTU Discovery as defined in RFC 1191. The table is formatted as - a list of 16-bit unsigned integers, ordered from smallest to largest. - The minimum MTU value cannot be smaller than 68. - - The code for this option is 25. Its minimum length is 2, and the - length MUST be a multiple of 2. - - Code Len Size 1 Size 2 - +-----+-----+-----+-----+-----+-----+--- - | 25 | n | s1 | s2 | s1 | s2 | ... - +-----+-----+-----+-----+-----+-----+--- - -5. IP Layer Parameters per Interface - - This section details the options that affect the operation of the IP - layer on a per-interface basis. It is expected that a client can - issue multiple requests, one per interface, in order to configure - interfaces with their specific parameters. - -5.1. Interface MTU Option - - This option specifies the MTU to use on this interface. The MTU is - specified as a 16-bit unsigned integer. The minimum legal value for - the MTU is 68. - - The code for this option is 26, and its length is 2. - - Code Len MTU - +-----+-----+-----+-----+ - | 26 | 2 | m1 | m2 | - +-----+-----+-----+-----+ - -5.2. All Subnets are Local Option - - This option specifies whether or not the client may assume that all - subnets of the IP network to which the client is connected use the - same MTU as the subnet of that network to which the client is - directly connected. A value of 1 indicates that all subnets share - the same MTU. A value of 0 means that the client should assume that - some subnets of the directly connected network may have smaller MTUs. - - - -Alexander & Droms Standards Track [Page 13] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - The code for this option is 27, and its length is 1. - - Code Len Value - +-----+-----+-----+ - | 27 | 1 | 0/1 | - +-----+-----+-----+ - -5.3. Broadcast Address Option - - This option specifies the broadcast address in use on the client's - subnet. Legal values for broadcast addresses are specified in - section 3.2.1.3 of [4]. - - The code for this option is 28, and its length is 4. - - Code Len Broadcast Address - +-----+-----+-----+-----+-----+-----+ - | 28 | 4 | b1 | b2 | b3 | b4 | - +-----+-----+-----+-----+-----+-----+ - -5.4. Perform Mask Discovery Option - - This option specifies whether or not the client should perform subnet - mask discovery using ICMP. A value of 0 indicates that the client - should not perform mask discovery. A value of 1 means that the - client should perform mask discovery. - - The code for this option is 29, and its length is 1. - - Code Len Value - +-----+-----+-----+ - | 29 | 1 | 0/1 | - +-----+-----+-----+ - -5.5. Mask Supplier Option - - This option specifies whether or not the client should respond to - subnet mask requests using ICMP. A value of 0 indicates that the - client should not respond. A value of 1 means that the client should - respond. - - The code for this option is 30, and its length is 1. - - Code Len Value - +-----+-----+-----+ - | 30 | 1 | 0/1 | - +-----+-----+-----+ - - - - -Alexander & Droms Standards Track [Page 14] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - -5.6. Perform Router Discovery Option - - This option specifies whether or not the client should solicit - routers using the Router Discovery mechanism defined in RFC 1256 - [13]. A value of 0 indicates that the client should not perform - router discovery. A value of 1 means that the client should perform - router discovery. - - The code for this option is 31, and its length is 1. - - Code Len Value - +-----+-----+-----+ - | 31 | 1 | 0/1 | - +-----+-----+-----+ - -5.7. Router Solicitation Address Option - - This option specifies the address to which the client should transmit - router solicitation requests. - - The code for this option is 32, and its length is 4. - - Code Len Address - +-----+-----+-----+-----+-----+-----+ - | 32 | 4 | a1 | a2 | a3 | a4 | - +-----+-----+-----+-----+-----+-----+ - -5.8. Static Route Option - - This option specifies a list of static routes that the client should - install in its routing cache. If multiple routes to the same - destination are specified, they are listed in descending order of - priority. - - The routes consist of a list of IP address pairs. The first address - is the destination address, and the second address is the router for - the destination. - - The default route (0.0.0.0) is an illegal destination for a static - route. See section 3.5 for information about the router option. - - The code for this option is 33. The minimum length of this option is - 8, and the length MUST be a multiple of 8. - - - - - - - - -Alexander & Droms Standards Track [Page 15] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - Code Len Destination 1 Router 1 - +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+ - | 33 | n | d1 | d2 | d3 | d4 | r1 | r2 | r3 | r4 | - +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+ - Destination 2 Router 2 - +-----+-----+-----+-----+-----+-----+-----+-----+--- - | d1 | d2 | d3 | d4 | r1 | r2 | r3 | r4 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+--- - -6. Link Layer Parameters per Interface - - This section lists the options that affect the operation of the data - link layer on a per-interface basis. - -6.1. Trailer Encapsulation Option - - This option specifies whether or not the client should negotiate the - use of trailers (RFC 893 [14]) when using the ARP protocol. A value - of 0 indicates that the client should not attempt to use trailers. A - value of 1 means that the client should attempt to use trailers. - - The code for this option is 34, and its length is 1. - - Code Len Value - +-----+-----+-----+ - | 34 | 1 | 0/1 | - +-----+-----+-----+ - -6.2. ARP Cache Timeout Option - - This option specifies the timeout in seconds for ARP cache entries. - The time is specified as a 32-bit unsigned integer. - - The code for this option is 35, and its length is 4. - - Code Len Time - +-----+-----+-----+-----+-----+-----+ - | 35 | 4 | t1 | t2 | t3 | t4 | - +-----+-----+-----+-----+-----+-----+ - -6.3. Ethernet Encapsulation Option - - This option specifies whether or not the client should use Ethernet - Version 2 (RFC 894 [15]) or IEEE 802.3 (RFC 1042 [16]) encapsulation - if the interface is an Ethernet. A value of 0 indicates that the - client should use RFC 894 encapsulation. A value of 1 means that the - client should use RFC 1042 encapsulation. - - - - -Alexander & Droms Standards Track [Page 16] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - The code for this option is 36, and its length is 1. - - Code Len Value - +-----+-----+-----+ - | 36 | 1 | 0/1 | - +-----+-----+-----+ - -7. TCP Parameters - - This section lists the options that affect the operation of the TCP - layer on a per-interface basis. - -7.1. TCP Default TTL Option - - This option specifies the default TTL that the client should use when - sending TCP segments. The value is represented as an 8-bit unsigned - integer. The minimum value is 1. - - The code for this option is 37, and its length is 1. - - Code Len TTL - +-----+-----+-----+ - | 37 | 1 | n | - +-----+-----+-----+ - -7.2. TCP Keepalive Interval Option - - This option specifies the interval (in seconds) that the client TCP - should wait before sending a keepalive message on a TCP connection. - The time is specified as a 32-bit unsigned integer. A value of zero - indicates that the client should not generate keepalive messages on - connections unless specifically requested by an application. - - The code for this option is 38, and its length is 4. - - Code Len Time - +-----+-----+-----+-----+-----+-----+ - | 38 | 4 | t1 | t2 | t3 | t4 | - +-----+-----+-----+-----+-----+-----+ - -7.3. TCP Keepalive Garbage Option - - This option specifies the whether or not the client should send TCP - keepalive messages with a octet of garbage for compatibility with - older implementations. A value of 0 indicates that a garbage octet - should not be sent. A value of 1 indicates that a garbage octet - should be sent. - - - - -Alexander & Droms Standards Track [Page 17] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - The code for this option is 39, and its length is 1. - - Code Len Value - +-----+-----+-----+ - | 39 | 1 | 0/1 | - +-----+-----+-----+ - -8. Application and Service Parameters - - This section details some miscellaneous options used to configure - miscellaneous applications and services. - -8.1. Network Information Service Domain Option - - This option specifies the name of the client's NIS [17] domain. The - domain is formatted as a character string consisting of characters - from the NVT ASCII character set. - - The code for this option is 40. Its minimum length is 1. - - Code Len NIS Domain Name - +-----+-----+-----+-----+-----+-----+--- - | 40 | n | n1 | n2 | n3 | n4 | ... - +-----+-----+-----+-----+-----+-----+--- - -8.2. Network Information Servers Option - - This option specifies a list of IP addresses indicating NIS servers - available to the client. Servers SHOULD be listed in order of - preference. - - The code for this option is 41. Its minimum length is 4, and the - length MUST be a multiple of 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 41 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -8.3. Network Time Protocol Servers Option - - This option specifies a list of IP addresses indicating NTP [18] - servers available to the client. Servers SHOULD be listed in order - of preference. - - The code for this option is 42. Its minimum length is 4, and the - length MUST be a multiple of 4. - - - - -Alexander & Droms Standards Track [Page 18] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 42 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -8.4. Vendor Specific Information - - This option is used by clients and servers to exchange vendor- - specific information. The information is an opaque object of n - octets, presumably interpreted by vendor-specific code on the clients - and servers. The definition of this information is vendor specific. - The vendor is indicated in the vendor class identifier option. - Servers not equipped to interpret the vendor-specific information - sent by a client MUST ignore it (although it may be reported). - Clients which do not receive desired vendor-specific information - SHOULD make an attempt to operate without it, although they may do so - (and announce they are doing so) in a degraded mode. - - If a vendor potentially encodes more than one item of information in - this option, then the vendor SHOULD encode the option using - "Encapsulated vendor-specific options" as described below: - - The Encapsulated vendor-specific options field SHOULD be encoded as a - sequence of code/length/value fields of identical syntax to the DHCP - options field with the following exceptions: - - 1) There SHOULD NOT be a "magic cookie" field in the encapsulated - vendor-specific extensions field. - - 2) Codes other than 0 or 255 MAY be redefined by the vendor within - the encapsulated vendor-specific extensions field, but SHOULD - conform to the tag-length-value syntax defined in section 2. - - 3) Code 255 (END), if present, signifies the end of the - encapsulated vendor extensions, not the end of the vendor - extensions field. If no code 255 is present, then the end of - the enclosing vendor-specific information field is taken as the - end of the encapsulated vendor-specific extensions field. - - The code for this option is 43 and its minimum length is 1. - - Code Len Vendor-specific information - +-----+-----+-----+-----+--- - | 43 | n | i1 | i2 | ... - +-----+-----+-----+-----+--- - - - - - - -Alexander & Droms Standards Track [Page 19] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - When encapsulated vendor-specific extensions are used, the - information bytes 1-n have the following format: - - Code Len Data item Code Len Data item Code - +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+ - | T1 | n | d1 | d2 | ... | T2 | n | D1 | D2 | ... | ... | - +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+ - -8.5. NetBIOS over TCP/IP Name Server Option - - The NetBIOS name server (NBNS) option specifies a list of RFC - 1001/1002 [19] [20] NBNS name servers listed in order of preference. - - The code for this option is 44. The minimum length of the option is - 4 octets, and the length must always be a multiple of 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+---- - | 44 | n | a1 | a2 | a3 | a4 | b1 | b2 | b3 | b4 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+---- - -8.6. NetBIOS over TCP/IP Datagram Distribution Server Option - - The NetBIOS datagram distribution server (NBDD) option specifies a - list of RFC 1001/1002 NBDD servers listed in order of preference. The - code for this option is 45. The minimum length of the option is 4 - octets, and the length must always be a multiple of 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+---- - | 45 | n | a1 | a2 | a3 | a4 | b1 | b2 | b3 | b4 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+---- - -8.7. NetBIOS over TCP/IP Node Type Option - - The NetBIOS node type option allows NetBIOS over TCP/IP clients which - are configurable to be configured as described in RFC 1001/1002. The - value is specified as a single octet which identifies the client type - as follows: - - Value Node Type - ----- --------- - 0x1 B-node - 0x2 P-node - 0x4 M-node - 0x8 H-node - - - - - -Alexander & Droms Standards Track [Page 20] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - In the above chart, the notation '0x' indicates a number in base-16 - (hexadecimal). - - The code for this option is 46. The length of this option is always - 1. - - Code Len Node Type - +-----+-----+-----------+ - | 46 | 1 | see above | - +-----+-----+-----------+ - -8.8. NetBIOS over TCP/IP Scope Option - - The NetBIOS scope option specifies the NetBIOS over TCP/IP scope - parameter for the client as specified in RFC 1001/1002. See [19], - [20], and [8] for character-set restrictions. - - The code for this option is 47. The minimum length of this option is - 1. - - Code Len NetBIOS Scope - +-----+-----+-----+-----+-----+-----+---- - | 47 | n | s1 | s2 | s3 | s4 | ... - +-----+-----+-----+-----+-----+-----+---- - -8.9. X Window System Font Server Option - - This option specifies a list of X Window System [21] Font servers - available to the client. Servers SHOULD be listed in order of - preference. - - The code for this option is 48. The minimum length of this option is - 4 octets, and the length MUST be a multiple of 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+--- - | 48 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+--- - -8.10. X Window System Display Manager Option - - This option specifies a list of IP addresses of systems that are - running the X Window System Display Manager and are available to the - client. - - Addresses SHOULD be listed in order of preference. - - - - - -Alexander & Droms Standards Track [Page 21] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - The code for the this option is 49. The minimum length of this option - is 4, and the length MUST be a multiple of 4. - - Code Len Address 1 Address 2 - - +-----+-----+-----+-----+-----+-----+-----+-----+--- - | 49 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+--- - -8.11. Network Information Service+ Domain Option - - This option specifies the name of the client's NIS+ [17] domain. The - domain is formatted as a character string consisting of characters - from the NVT ASCII character set. - - The code for this option is 64. Its minimum length is 1. - - Code Len NIS Client Domain Name - +-----+-----+-----+-----+-----+-----+--- - | 64 | n | n1 | n2 | n3 | n4 | ... - +-----+-----+-----+-----+-----+-----+--- - -8.12. Network Information Service+ Servers Option - - This option specifies a list of IP addresses indicating NIS+ servers - available to the client. Servers SHOULD be listed in order of - preference. - - The code for this option is 65. Its minimum length is 4, and the - length MUST be a multiple of 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 65 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -8.13. Mobile IP Home Agent option - - This option specifies a list of IP addresses indicating mobile IP - home agents available to the client. Agents SHOULD be listed in - order of preference. - - The code for this option is 68. Its minimum length is 0 (indicating - no home agents are available) and the length MUST be a multiple of 4. - It is expected that the usual length will be four octets, containing - a single home agent's address. - - - - - -Alexander & Droms Standards Track [Page 22] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - Code Len Home Agent Addresses (zero or more) - +-----+-----+-----+-----+-----+-----+-- - | 68 | n | a1 | a2 | a3 | a4 | ... - +-----+-----+-----+-----+-----+-----+-- - -8.14. Simple Mail Transport Protocol (SMTP) Server Option - - The SMTP server option specifies a list of SMTP servers available to - the client. Servers SHOULD be listed in order of preference. - - The code for the SMTP server option is 69. The minimum length for - this option is 4 octets, and the length MUST always be a multiple of - 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 69 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -8.15. Post Office Protocol (POP3) Server Option - - The POP3 server option specifies a list of POP3 available to the - client. Servers SHOULD be listed in order of preference. - - The code for the POP3 server option is 70. The minimum length for - this option is 4 octets, and the length MUST always be a multiple of - 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 70 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -8.16. Network News Transport Protocol (NNTP) Server Option - - The NNTP server option specifies a list of NNTP available to the - client. Servers SHOULD be listed in order of preference. - - The code for the NNTP server option is 71. The minimum length for - this option is 4 octets, and the length MUST always be a multiple of - 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 71 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - - - - - -Alexander & Droms Standards Track [Page 23] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - -8.17. Default World Wide Web (WWW) Server Option - - The WWW server option specifies a list of WWW available to the - client. Servers SHOULD be listed in order of preference. - - The code for the WWW server option is 72. The minimum length for - this option is 4 octets, and the length MUST always be a multiple of - 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 72 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -8.18. Default Finger Server Option - - The Finger server option specifies a list of Finger available to the - client. Servers SHOULD be listed in order of preference. - - The code for the Finger server option is 73. The minimum length for - this option is 4 octets, and the length MUST always be a multiple of - 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 73 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -8.19. Default Internet Relay Chat (IRC) Server Option - - The IRC server option specifies a list of IRC available to the - client. Servers SHOULD be listed in order of preference. - - The code for the IRC server option is 74. The minimum length for - this option is 4 octets, and the length MUST always be a multiple of - 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 74 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -8.20. StreetTalk Server Option - - The StreetTalk server option specifies a list of StreetTalk servers - available to the client. Servers SHOULD be listed in order of - preference. - - - - -Alexander & Droms Standards Track [Page 24] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - The code for the StreetTalk server option is 75. The minimum length - for this option is 4 octets, and the length MUST always be a multiple - of 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 75 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -8.21. StreetTalk Directory Assistance (STDA) Server Option - - The StreetTalk Directory Assistance (STDA) server option specifies a - list of STDA servers available to the client. Servers SHOULD be - listed in order of preference. - - The code for the StreetTalk Directory Assistance server option is 76. - The minimum length for this option is 4 octets, and the length MUST - always be a multiple of 4. - - Code Len Address 1 Address 2 - +-----+-----+-----+-----+-----+-----+-----+-----+-- - | 76 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... - +-----+-----+-----+-----+-----+-----+-----+-----+-- - -9. DHCP Extensions - - This section details the options that are specific to DHCP. - -9.1. Requested IP Address - - This option is used in a client request (DHCPDISCOVER) to allow the - client to request that a particular IP address be assigned. - - The code for this option is 50, and its length is 4. - - Code Len Address - +-----+-----+-----+-----+-----+-----+ - | 50 | 4 | a1 | a2 | a3 | a4 | - +-----+-----+-----+-----+-----+-----+ - -9.2. IP Address Lease Time - - This option is used in a client request (DHCPDISCOVER or DHCPREQUEST) - to allow the client to request a lease time for the IP address. In a - server reply (DHCPOFFER), a DHCP server uses this option to specify - the lease time it is willing to offer. - - - - - -Alexander & Droms Standards Track [Page 25] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - The time is in units of seconds, and is specified as a 32-bit - unsigned integer. - - The code for this option is 51, and its length is 4. - - Code Len Lease Time - +-----+-----+-----+-----+-----+-----+ - | 51 | 4 | t1 | t2 | t3 | t4 | - +-----+-----+-----+-----+-----+-----+ - -9.3. Option Overload - - This option is used to indicate that the DHCP 'sname' or 'file' - fields are being overloaded by using them to carry DHCP options. A - DHCP server inserts this option if the returned parameters will - exceed the usual space allotted for options. - - If this option is present, the client interprets the specified - additional fields after it concludes interpretation of the standard - option fields. - - The code for this option is 52, and its length is 1. Legal values - for this option are: - - Value Meaning - ----- -------- - 1 the 'file' field is used to hold options - 2 the 'sname' field is used to hold options - 3 both fields are used to hold options - - Code Len Value - +-----+-----+-----+ - | 52 | 1 |1/2/3| - +-----+-----+-----+ - -9.4 TFTP server name - - This option is used to identify a TFTP server when the 'sname' field - in the DHCP header has been used for DHCP options. - - The code for this option is 66, and its minimum length is 1. - - Code Len TFTP server - +-----+-----+-----+-----+-----+--- - | 66 | n | c1 | c2 | c3 | ... - +-----+-----+-----+-----+-----+--- - - - - - -Alexander & Droms Standards Track [Page 26] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - -9.5 Bootfile name - - This option is used to identify a bootfile when the 'file' field in - the DHCP header has been used for DHCP options. - - The code for this option is 67, and its minimum length is 1. - - Code Len Bootfile name - +-----+-----+-----+-----+-----+--- - | 67 | n | c1 | c2 | c3 | ... - +-----+-----+-----+-----+-----+--- - -9.6. DHCP Message Type - - This option is used to convey the type of the DHCP message. The code - for this option is 53, and its length is 1. Legal values for this - option are: - - Value Message Type - ----- ------------ - 1 DHCPDISCOVER - 2 DHCPOFFER - 3 DHCPREQUEST - 4 DHCPDECLINE - 5 DHCPACK - 6 DHCPNAK - 7 DHCPRELEASE - 8 DHCPINFORM - - Code Len Type - +-----+-----+-----+ - | 53 | 1 | 1-9 | - +-----+-----+-----+ - -9.7. Server Identifier - - This option is used in DHCPOFFER and DHCPREQUEST messages, and may - optionally be included in the DHCPACK and DHCPNAK messages. DHCP - servers include this option in the DHCPOFFER in order to allow the - client to distinguish between lease offers. DHCP clients use the - contents of the 'server identifier' field as the destination address - for any DHCP messages unicast to the DHCP server. DHCP clients also - indicate which of several lease offers is being accepted by including - this option in a DHCPREQUEST message. - - The identifier is the IP address of the selected server. - - The code for this option is 54, and its length is 4. - - - -Alexander & Droms Standards Track [Page 27] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - Code Len Address - +-----+-----+-----+-----+-----+-----+ - | 54 | 4 | a1 | a2 | a3 | a4 | - +-----+-----+-----+-----+-----+-----+ - -9.8. Parameter Request List - - This option is used by a DHCP client to request values for specified - configuration parameters. The list of requested parameters is - specified as n octets, where each octet is a valid DHCP option code - as defined in this document. - - The client MAY list the options in order of preference. The DHCP - server is not required to return the options in the requested order, - but MUST try to insert the requested options in the order requested - by the client. - - The code for this option is 55. Its minimum length is 1. - - Code Len Option Codes - +-----+-----+-----+-----+--- - | 55 | n | c1 | c2 | ... - +-----+-----+-----+-----+--- - -9.9. Message - - This option is used by a DHCP server to provide an error message to a - DHCP client in a DHCPNAK message in the event of a failure. A client - may use this option in a DHCPDECLINE message to indicate the why the - client declined the offered parameters. The message consists of n - octets of NVT ASCII text, which the client may display on an - available output device. - - The code for this option is 56 and its minimum length is 1. - - Code Len Text - +-----+-----+-----+-----+--- - | 56 | n | c1 | c2 | ... - +-----+-----+-----+-----+--- - -9.10. Maximum DHCP Message Size - - This option specifies the maximum length DHCP message that it is - willing to accept. The length is specified as an unsigned 16-bit - integer. A client may use the maximum DHCP message size option in - DHCPDISCOVER or DHCPREQUEST messages, but should not use the option - in DHCPDECLINE messages. - - - - -Alexander & Droms Standards Track [Page 28] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - The code for this option is 57, and its length is 2. The minimum - legal value is 576 octets. - - Code Len Length - +-----+-----+-----+-----+ - | 57 | 2 | l1 | l2 | - +-----+-----+-----+-----+ - -9.11. Renewal (T1) Time Value - - This option specifies the time interval from address assignment until - the client transitions to the RENEWING state. - - The value is in units of seconds, and is specified as a 32-bit - unsigned integer. - - The code for this option is 58, and its length is 4. - - Code Len T1 Interval - +-----+-----+-----+-----+-----+-----+ - | 58 | 4 | t1 | t2 | t3 | t4 | - +-----+-----+-----+-----+-----+-----+ - -9.12. Rebinding (T2) Time Value - - This option specifies the time interval from address assignment until - the client transitions to the REBINDING state. - - The value is in units of seconds, and is specified as a 32-bit - unsigned integer. - - The code for this option is 59, and its length is 4. - - Code Len T2 Interval - +-----+-----+-----+-----+-----+-----+ - | 59 | 4 | t1 | t2 | t3 | t4 | - +-----+-----+-----+-----+-----+-----+ - -9.13. Vendor class identifier - - This option is used by DHCP clients to optionally identify the vendor - type and configuration of a DHCP client. The information is a string - of n octets, interpreted by servers. Vendors may choose to define - specific vendor class identifiers to convey particular configuration - or other identification information about a client. For example, the - identifier may encode the client's hardware configuration. Servers - not equipped to interpret the class-specific information sent by a - client MUST ignore it (although it may be reported). Servers that - - - -Alexander & Droms Standards Track [Page 29] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - respond SHOULD only use option 43 to return the vendor-specific - information to the client. - - The code for this option is 60, and its minimum length is 1. - - Code Len Vendor class Identifier - +-----+-----+-----+-----+--- - | 60 | n | i1 | i2 | ... - +-----+-----+-----+-----+--- - -9.14. Client-identifier - - This option is used by DHCP clients to specify their unique - identifier. DHCP servers use this value to index their database of - address bindings. This value is expected to be unique for all - clients in an administrative domain. - - Identifiers SHOULD be treated as opaque objects by DHCP servers. - - The client identifier MAY consist of type-value pairs similar to the - 'htype'/'chaddr' fields defined in [3]. For instance, it MAY consist - of a hardware type and hardware address. In this case the type field - SHOULD be one of the ARP hardware types defined in STD2 [22]. A - hardware type of 0 (zero) should be used when the value field - contains an identifier other than a hardware address (e.g. a fully - qualified domain name). - - For correct identification of clients, each client's client- - identifier MUST be unique among the client-identifiers used on the - subnet to which the client is attached. Vendors and system - administrators are responsible for choosing client-identifiers that - meet this requirement for uniqueness. - - The code for this option is 61, and its minimum length is 2. - - Code Len Type Client-Identifier - +-----+-----+-----+-----+-----+--- - | 61 | n | t1 | i1 | i2 | ... - +-----+-----+-----+-----+-----+--- - - - - - - - - - - - - -Alexander & Droms Standards Track [Page 30] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - -10. Defining new extensions - - The author of a new DHCP option will follow these steps to obtain - acceptance of the option as a part of the DHCP Internet Standard: - - 1. The author devises the new option. - 2. The author requests a number for the new option from IANA by - contacting: - Internet Assigned Numbers Authority (IANA) - USC/Information Sciences Institute - 4676 Admiralty Way - Marina del Rey, California 90292-6695 - - or by email as: iana@iana.org - - 3. The author documents the new option, using the newly obtained - option number, as an Internet Draft. - 4. The author submits the Internet Draft for review through the IETF - standards process as defined in "Internet Official Protocol - Standards" (STD 1). The new option will be submitted for eventual - acceptance as an Internet Standard. - 5. The new option progresses through the IETF standards process; the - new option will be reviewed by the Dynamic Host Configuration - Working Group (if that group still exists), or as an Internet - Draft not submitted by an IETF working group. - 6. If the new option fails to gain acceptance as an Internet - Standard, the assigned option number will be returned to IANA for - reassignment. - - This procedure for defining new extensions will ensure that: - - * allocation of new option numbers is coordinated from a single - authority, - * new options are reviewed for technical correctness and - appropriateness, and - * documentation for new options is complete and published. - -11. Acknowledgements - - The author thanks the many (and too numerous to mention!) members of - the DHC WG for their tireless and ongoing efforts in the development - of DHCP and this document. - - The efforts of J Allard, Mike Carney, Dave Lapp, Fred Lien and John - Mendonca in organizing DHCP interoperability testing sessions are - gratefully acknowledged. - - - - - -Alexander & Droms Standards Track [Page 31] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - The development of this document was supported in part by grants from - the Corporation for National Research Initiatives (CNRI), Bucknell - University and Sun Microsystems. - -12. References - - [1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, - Bucknell University, March 1997. - - [2] Reynolds, J., "BOOTP Vendor Information Extensions", RFC 1497, - USC/Information Sciences Institute, August 1993. - - [3] Croft, W., and J. Gilmore, "Bootstrap Protocol", RFC 951, - Stanford University and Sun Microsystems, September 1985. - - [4] Braden, R., Editor, "Requirements for Internet Hosts - - Communication Layers", STD 3, RFC 1122, USC/Information Sciences - Institute, October 1989. - - [5] Mogul, J., and J. Postel, "Internet Standard Subnetting - Procedure", STD 5, RFC 950, USC/Information Sciences Institute, - August 1985. - - [6] Postel, J., and K. Harrenstien, "Time Protocol", STD 26, RFC - 868, USC/Information Sciences Institute, SRI, May 1983. - - [7] Postel, J., "Name Server", IEN 116, USC/Information Sciences - Institute, August 1979. - - [8] Mockapetris, P., "Domain Names - Implementation and - Specification", STD 13, RFC 1035, USC/Information Sciences - Institute, November 1987. - - [9] Postel, J., "Quote of the Day Protocol", STD 23, RFC 865, - USC/Information Sciences Institute, May 1983. - - [10] McLaughlin, L., "Line Printer Daemon Protocol", RFC 1179, The - Wollongong Group, August 1990. - - [11] Accetta, M., "Resource Location Protocol", RFC 887, CMU, - December 1983. - - [12] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191, - DECWRL, Stanford University, November 1990. - - [13] Deering, S., "ICMP Router Discovery Messages", RFC 1256, - Xerox PARC, September 1991. - - - - -Alexander & Droms Standards Track [Page 32] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - - [14] Leffler, S. and M. Karels, "Trailer Encapsulations", RFC 893, - U. C. Berkeley, April 1984. - - [15] Hornig, C., "Standard for the Transmission of IP Datagrams over - Ethernet Networks", RFC 894, Symbolics, April 1984. - - [16] Postel, J. and J. Reynolds, "Standard for the Transmission of - IP Datagrams Over IEEE 802 Networks", RFC 1042, USC/Information - Sciences Institute, February 1988. - - [17] Sun Microsystems, "System and Network Administration", March - 1990. - - [18] Mills, D., "Internet Time Synchronization: The Network Time - Protocol", RFC 1305, UDEL, March 1992. - - [19] NetBIOS Working Group, "Protocol Standard for a NetBIOS Service - on a TCP/UDP transport: Concepts and Methods", STD 19, RFC 1001, - March 1987. - - [20] NetBIOS Working Group, "Protocol Standard for a NetBIOS Service - on a TCP/UDP transport: Detailed Specifications", STD 19, RFC - 1002, March 1987. - - [21] Scheifler, R., "FYI On the X Window System", FYI 6, RFC 1198, - MIT Laboratory for Computer Science, January 1991. - - [22] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1700, - USC/Information Sciences Institute, July 1992. - -13. Security Considerations - - Security issues are not discussed in this memo. - - - - - - - - - - - - - - - - - - -Alexander & Droms Standards Track [Page 33] - -RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997 - - -14. Authors' Addresses - - Steve Alexander - Silicon Graphics, Inc. - 2011 N. Shoreline Boulevard - Mailstop 510 - Mountain View, CA 94043-1389 - - Phone: (415) 933-6172 - EMail: sca@engr.sgi.com - - - Ralph Droms - Bucknell University - Lewisburg, PA 17837 - - Phone: (717) 524-1145 - EMail: droms@bucknell.edu - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Alexander & Droms Standards Track [Page 34] - diff --git a/contrib/isc-dhcp/doc/rfc951.txt b/contrib/isc-dhcp/doc/rfc951.txt deleted file mode 100644 index 86cd69e..0000000 --- a/contrib/isc-dhcp/doc/rfc951.txt +++ /dev/null @@ -1,684 +0,0 @@ - - -Network Working Group Bill Croft (Stanford University) -Request for Comments: 951 John Gilmore (Sun Microsystems) - September 1985 - - BOOTSTRAP PROTOCOL (BOOTP) - - -1. Status of this Memo - - This RFC suggests a proposed protocol for the ARPA-Internet - community, and requests discussion and suggestions for improvements. - Distribution of this memo is unlimited. - -2. Overview - - This RFC describes an IP/UDP bootstrap protocol (BOOTP) which allows - a diskless client machine to discover its own IP address, the address - of a server host, and the name of a file to be loaded into memory and - executed. The bootstrap operation can be thought of as consisting of - TWO PHASES. This RFC describes the first phase, which could be - labeled 'address determination and bootfile selection'. After this - address and filename information is obtained, control passes to the - second phase of the bootstrap where a file transfer occurs. The file - transfer will typically use the TFTP protocol [9], since it is - intended that both phases reside in PROM on the client. However - BOOTP could also work with other protocols such as SFTP [3] or - FTP [6]. - - We suggest that the client's PROM software provide a way to do a - complete bootstrap without 'user' interaction. This is the type of - boot that would occur during an unattended power-up. A mechanism - should be provided for the user to manually supply the necessary - address and filename information to bypass the BOOTP protocol and - enter the file transfer phase directly. If non-volatile storage is - available, we suggest keeping default settings there and bypassing - the BOOTP protocol unless these settings cause the file transfer - phase to fail. If the cached information fails, the bootstrap should - fall back to phase 1 and use BOOTP. - - Here is a brief outline of the protocol: - - 1. A single packet exchange is performed. Timeouts are used to - retransmit until a reply is received. The same packet field - layout is used in both directions. Fixed length fields of maximum - reasonable length are used to simplify structure definition and - parsing. - - 2. An 'opcode' field exists with two values. The client - broadcasts a 'bootrequest' packet. The server then answers with a - 'bootreply' packet. The bootrequest contains the client's - hardware address and its IP address, if known. - - -Croft & Gilmore [Page 1] - - - -RFC 951 September 1985 -Bootstrap Protocol - - - 3. The request can optionally contain the name of the server the - client wishes to respond. This is so the client can force the - boot to occur from a specific host (e.g. if multiple versions of - the same bootfile exist or if the server is in a far distant - net/domain). The client does not have to deal with name / domain - services; instead this function is pushed off to the BOOTP server. - - 4. The request can optionally contain the 'generic' filename to be - booted. For example 'unix' or 'ethertip'. When the server sends - the bootreply, it replaces this field with the fully qualified - path name of the appropriate boot file. In determining this name, - the server may consult his own database correlating the client's - address and filename request, with a particular boot file - customized for that client. If the bootrequest filename is a null - string, then the server returns a filename field indicating the - 'default' file to be loaded for that client. - - 5. In the case of clients who do not know their IP addresses, the - server must also have a database relating hardware address to IP - address. This client IP address is then placed into a field in - the bootreply. - - 6. Certain network topologies (such as Stanford's) may be such - that a given physical cable does not have a TFTP server directly - attached to it (e.g. all the gateways and hosts on a certain cable - may be diskless). With the cooperation of neighboring gateways, - BOOTP can allow clients to boot off of servers several hops away, - through these gateways. See the section 'Booting Through - Gateways' below. This part of the protocol requires no special - action on the part of the client. Implementation is optional and - requires a small amount of additional code in gateways and - servers. - -3. Packet Format - - All numbers shown are decimal, unless indicated otherwise. The BOOTP - packet is enclosed in a standard IP [8] UDP [7] datagram. For - simplicity it is assumed that the BOOTP packet is never fragmented. - Any numeric fields shown are packed in 'standard network byte order', - i.e. high order bits are sent first. - - In the IP header of a bootrequest, the client fills in its own IP - source address if known, otherwise zero. When the server address is - unknown, the IP destination address will be the 'broadcast address' - 255.255.255.255. This address means 'broadcast on the local cable, - (I don't know my net number)' [4]. - - - -Croft & Gilmore [Page 2] - - - -RFC 951 September 1985 -Bootstrap Protocol - - - The UDP header contains source and destination port numbers. The - BOOTP protocol uses two reserved port numbers, 'BOOTP client' (68) - and 'BOOTP server' (67). The client sends requests using 'BOOTP - server' as the destination port; this is usually a broadcast. The - server sends replies using 'BOOTP client' as the destination port; - depending on the kernel or driver facilities in the server, this may - or may not be a broadcast (this is explained further in the section - titled 'Chicken/Egg issues' below). The reason TWO reserved ports - are used, is to avoid 'waking up' and scheduling the BOOTP server - daemons, when a bootreply must be broadcast to a client. Since the - server and other hosts won't be listening on the 'BOOTP client' port, - any such incoming broadcasts will be filtered out at the kernel - level. We could not simply allow the client to pick a 'random' port - number for the UDP source port field; since the server reply may be - broadcast, a randomly chosen port number could confuse other hosts - that happened to be listening on that port. - - The UDP length field is set to the length of the UDP plus BOOTP - portions of the packet. The UDP checksum field can be set to zero by - the client (or server) if desired, to avoid this extra overhead in a - PROM implementation. In the 'Packet Processing' section below the - phrase '[UDP checksum.]' is used whenever the checksum might be - verified/computed. - - FIELD BYTES DESCRIPTION - ----- ----- ----------- - - op 1 packet op code / message type. - 1 = BOOTREQUEST, 2 = BOOTREPLY - - htype 1 hardware address type, - see ARP section in "Assigned Numbers" RFC. - '1' = 10mb ethernet - - hlen 1 hardware address length - (eg '6' for 10mb ethernet). - - hops 1 client sets to zero, - optionally used by gateways - in cross-gateway booting. - - xid 4 transaction ID, a random number, - used to match this boot request with the - responses it generates. - - secs 2 filled in by client, seconds elapsed since - client started trying to boot. - - -Croft & Gilmore [Page 3] - - - -RFC 951 September 1985 -Bootstrap Protocol - - - -- 2 unused - - ciaddr 4 client IP address; - filled in by client in bootrequest if known. - - yiaddr 4 'your' (client) IP address; - filled by server if client doesn't - know its own address (ciaddr was 0). - - siaddr 4 server IP address; - returned in bootreply by server. - - giaddr 4 gateway IP address, - used in optional cross-gateway booting. - - chaddr 16 client hardware address, - filled in by client. - - sname 64 optional server host name, - null terminated string. - - file 128 boot file name, null terminated string; - 'generic' name or null in bootrequest, - fully qualified directory-path - name in bootreply. - - vend 64 optional vendor-specific area, - e.g. could be hardware type/serial on request, - or 'capability' / remote file system handle - on reply. This info may be set aside for use - by a third phase bootstrap or kernel. - -4. Chicken / Egg Issues - - How can the server send an IP datagram to the client, if the client - doesnt know its own IP address (yet)? Whenever a bootreply is being - sent, the transmitting machine performs the following operations: - - 1. If the client knows its own IP address ('ciaddr' field is - nonzero), then the IP can be sent 'as normal', since the client - will respond to ARPs [5]. - - 2. If the client does not yet know its IP address (ciaddr zero), - then the client cannot respond to ARPs sent by the transmitter of - the bootreply. There are two options: - - a. If the transmitter has the necessary kernel or driver hooks - - -Croft & Gilmore [Page 4] - - - -RFC 951 September 1985 -Bootstrap Protocol - - - to 'manually' construct an ARP address cache entry, then it can - fill in an entry using the 'chaddr' and 'yiaddr' fields. Of - course, this entry should have a timeout on it, just like any - other entry made by the normal ARP code itself. The - transmitter of the bootreply can then simply send the bootreply - to the client's IP address. UNIX (4.2 BSD) has this - capability. - - b. If the transmitter lacks these kernel hooks, it can simply - send the bootreply to the IP broadcast address on the - appropriate interface. This is only one additional broadcast - over the previous case. - -5. Client Use of ARP - - The client PROM must contain a simple implementation of ARP, e.g. the - address cache could be just one entry in size. This will allow a - second-phase-only boot (TFTP) to be performed when the client knows - the IP addresses and bootfile name. - - Any time the client is expecting to receive a TFTP or BOOTP reply, it - should be prepared to answer an ARP request for its own IP to - hardware address mapping (if known). - - Since the bootreply will contain (in the hardware encapsulation) the - hardware source address of the server/gateway, the client MAY be able - to avoid sending an ARP request for the server/gateway IP address to - be used in the following TFTP phase. However this should be treated - only as a special case, since it is desirable to still allow a - second-phase-only boot as described above. - -6. Comparison to RARP - - An earlier protocol, Reverse Address Resolution Protocol (RARP) [1] - was proposed to allow a client to determine its IP address, given - that it knew its hardware address. However RARP had the disadvantage - that it was a hardware link level protocol (not IP/UDP based). This - means that RARP could only be implemented on hosts containing special - kernel or driver modifications to access these 'raw' packets. Since - there are many network kernels existent now, with each source - maintained by different organizations, a boot protocol that does not - require kernel modifications is a decided advantage. - - BOOTP provides this hardware to IP address lookup function, in - addition to the other useful features described in the sections - above. - - - -Croft & Gilmore [Page 5] - - - -RFC 951 September 1985 -Bootstrap Protocol - - -7. Packet Processing - - 7.1. Client Transmission - - Before setting up the packet for the first time, it is a good idea - to clear the entire packet buffer to all zeros; this will place - all fields in their default state. The client then creates a - packet with the following fields. - - The IP destination address is set to 255.255.255.255. (the - broadcast address) or to the server's IP address (if known). The - IP source address and 'ciaddr' are set to the client's IP address - if known, else 0. The UDP header is set with the proper length; - source port = 'BOOTP client' port destination port = 'BOOTP - server' port. - - 'op' is set to '1', BOOTREQUEST. 'htype' is set to the hardware - address type as assigned in the ARP section of the "Assigned - Numbers" RFC. 'hlen' is set to the length of the hardware address, - e.g. '6' for 10mb ethernet. - - 'xid' is set to a 'random' transaction id. 'secs' is set to the - number of seconds that have elapsed since the client has started - booting. This will let the servers know how long a client has - been trying. As the number gets larger, certain servers may feel - more 'sympathetic' towards a client they don't normally service. - If a client lacks a suitable clock, it could construct a rough - estimate using a loop timer. Or it could choose to simply send - this field as always a fixed value, say 100 seconds. - - If the client knows its IP address, 'ciaddr' (and the IP source - address) are set to this value. 'chaddr' is filled in with the - client's hardware address. - - If the client wishes to restrict booting to a particular server - name, it may place a null-terminated string in 'sname'. The name - used should be any of the allowable names or nicknames of the - desired host. - - The client has several options for filling the 'file' name field. - If left null, the meaning is 'I want to boot the default file for - my machine'. A null file name can also mean 'I am only interested - in finding out client/server/gateway IP addresses, I dont care - about file names'. - - The field can also be a 'generic' name such as 'unix' or - - - -Croft & Gilmore [Page 6] - - - -RFC 951 September 1985 -Bootstrap Protocol - - - 'gateway'; this means 'boot the named program configured for my - machine'. Finally the field can be a fully directory qualified - path name. - - The 'vend' field can be filled in by the client with - vendor-specific strings or structures. For example the machine - hardware type or serial number may be placed here. However the - operation of the BOOTP server should not DEPEND on this - information existing. - - If the 'vend' field is used, it is recommended that a 4 byte - 'magic number' be the first item within 'vend'. This lets a - server determine what kind of information it is seeing in this - field. Numbers can be assigned by the usual 'magic number' - process --you pick one and it's magic. A different magic number - could be used for bootreply's than bootrequest's to allow the - client to take special action with the reply information. - - [UDP checksum.] - - 7.2. Client Retransmission Strategy - - If no reply is received for a certain length of time, the client - should retransmit the request. The time interval must be chosen - carefully so as not to flood the network. Consider the case of a - cable containing 100 machines that are just coming up after a - power failure. Simply retransmitting the request every four - seconds will inundate the net. - - As a possible strategy, you might consider backing off - exponentially, similar to the way ethernet backs off on a - collision. So for example if the first packet is at time 0:00, - the second would be at :04, then :08, then :16, then :32, then - :64. You should also randomize each time; this would be done - similar to the ethernet specification by starting with a mask and - 'and'ing that with with a random number to get the first backoff. - On each succeeding backoff, the mask is increased in length by one - bit. This doubles the average delay on each backoff. - - After the 'average' backoff reaches about 60 seconds, it should be - increased no further, but still randomized. - - Before each retransmission, the client should update the 'secs' - field. [UDP checksum.] - - - - - -Croft & Gilmore [Page 7] - - - -RFC 951 September 1985 -Bootstrap Protocol - - - 7.3. Server Receives BOOTREQUEST - - [UDP checksum.] If the UDP destination port does not match the - 'BOOTP server' port, discard the packet. - - If the server name field (sname) is null (no particular server - specified), or sname is specified and matches our name or - nickname, then continue with packet processing. - - If the sname field is specified, but does not match 'us', then - there are several options: - - 1. You may choose to simply discard this packet. - - 2. If a name lookup on sname shows it to be on this same cable, - discard the packet. - - 3. If sname is on a different net, you may choose to forward - the packet to that address. If so, check the 'giaddr' (gateway - address) field. If 'giaddr' is zero, fill it in with my - address or the address of a gateway that can be used to get to - that net. Then forward the packet. - - If the client IP address (ciaddr) is zero, then the client does - not know its own IP address. Attempt to lookup the client - hardware address (chaddr, hlen, htype) in our database. If no - match is found, discard the packet. Otherwise we now have an IP - address for this client; fill it into the 'yiaddr' (your IP - address) field. - - We now check the boot file name field (file). The field will be - null if the client is not interested in filenames, or wants the - default bootfile. If the field is non-null, it is used as a - lookup key in a database, along with the client's IP address. If - there is a default file or generic file (possibly indexed by the - client address) or a fully-specified path name that matches, then - replace the 'file' field with the fully-specified path name of the - selected boot file. If the field is non-null and no match was - found, then the client is asking for a file we dont have; discard - the packet, perhaps some other BOOTP server will have it. - - The 'vend' vendor-specific data field should now be checked and if - a recognized type of data is provided, client-specific actions - should be taken, and a response placed in the 'vend' data field of - the reply packet. For example, a workstation client could provide - - - - -Croft & Gilmore [Page 8] - - - -RFC 951 September 1985 -Bootstrap Protocol - - - an authentication key and receive from the server a capability for - remote file access, or a set of configuration options, which can - be passed to the operating system that will shortly be booted in. - - Place my (server) IP address in the 'siaddr' field. Set the 'op' - field to BOOTREPLY. The UDP destination port is set to 'BOOTP - client'. If the client address 'ciaddr' is nonzero, send the - packet there; else if the gateway address 'giaddr' is nonzero, set - the UDP destination port to 'BOOTP server' and send the packet to - 'giaddr'; else the client is on one of our cables but it doesnt - know its own IP address yet --use a method described in the 'Egg' - section above to send it to the client. If 'Egg' is used and we - have multiple interfaces on this host, use the 'yiaddr' (your IP - address) field to figure out which net (cable/interface) to send - the packet to. [UDP checksum.] - - 7.4. Server/Gateway Receives BOOTREPLY - - [UDP checksum.] If 'yiaddr' (your [the client's] IP address) - refers to one of our cables, use one of the 'Egg' methods above to - forward it to the client. Be sure to send it to the 'BOOTP - client' UDP destination port. - - 7.5. Client Reception - - Don't forget to process ARP requests for my own IP address (if I - know it). [UDP checksum.] The client should discard incoming - packets that: are not IP/UDPs addressed to the boot port; are not - BOOTREPLYs; do not match my IP address (if I know it) or my - hardware address; do not match my transaction id. Otherwise we - have received a successful reply. 'yiaddr' will contain my IP - address, if I didnt know it before. 'file' is the name of the - file name to TFTP 'read request'. The server address is in - 'siaddr'. If 'giaddr' (gateway address) is nonzero, then the - packets should be forwarded there first, in order to get to the - server. - -8. Booting Through Gateways - - This part of the protocol is optional and requires some additional - code in cooperating gateways and servers, but it allows cross-gateway - booting. This is mainly useful when gateways are diskless machines. - Gateways containing disks (e.g. a UNIX machine acting as a gateway), - might as well run their own BOOTP/TFTP servers. - - Gateways listening to broadcast BOOTREQUESTs may decide to forward or - rebroadcast these requests 'when appropriate'. For example, the - - -Croft & Gilmore [Page 9] - - - -RFC 951 September 1985 -Bootstrap Protocol - - - gateway could have, as part of his configuration tables, a list of - other networks or hosts to receive a copy of any broadcast - BOOTREQUESTs. Even though a 'hops' field exists, it is a poor idea - to simply globally rebroadcast the requests, since broadcast loops - will almost certainly occur. - - The forwarding could begin immediately, or wait until the 'secs' - (seconds client has been trying) field passes a certain threshold. - - If a gateway does decide to forward the request, it should look at - the 'giaddr' (gateway IP address) field. If zero, it should plug its - own IP address (on the receiving cable) into this field. It may also - use the 'hops' field to optionally control how far the packet is - reforwarded. Hops should be incremented on each forwarding. For - example, if hops passes '3', the packet should probably be discarded. - [UDP checksum.] - - Here we have recommended placing this special forwarding function in - the gateways. But that does not have to be the case. As long as - some 'BOOTP forwarding agent' exists on the net with the booting - client, the agent can do the forwarding when appropriate. Thus this - service may or may not be co-located with the gateway. - - In the case of a forwarding agent not located in the gateway, the - agent could save himself some work by plugging the broadcast address - of the interface receiving the bootrequest into the 'giaddr' field. - Thus the reply would get forwarded using normal gateways, not - involving the forwarding agent. Of course the disadvantage here is - that you lose the ability to use the 'Egg' non-broadcast method of - sending the reply, causing extra overhead for every host on the - client cable. - -9. Sample BOOTP Server Database - - As a suggestion, we show a sample text file database that the BOOTP - server program might use. The database has two sections, delimited - by a line containing an percent in column 1. The first section - contains a 'default directory' and mappings from generic names to - directory/pathnames. The first generic name in this section is the - 'default file' you get when the bootrequest contains a null 'file' - string. - - The second section maps hardware addresstype/address into an - ipaddress. Optionally you can also overide the default generic name - by supplying a ipaddress specific genericname. A 'suffix' item is - also an option; if supplied, any generic names specified by the - client will be accessed by first appending 'suffix' to the 'pathname' - - -Croft & Gilmore [Page 10] - - - -RFC 951 September 1985 -Bootstrap Protocol - - - appropriate to that generic name. If that file is not found, then - the plain 'pathname' will be tried. This 'suffix' option allows a - whole set of custom generics to be setup without a lot of effort. - Below is shown the general format; fields are delimited by one or - more spaces or tabs; trailing empty fields may be omitted; blank - lines and lines beginning with '#' are ignored. - - # comment line - - homedirectory - genericname1 pathname1 - genericname2 pathname2 - ... - - % end of generic names, start of address mappings - - hostname1 hardwaretype hardwareaddr1 ipaddr1 genericname suffix - hostname2 hardwaretype hardwareaddr2 ipaddr2 genericname suffix - ... - - Here is a specific example. Note the 'hardwaretype' number is the - same as that shown in the ARP section of the 'Assigned Numbers' RFC. - The 'hardwaretype' and 'ipaddr' numbers are in decimal; - 'hardwareaddr' is in hex. - - # last updated by smith - - /usr/boot - vmunix vmunix - tip ethertip - watch /usr/diag/etherwatch - gate gate. - - % end of generic names, start of address mappings - - hamilton 1 02.60.8c.06.34.98 36.19.0.5 - burr 1 02.60.8c.34.11.78 36.44.0.12 - 101-gateway 1 02.60.8c.23.ab.35 36.44.0.32 gate 101 - mjh-gateway 1 02.60.8c.12.32.bc 36.42.0.64 gate mjh - welch-tipa 1 02.60.8c.22.65.32 36.47.0.14 tip - welch-tipb 1 02.60.8c.12.15.c8 36.46.0.12 tip - - In the example above, if 'mjh-gateway' does a default boot, it will - get the file '/usr/boot/gate.mjh'. - - - - - -Croft & Gilmore [Page 11] - - - -RFC 951 September 1985 -Bootstrap Protocol - - -10. Acknowledgements - - Ross Finlayson (et. al.) produced two earlier RFC's discussing TFTP - bootstraping [2] using RARP [1]. - - We would also like to acknowledge the previous work and comments of - Noel Chiappa, Bob Lyon, Jeff Mogul, Mark Lewis, and David Plummer. - -REFERENCES - - 1. Ross Finlayson, Timothy Mann, Jeffrey Mogul, Marvin Theimer. A - Reverse Address Resolution Protocol. RFC 903, NIC, June, 1984. - - 2. Ross Finlayson. Bootstrap Loading using TFTP. RFC 906, NIC, - June, 1984. - - 3. Mark Lottor. Simple File Transfer Protocol. RFC 913, NIC, - September, 1984. - - 4. Jeffrey Mogul. Broadcasting Internet Packets. RFC 919, NIC, - October, 1984. - - 5. David Plummer. An Ethernet Address Resolution Protocol. RFC - 826, NIC, September, 1982. - - 6. Jon Postel. File Transfer Protocol. RFC 765, NIC, June, 1980. - - 7. Jon Postel. User Datagram Protocol. RFC 768, NIC, August, 1980. - - 8. Jon Postel. Internet Protocol. RFC 791, NIC, September, 1981. - - 9. K. R. Sollins, Noel Chiappa. The TFTP Protocol. RFC 783, NIC, - June, 1981. - - - - - - - - - - - - - - - - -Croft & Gilmore [Page 12] - |
