diff options
Diffstat (limited to 'usr.sbin/sendmail/doc/intro/intro.me')
| -rw-r--r-- | usr.sbin/sendmail/doc/intro/intro.me | 1478 |
1 files changed, 0 insertions, 1478 deletions
diff --git a/usr.sbin/sendmail/doc/intro/intro.me b/usr.sbin/sendmail/doc/intro/intro.me deleted file mode 100644 index 0406bb1..0000000 --- a/usr.sbin/sendmail/doc/intro/intro.me +++ /dev/null @@ -1,1478 +0,0 @@ -.\" Copyright (c) 1983 Eric P. Allman -.\" Copyright (c) 1988, 1993 -.\" The Regents of the University of California. All rights reserved. -.\" -.\" Redistribution and use in source and binary forms, with or without -.\" modification, are permitted provided that the following conditions -.\" are met: -.\" 1. Redistributions of source code must retain the above copyright -.\" notice, this list of conditions and the following disclaimer. -.\" 2. Redistributions in binary form must reproduce the above copyright -.\" notice, this list of conditions and the following disclaimer in the -.\" documentation and/or other materials provided with the distribution. -.\" 3. All advertising materials mentioning features or use of this software -.\" must display the following acknowledgement: -.\" This product includes software developed by the University of -.\" California, Berkeley and its contributors. -.\" 4. Neither the name of the University nor the names of its contributors -.\" may be used to endorse or promote products derived from this software -.\" without specific prior written permission. -.\" -.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND -.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE -.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE -.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE -.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL -.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS -.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) -.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT -.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY -.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF -.\" SUCH DAMAGE. -.\" -.\" @(#)intro.me 8.2 (Berkeley) 11/27/93 -.\" -.\" pic -Pxx intro.me | ditroff -me -Pxx -.eh 'SMM:9-%''SENDMAIL \*- An Internetwork Mail Router' -.oh 'SENDMAIL \*- An Internetwork Mail Router''SMM:9-%' -.nr si 3n -.if n .ls 2 -.+c -.(l C -.sz 14 -SENDMAIL \*- An Internetwork Mail Router -.sz -.sp -Eric Allman* -.sp 0.5 -.i -University of California, Berkeley -Mammoth Project -.)l -.sp -.(l F -.ce -ABSTRACT -.sp \n(psu -Routing mail through a heterogenous internet presents many new -problems. Among the worst of these is that of address mapping. -Historically, this has been handled on an -.i "ad hoc" -basis. However, -this approach has become unmanageable as internets grow. -.sp \n(psu -Sendmail acts a unified "post office" to which all mail can be -submitted. Address interpretation is controlled by a production -system, which can parse both domain-based addressing and old-style -.i "ad hoc" -addresses. -The production system is powerful -enough to rewrite addresses in the message header to conform to the -standards of a number of common target networks, including old -(NCP/RFC733) Arpanet, new (TCP/RFC822) Arpanet, UUCP, and Phonenet. -Sendmail also implements an SMTP server, message -queueing, and aliasing. -.)l -.sp 2 -.(f -*A considerable part of this work -was done while under the employ -of the INGRES Project -at the University of California at Berkeley -and at Britton Lee. -.)f -.pp -.i Sendmail -implements a general internetwork mail routing facility, -featuring aliasing and forwarding, -automatic routing to network gateways, -and flexible configuration. -.pp -In a simple network, -each node has an address, -and resources can be identified -with a host-resource pair; -in particular, -the mail system can refer to users -using a host-username pair. -Host names and numbers have to be administered by a central authority, -but usernames can be assigned locally to each host. -.pp -In an internet, -multiple networks with different characterstics -and managements -must communicate. -In particular, -the syntax and semantics of resource identification change. -Certain special cases can be handled trivially -by -.i "ad hoc" -techniques, -such as -providing network names that appear local to hosts -on other networks, -as with the Ethernet at Xerox PARC. -However, the general case is extremely complex. -For example, -some networks require point-to-point routing, -which simplifies the database update problem -since only adjacent hosts must be entered -into the system tables, -while others use end-to-end addressing. -Some networks use a left-associative syntax -and others use a right-associative syntax, -causing ambiguity in mixed addresses. -.pp -Internet standards seek to eliminate these problems. -Initially, these proposed expanding the address pairs -to address triples, -consisting of -{network, host, resource} -triples. -Network numbers must be universally agreed upon, -and hosts can be assigned locally -on each network. -The user-level presentation was quickly expanded -to address domains, -comprised of a local resource identification -and a hierarchical domain specification -with a common static root. -The domain technique -separates the issue of physical versus logical addressing. -For example, -an address of the form -.q "eric@a.cc.berkeley.arpa" -describes only the logical -organization of the address space. -.pp -.i Sendmail -is intended to help bridge the gap -between the totally -.i "ad hoc" -world -of networks that know nothing of each other -and the clean, tightly-coupled world -of unique network numbers. -It can accept old arbitrary address syntaxes, -resolving ambiguities using heuristics -specified by the system administrator, -as well as domain-based addressing. -It helps guide the conversion of message formats -between disparate networks. -In short, -.i sendmail -is designed to assist a graceful transition -to consistent internetwork addressing schemes. -.sp -.pp -Section 1 discusses the design goals for -.i sendmail . -Section 2 gives an overview of the basic functions of the system. -In section 3, -details of usage are discussed. -Section 4 compares -.i sendmail -to other internet mail routers, -and an evaluation of -.i sendmail -is given in section 5, -including future plans. -.sh 1 "DESIGN GOALS" -.pp -Design goals for -.i sendmail -include: -.np -Compatibility with the existing mail programs, -including Bell version 6 mail, -Bell version 7 mail -[UNIX83], -Berkeley -.i Mail -[Shoens79], -BerkNet mail -[Schmidt79], -and hopefully UUCP mail -[Nowitz78a, Nowitz78b]. -ARPANET mail -[Crocker77a, Postel77] -was also required. -.np -Reliability, in the sense of guaranteeing -that every message is correctly delivered -or at least brought to the attention of a human -for correct disposal; -no message should ever be completely lost. -This goal was considered essential -because of the emphasis on mail in our environment. -It has turned out to be one of the hardest goals to satisfy, -especially in the face of the many anomalous message formats -produced by various ARPANET sites. -For example, -certain sites generate improperly formated addresses, -occasionally -causing error-message loops. -Some hosts use blanks in names, -causing problems with -UNIX mail programs that assume that an address -is one word. -The semantics of some fields -are interpreted slightly differently -by different sites. -In summary, -the obscure features of the ARPANET mail protocol -really -.i are -used and -are difficult to support, -but must be supported. -.np -Existing software to do actual delivery -should be used whenever possible. -This goal derives as much from political and practical considerations -as technical. -.np -Easy expansion to -fairly complex environments, -including multiple -connections to a single network type -(such as with multiple UUCP or Ether nets -[Metcalfe76]). -This goal requires consideration of the contents of an address -as well as its syntax -in order to determine which gateway to use. -For example, -the ARPANET is bringing up the -TCP protocol to replace the old NCP protocol. -No host at Berkeley runs both TCP and NCP, -so it is necessary to look at the ARPANET host name -to determine whether to route mail to an NCP gateway -or a TCP gateway. -.np -Configuration should not be compiled into the code. -A single compiled program should be able to run as is at any site -(barring such basic changes as the CPU type or the operating system). -We have found this seemingly unimportant goal -to be critical in real life. -Besides the simple problems that occur when any program gets recompiled -in a different environment, -many sites like to -.q fiddle -with anything that they will be recompiling anyway. -.np -.i Sendmail -must be able to let various groups maintain their own mailing lists, -and let individuals specify their own forwarding, -without modifying the system alias file. -.np -Each user should be able to specify which mailer to execute -to process mail being delivered for him. -This feature allows users who are using specialized mailers -that use a different format to build their environment -without changing the system, -and facilitates specialized functions -(such as returning an -.q "I am on vacation" -message). -.np -Network traffic should be minimized -by batching addresses to a single host where possible, -without assistance from the user. -.pp -These goals motivated the architecture illustrated in figure 1. -.(z -.hl -.ie t \ -\{\ -.ie !"\*(.T"" \ -\{\ -.PS -boxht = 0.5i -boxwid = 1.0i - - down -S: [ - right - S1: box "sender1" - move - box "sender2" - move - S3: box "sender3" - ] - arrow -SM: box "sendmail" wid 2i ht boxht - arrow -M: [ - right - M1: box "mailer1" - move - box "mailer2" - move - M3: box "mailer3" - ] - - arrow from S.S1.s to 1/2 between SM.nw and SM.n - arrow from S.S3.s to 1/2 between SM.n and SM.ne - - arrow from 1/2 between SM.sw and SM.s to M.M1.n - arrow from 1/2 between SM.s and SM.se to M.M3.n -.PE -.\} -.el \ -. sp 18 -.\} -.el \{\ -.(c -+---------+ +---------+ +---------+ -| sender1 | | sender2 | | sender3 | -+---------+ +---------+ +---------+ - | | | - +----------+ + +----------+ - | | | - v v v - +-------------+ - | sendmail | - +-------------+ - | | | - +----------+ + +----------+ - | | | - v v v -+---------+ +---------+ +---------+ -| mailer1 | | mailer2 | | mailer3 | -+---------+ +---------+ +---------+ -.)c -.\} - -.ce -Figure 1 \*- Sendmail System Structure. -.hl -.)z -The user interacts with a mail generating and sending program. -When the mail is created, -the generator calls -.i sendmail , -which routes the message to the correct mailer(s). -Since some of the senders may be network servers -and some of the mailers may be network clients, -.i sendmail -may be used as an internet mail gateway. -.sh 1 "OVERVIEW" -.sh 2 "System Organization" -.pp -.i Sendmail -neither interfaces with the user -nor does actual mail delivery. -Rather, -it collects a message -generated by a user interface program (UIP) -such as Berkeley -.i Mail , -MS -[Crocker77b], -or MH -[Borden79], -edits the message as required by the destination network, -and calls appropriate mailers -to do mail delivery or queueing for network transmission\**. -.(f -\**except when mailing to a file, -when -.i sendmail -does the delivery directly. -.)f -This discipline allows the insertion of new mailers -at minimum cost. -In this sense -.i sendmail -resembles the Message Processing Module (MPM) -of [Postel79b]. -.sh 2 "Interfaces to the Outside World" -.pp -There are three ways -.i sendmail -can communicate with the outside world, -both in receiving and in sending mail. -These are using the conventional UNIX -argument vector/return status, -speaking SMTP over a pair of UNIX pipes, -and speaking SMTP over an interprocess(or) channel. -.sh 3 "Argument vector/exit status" -.pp -This technique is the standard UNIX method -for communicating with the process. -A list of recipients is sent in the argument vector, -and the message body is sent on the standard input. -Anything that the mailer prints -is simply collected and sent back to the sender -if there were any problems. -The exit status from the mailer is collected -after the message is sent, -and a diagnostic is printed if appropriate. -.sh 3 "SMTP over pipes" -.pp -The SMTP protocol -[Postel82] -can be used to run an interactive lock-step interface -with the mailer. -A subprocess is still created, -but no recipient addresses are passed to the mailer -via the argument list. -Instead, they are passed one at a time -in commands sent to the processes standard input. -Anything appearing on the standard output -must be a reply code -in a special format. -.sh 3 "SMTP over an IPC connection" -.pp -This technique is similar to the previous technique, -except that it uses a 4.2bsd IPC channel -[UNIX83]. -This method is exceptionally flexible -in that the mailer need not reside -on the same machine. -It is normally used to connect to a sendmail process -on another machine. -.sh 2 "Operational Description" -.pp -When a sender wants to send a message, -it issues a request to -.i sendmail -using one of the three methods described above. -.i Sendmail -operates in two distinct phases. -In the first phase, -it collects and stores the message. -In the second phase, -message delivery occurs. -If there were errors during processing -during the second phase, -.i sendmail -creates and returns a new message describing the error -and/or returns an status code -telling what went wrong. -.sh 3 "Argument processing and address parsing" -.pp -If -.i sendmail -is called using one of the two subprocess techniques, -the arguments -are first scanned -and option specifications are processed. -Recipient addresses are then collected, -either from the command line -or from the SMTP -RCPT command, -and a list of recipients is created. -Aliases are expanded at this step, -including mailing lists. -As much validation as possible of the addresses -is done at this step: -syntax is checked, and local addresses are verified, -but detailed checking of host names and addresses -is deferred until delivery. -Forwarding is also performed -as the local addresses are verified. -.pp -.i Sendmail -appends each address -to the recipient list after parsing. -When a name is aliased or forwarded, -the old name is retained in the list, -and a flag is set that tells the delivery phase -to ignore this recipient. -This list is kept free from duplicates, -preventing alias loops -and duplicate messages deliverd to the same recipient, -as might occur if a person is in two groups. -.sh 3 "Message collection" -.pp -.i Sendmail -then collects the message. -The message should have a header at the beginning. -No formatting requirements are imposed on the message -except that they must be lines of text -(i.e., binary data is not allowed). -The header is parsed and stored in memory, -and the body of the message is saved -in a temporary file. -.pp -To simplify the program interface, -the message is collected even if no addresses were valid. -The message will be returned with an error. -.sh 3 "Message delivery" -.pp -For each unique mailer and host in the recipient list, -.i sendmail -calls the appropriate mailer. -Each mailer invocation sends to all users receiving the message on one host. -Mailers that only accept one recipient at a time -are handled properly. -.pp -The message is sent to the mailer -using one of the same three interfaces -used to submit a message to sendmail. -Each copy of the message is -prepended by a customized header. -The mailer status code is caught and checked, -and a suitable error message given as appropriate. -The exit code must conform to a system standard -or a generic message -(\c -.q "Service unavailable" ) -is given. -.sh 3 "Queueing for retransmission" -.pp -If the mailer returned an status that -indicated that it might be able to handle the mail later, -.i sendmail -will queue the mail and try again later. -.sh 3 "Return to sender" -.pp -If errors occur during processing, -.i sendmail -returns the message to the sender for retransmission. -The letter can be mailed back -or written in the file -.q dead.letter -in the sender's home directory\**. -.(f -\**Obviously, if the site giving the error is not the originating -site, the only reasonable option is to mail back to the sender. -Also, there are many more error disposition options, -but they only effect the error message \*- the -.q "return to sender" -function is always handled in one of these two ways. -.)f -.sh 2 "Message Header Editing" -.pp -Certain editing of the message header -occurs automatically. -Header lines can be inserted -under control of the configuration file. -Some lines can be merged; -for example, -a -.q From: -line and a -.q Full-name: -line can be merged under certain circumstances. -.sh 2 "Configuration File" -.pp -Almost all configuration information is read at runtime -from an ASCII file, -encoding -macro definitions -(defining the value of macros used internally), -header declarations -(telling sendmail the format of header lines that it will process specially, -i.e., lines that it will add or reformat), -mailer definitions -(giving information such as the location and characteristics -of each mailer), -and address rewriting rules -(a limited production system to rewrite addresses -which is used to parse and rewrite the addresses). -.pp -To improve performance when reading the configuration file, -a memory image can be provided. -This provides a -.q compiled -form of the configuration file. -.sh 1 "USAGE AND IMPLEMENTATION" -.sh 2 "Arguments" -.pp -Arguments may be flags and addresses. -Flags set various processing options. -Following flag arguments, -address arguments may be given, -unless we are running in SMTP mode. -Addresses follow the syntax in RFC822 -[Crocker82] -for ARPANET -address formats. -In brief, the format is: -.np -Anything in parentheses is thrown away -(as a comment). -.np -Anything in angle brackets (\c -.q "<\|>" ) -is preferred -over anything else. -This rule implements the ARPANET standard that addresses of the form -.(b -user name <machine-address> -.)b -will send to the electronic -.q machine-address -rather than the human -.q "user name." -.np -Double quotes -(\ "\ ) -quote phrases; -backslashes quote characters. -Backslashes are more powerful -in that they will cause otherwise equivalent phrases -to compare differently \*- for example, -.i user -and -.i -"user" -.r -are equivalent, -but -.i \euser -is different from either of them. -.pp -Parentheses, angle brackets, and double quotes -must be properly balanced and nested. -The rewriting rules control remaining parsing\**. -.(f -\**Disclaimer: Some special processing is done -after rewriting local names; see below. -.)f -.sh 2 "Mail to Files and Programs" -.pp -Files and programs are legitimate message recipients. -Files provide archival storage of messages, -useful for project administration and history. -Programs are useful as recipients in a variety of situations, -for example, -to maintain a public repository of systems messages -(such as the Berkeley -.i msgs -program, -or the MARS system -[Sattley78]). -.pp -Any address passing through the initial parsing algorithm -as a local address -(i.e, not appearing to be a valid address for another mailer) -is scanned for two special cases. -If prefixed by a vertical bar (\c -.q \^|\^ ) -the rest of the address is processed as a shell command. -If the user name begins with a slash mark (\c -.q /\^ ) -the name is used as a file name, -instead of a login name. -.pp -Files that have setuid or setgid bits set -but no execute bits set -have those bits honored if -.i sendmail -is running as root. -.sh 2 "Aliasing, Forwarding, Inclusion" -.pp -.i Sendmail -reroutes mail three ways. -Aliasing applies system wide. -Forwarding allows each user to reroute incoming mail -destined for that account. -Inclusion directs -.i sendmail -to read a file for a list of addresses, -and is normally used -in conjunction with aliasing. -.sh 3 "Aliasing" -.pp -Aliasing maps names to address lists using a system-wide file. -This file is indexed to speed access. -Only names that parse as local -are allowed as aliases; -this guarantees a unique key -(since there are no nicknames for the local host). -.sh 3 "Forwarding" -.pp -After aliasing, -recipients that are local and valid -are checked for the existence of a -.q .forward -file in their home directory. -If it exists, -the message is -.i not -sent to that user, -but rather to the list of users in that file. -Often -this list will contain only one address, -and the feature will be used for network mail forwarding. -.pp -Forwarding also permits a user to specify a private incoming mailer. -For example, -forwarding to: -.(b -"\^|\|/usr/local/newmail myname" -.)b -will use a different incoming mailer. -.sh 3 "Inclusion" -.pp -Inclusion is specified in RFC 733 [Crocker77a] syntax: -.(b -:Include: pathname -.)b -An address of this form reads the file specified by -.i pathname -and sends to all users listed in that file. -.pp -The intent is -.i not -to support direct use of this feature, -but rather to use this as a subset of aliasing. -For example, -an alias of the form: -.(b -project: :include:/usr/project/userlist -.)b -is a method of letting a project maintain a mailing list -without interaction with the system administration, -even if the alias file is protected. -.pp -It is not necessary to rebuild the index on the alias database -when a :include: list is changed. -.sh 2 "Message Collection" -.pp -Once all recipient addresses are parsed and verified, -the message is collected. -The message comes in two parts: -a message header and a message body, -separated by a blank line. -.pp -The header is formatted as a series of lines -of the form -.(b - field-name: field-value -.)b -Field-value can be split across lines by starting the following -lines with a space or a tab. -Some header fields have special internal meaning, -and have appropriate special processing. -Other headers are simply passed through. -Some header fields may be added automatically, -such as time stamps. -.pp -The body is a series of text lines. -It is completely uninterpreted and untouched, -except that lines beginning with a dot -have the dot doubled -when transmitted over an SMTP channel. -This extra dot is stripped by the receiver. -.sh 2 "Message Delivery" -.pp -The send queue is ordered by receiving host -before transmission -to implement message batching. -Each address is marked as it is sent -so rescanning the list is safe. -An argument list is built as the scan proceeds. -Mail to files is detected during the scan of the send list. -The interface to the mailer -is performed using one of the techniques -described in section 2.2. -.pp -After a connection is established, -.i sendmail -makes the per-mailer changes to the header -and sends the result to the mailer. -If any mail is rejected by the mailer, -a flag is set to invoke the return-to-sender function -after all delivery completes. -.sh 2 "Queued Messages" -.pp -If the mailer returns a -.q "temporary failure" -exit status, -the message is queued. -A control file is used to describe the recipients to be sent to -and various other parameters. -This control file is formatted as a series of lines, -each describing a sender, -a recipient, -the time of submission, -or some other salient parameter of the message. -The header of the message is stored -in the control file, -so that the associated data file in the queue -is just the temporary file that was originally collected. -.sh 2 "Configuration" -.pp -Configuration is controlled primarily by a configuration file -read at startup. -.i Sendmail -should not need to be recomplied except -.np -To change operating systems -(V6, V7/32V, 4BSD). -.np -To remove or insert the DBM -(UNIX database) -library. -.np -To change ARPANET reply codes. -.np -To add headers fields requiring special processing. -.lp -Adding mailers or changing parsing -(i.e., rewriting) -or routing information -does not require recompilation. -.pp -If the mail is being sent by a local user, -and the file -.q .mailcf -exists in the sender's home directory, -that file is read as a configuration file -after the system configuration file. -The primary use of this feature is to add header lines. -.pp -The configuration file encodes macro definitions, -header definitions, -mailer definitions, -rewriting rules, -and options. -.sh 3 Macros -.pp -Macros can be used in three ways. -Certain macros transmit -unstructured textual information -into the mail system, -such as the name -.i sendmail -will use to identify itself in error messages. -Other macros transmit information from -.i sendmail -to the configuration file -for use in creating other fields -(such as argument vectors to mailers); -e.g., the name of the sender, -and the host and user -of the recipient. -Other macros are unused internally, -and can be used as shorthand in the configuration file. -.sh 3 "Header declarations" -.pp -Header declarations inform -.i sendmail -of the format of known header lines. -Knowledge of a few header lines -is built into -.i sendmail , -such as the -.q From: -and -.q Date: -lines. -.pp -Most configured headers -will be automatically inserted -in the outgoing message -if they don't exist in the incoming message. -Certain headers are suppressed by some mailers. -.sh 3 "Mailer declarations" -.pp -Mailer declarations tell -.i sendmail -of the various mailers available to it. -The definition specifies the internal name of the mailer, -the pathname of the program to call, -some flags associated with the mailer, -and an argument vector to be used on the call; -this vector is macro-expanded before use. -.sh 3 "Address rewriting rules" -.pp -The heart of address parsing in -.i sendmail -is a set of rewriting rules. -These are an ordered list of pattern-replacement rules, -(somewhat like a production system, -except that order is critical), -which are applied to each address. -The address is rewritten textually until it is either rewritten -into a special canonical form -(i.e., -a (mailer, host, user) -3-tuple, -such as {arpanet, usc-isif, postel} -representing the address -.q "postel@usc-isif" ), -or it falls off the end. -When a pattern matches, -the rule is reapplied until it fails. -.pp -The configuration file also supports the editing of addresses -into different formats. -For example, -an address of the form: -.(b -ucsfcgl!tef -.)b -might be mapped into: -.(b -tef@ucsfcgl.UUCP -.)b -to conform to the domain syntax. -Translations can also be done in the other direction. -.sh 3 "Option setting" -.pp -There are several options that can be set -from the configuration file. -These include the pathnames of various support files, -timeouts, -default modes, -etc. -.sh 1 "COMPARISON WITH OTHER MAILERS" -.sh 2 "Delivermail" -.pp -.i Sendmail -is an outgrowth of -.i delivermail . -The primary differences are: -.np -Configuration information is not compiled in. -This change simplifies many of the problems -of moving to other machines. -It also allows easy debugging of new mailers. -.np -Address parsing is more flexible. -For example, -.i delivermail -only supported one gateway to any network, -whereas -.i sendmail -can be sensitive to host names -and reroute to different gateways. -.np -Forwarding and -:include: -features eliminate the requirement that the system alias file -be writable by any user -(or that an update program be written, -or that the system administration make all changes). -.np -.i Sendmail -supports message batching across networks -when a message is being sent to multiple recipients. -.np -A mail queue is provided in -.i sendmail. -Mail that cannot be delivered immediately -but can potentially be delivered later -is stored in this queue for a later retry. -The queue also provides a buffer against system crashes; -after the message has been collected -it may be reliably redelivered -even if the system crashes during the initial delivery. -.np -.i Sendmail -uses the networking support provided by 4.2BSD -to provide a direct interface networks such as the ARPANET -and/or Ethernet -using SMTP (the Simple Mail Transfer Protocol) -over a TCP/IP connection. -.sh 2 "MMDF" -.pp -MMDF -[Crocker79] -spans a wider problem set than -.i sendmail . -For example, -the domain of -MMDF includes a -.q "phone network" -mailer, whereas -.i sendmail -calls on preexisting mailers in most cases. -.pp -MMDF and -.i sendmail -both support aliasing, -customized mailers, -message batching, -automatic forwarding to gateways, -queueing, -and retransmission. -MMDF supports two-stage timeout, -which -.i sendmail -does not support. -.pp -The configuration for MMDF -is compiled into the code\**. -.(f -\**Dynamic configuration tables are currently being considered -for MMDF; -allowing the installer to select either compiled -or dynamic tables. -.)f -.pp -Since MMDF does not consider backwards compatibility -as a design goal, -the address parsing is simpler but much less flexible. -.pp -It is somewhat harder to integrate a new channel\** -.(f -\**The MMDF equivalent of a -.i sendmail -.q mailer. -.)f -into MMDF. -In particular, -MMDF must know the location and format -of host tables for all channels, -and the channel must speak a special protocol. -This allows MMDF to do additional verification -(such as verifying host names) -at submission time. -.pp -MMDF strictly separates the submission and delivery phases. -Although -.i sendmail -has the concept of each of these stages, -they are integrated into one program, -whereas in MMDF they are split into two programs. -.sh 2 "Message Processing Module" -.pp -The Message Processing Module (MPM) -discussed by Postel [Postel79b] -matches -.i sendmail -closely in terms of its basic architecture. -However, -like MMDF, -the MPM includes the network interface software -as part of its domain. -.pp -MPM also postulates a duplex channel to the receiver, -as does MMDF, -thus allowing simpler handling of errors -by the mailer -than is possible in -.i sendmail . -When a message queued by -.i sendmail -is sent, -any errors must be returned to the sender -by the mailer itself. -Both MPM and MMDF mailers -can return an immediate error response, -and a single error processor can create an appropriate response. -.pp -MPM prefers passing the message as a structured object, -with type-length-value tuples\**. -.(f -\**This is similar to the NBS standard. -.)f -Such a convention requires a much higher degree of cooperation -between mailers than is required by -.i sendmail . -MPM also assumes a universally agreed upon internet name space -(with each address in the form of a net-host-user tuple), -which -.i sendmail -does not. -.sh 1 "EVALUATIONS AND FUTURE PLANS" -.pp -.i Sendmail -is designed to work in a nonhomogeneous environment. -Every attempt is made to avoid imposing unnecessary constraints -on the underlying mailers. -This goal has driven much of the design. -One of the major problems -has been the lack of a uniform address space, -as postulated in [Postel79a] -and [Postel79b]. -.pp -A nonuniform address space implies that a path will be specified -in all addresses, -either explicitly (as part of the address) -or implicitly -(as with implied forwarding to gateways). -This restriction has the unpleasant effect of making replying to messages -exceedingly difficult, -since there is no one -.q address -for any person, -but only a way to get there from wherever you are. -.pp -Interfacing to mail programs -that were not initially intended to be applied -in an internet environment -has been amazingly successful, -and has reduced the job to a manageable task. -.pp -.i Sendmail -has knowledge of a few difficult environments -built in. -It generates ARPANET FTP/SMTP compatible error messages -(prepended with three-digit numbers -[Neigus73, Postel74, Postel82]) -as necessary, -optionally generates UNIX-style -.q From -lines on the front of messages for some mailers, -and knows how to parse the same lines on input. -Also, -error handling has an option customized for BerkNet. -.pp -The decision to avoid doing any type of delivery where possible -(even, or perhaps especially, local delivery) -has turned out to be a good idea. -Even with local delivery, -there are issues of the location of the mailbox, -the format of the mailbox, -the locking protocol used, -etc., -that are best decided by other programs. -One surprisingly major annoyance in many internet mailers -is that the location and format of local mail is built in. -The feeling seems to be that local mail is so common -that it should be efficient. -This feeling is not born out by -our experience; -on the contrary, -the location and format of mailboxes seems to vary widely -from system to system. -.pp -The ability to automatically generate a response to incoming mail -(by forwarding mail to a program) -seems useful -(\c -.q "I am on vacation until late August...." ) -but can create problems -such as forwarding loops -(two people on vacation whose programs send notes back and forth, -for instance) -if these programs are not well written. -A program could be written to do standard tasks correctly, -but this would solve the general case. -.pp -It might be desirable to implement some form of load limiting. -I am unaware of any mail system that addresses this problem, -nor am I aware of any reasonable solution at this time. -.pp -The configuration file is currently practically inscrutable; -considerable convenience could be realized -with a higher-level format. -.pp -It seems clear that common protocols will be changing soon -to accommodate changing requirements and environments. -These changes will include modifications to the message header -(e.g., [NBS80]) -or to the body of the message itself -(such as for multimedia messages -[Postel80]). -Experience indicates that -these changes should be relatively trivial to integrate -into the existing system. -.pp -In tightly coupled environments, -it would be nice to have a name server -such as Grapvine -[Birrell82] -integrated into the mail system. -This would allow a site such as -.q Berkeley -to appear as a single host, -rather than as a collection of hosts, -and would allow people to move transparently among machines -without having to change their addresses. -Such a facility -would require an automatically updated database -and some method of resolving conflicts. -Ideally this would be effective even without -all hosts being under -a single management. -However, -it is not clear whether this feature -should be integrated into the -aliasing facility -or should be considered a -.q "value added" -feature outside -.i sendmail -itself. -.pp -As a more interesting case, -the CSNET name server -[Solomon81] -provides an facility that goes beyond a single -tightly-coupled environment. -Such a facility would normally exist outside of -.i sendmail -however. -.sh 0 "ACKNOWLEDGEMENTS" -.pp -Thanks are due to Kurt Shoens for his continual cheerful -assistance and good advice, -Bill Joy for pointing me in the correct direction -(over and over), -and Mark Horton for more advice, -prodding, -and many of the good ideas. -Kurt and Eric Schmidt are to be credited -for using -.i delivermail -as a server for their programs -(\c -.i Mail -and BerkNet respectively) -before any sane person should have, -and making the necessary modifications -promptly and happily. -Eric gave me considerable advice about the perils -of network software which saved me an unknown -amount of work and grief. -Mark did the original implementation of the DBM version -of aliasing, installed the VFORK code, -wrote the current version of -.i rmail , -and was the person who really convinced me -to put the work into -.i delivermail -to turn it into -.i sendmail . -Kurt deserves accolades for using -.i sendmail -when I was myself afraid to take the risk; -how a person can continue to be so enthusiastic -in the face of so much bitter reality is beyond me. -.pp -Kurt, -Mark, -Kirk McKusick, -Marvin Solomon, -and many others have reviewed this paper, -giving considerable useful advice. -.pp -Special thanks are reserved for Mike Stonebraker at Berkeley -and Bob Epstein at Britton-Lee, -who both knowingly allowed me to put so much work into this -project -when there were so many other things I really should -have been working on. -.+c -.ce -REFERENCES -.nr ii 1.5i -.ip [Birrell82] -Birrell, A. D., -Levin, R., -Needham, R. M., -and -Schroeder, M. D., -.q "Grapevine: An Exercise in Distributed Computing." -In -.ul -Comm. A.C.M. 25, -4, -April 82. -.ip [Borden79] -Borden, S., -Gaines, R. S., -and -Shapiro, N. Z., -.ul -The MH Message Handling System: Users' Manual. -R-2367-PAF. -Rand Corporation. -October 1979. -.ip [Crocker77a] -Crocker, D. H., -Vittal, J. J., -Pogran, K. T., -and -Henderson, D. A. Jr., -.ul -Standard for the Format of ARPA Network Text Messages. -RFC 733, -NIC 41952. -In [Feinler78]. -November 1977. -.ip [Crocker77b] -Crocker, D. H., -.ul -Framework and Functions of the MS Personal Message System. -R-2134-ARPA, -Rand Corporation, -Santa Monica, California. -1977. -.ip [Crocker79] -Crocker, D. H., -Szurkowski, E. S., -and -Farber, D. J., -.ul -An Internetwork Memo Distribution Facility \*- MMDF. -6th Data Communication Symposium, -Asilomar. -November 1979. -.ip [Crocker82] -Crocker, D. H., -.ul -Standard for the Format of Arpa Internet Text Messages. -RFC 822. -Network Information Center, -SRI International, -Menlo Park, California. -August 1982. -.ip [Metcalfe76] -Metcalfe, R., -and -Boggs, D., -.q "Ethernet: Distributed Packet Switching for Local Computer Networks" , -.ul -Communications of the ACM 19, -7. -July 1976. -.ip [Feinler78] -Feinler, E., -and -Postel, J. -(eds.), -.ul -ARPANET Protocol Handbook. -NIC 7104, -Network Information Center, -SRI International, -Menlo Park, California. -1978. -.ip [NBS80] -National Bureau of Standards, -.ul -Specification of a Draft Message Format Standard. -Report No. ICST/CBOS 80-2. -October 1980. -.ip [Neigus73] -Neigus, N., -.ul -File Transfer Protocol for the ARPA Network. -RFC 542, NIC 17759. -In [Feinler78]. -August, 1973. -.ip [Nowitz78a] -Nowitz, D. A., -and -Lesk, M. E., -.ul -A Dial-Up Network of UNIX Systems. -Bell Laboratories. -In -UNIX Programmer's Manual, Seventh Edition, -Volume 2. -August, 1978. -.ip [Nowitz78b] -Nowitz, D. A., -.ul -Uucp Implementation Description. -Bell Laboratories. -In -UNIX Programmer's Manual, Seventh Edition, -Volume 2. -October, 1978. -.ip [Postel74] -Postel, J., -and -Neigus, N., -Revised FTP Reply Codes. -RFC 640, NIC 30843. -In [Feinler78]. -June, 1974. -.ip [Postel77] -Postel, J., -.ul -Mail Protocol. -NIC 29588. -In [Feinler78]. -November 1977. -.ip [Postel79a] -Postel, J., -.ul -Internet Message Protocol. -RFC 753, -IEN 85. -Network Information Center, -SRI International, -Menlo Park, California. -March 1979. -.ip [Postel79b] -Postel, J. B., -.ul -An Internetwork Message Structure. -In -.ul -Proceedings of the Sixth Data Communications Symposium, -IEEE. -New York. -November 1979. -.ip [Postel80] -Postel, J. B., -.ul -A Structured Format for Transmission of Multi-Media Documents. -RFC 767. -Network Information Center, -SRI International, -Menlo Park, California. -August 1980. -.ip [Postel82] -Postel, J. B., -.ul -Simple Mail Transfer Protocol. -RFC821 -(obsoleting RFC788). -Network Information Center, -SRI International, -Menlo Park, California. -August 1982. -.ip [Schmidt79] -Schmidt, E., -.ul -An Introduction to the Berkeley Network. -University of California, Berkeley California. -1979. -.ip [Shoens79] -Shoens, K., -.ul -Mail Reference Manual. -University of California, Berkeley. -In UNIX Programmer's Manual, -Seventh Edition, -Volume 2C. -December 1979. -.ip [Sluizer81] -Sluizer, S., -and -Postel, J. B., -.ul -Mail Transfer Protocol. -RFC 780. -Network Information Center, -SRI International, -Menlo Park, California. -May 1981. -.ip [Solomon81] -Solomon, M., Landweber, L., and Neuhengen, D., -.q "The Design of the CSNET Name Server." -CS-DN-2, -University of Wisconsin, Madison. -November 1981. -.ip [Su82] -Su, Zaw-Sing, -and -Postel, Jon, -.ul -The Domain Naming Convention for Internet User Applications. -RFC819. -Network Information Center, -SRI International, -Menlo Park, California. -August 1982. -.ip [UNIX83] -.ul -The UNIX Programmer's Manual, Seventh Edition, -Virtual VAX-11 Version, -Volume 1. -Bell Laboratories, -modified by the University of California, -Berkeley, California. -March, 1983. |
