diff options
Diffstat (limited to 'crypto/openssh/PROTOCOL')
| -rw-r--r-- | crypto/openssh/PROTOCOL | 254 |
1 files changed, 254 insertions, 0 deletions
diff --git a/crypto/openssh/PROTOCOL b/crypto/openssh/PROTOCOL new file mode 100644 index 0000000..5aada63 --- /dev/null +++ b/crypto/openssh/PROTOCOL @@ -0,0 +1,254 @@ +This documents OpenSSH's deviations and extensions to the published SSH +protocol. + +Note that OpenSSH's sftp and sftp-server implement revision 3 of the SSH +filexfer protocol described in: + +http://www.openssh.com/txt/draft-ietf-secsh-filexfer-02.txt + +Features from newer versions of the draft are not supported, unless +explicitly implemented as extensions described below. + +The protocol used by OpenSSH's ssh-agent is described in the file +PROTOCOL.agent + +1. transport: Protocol 2 MAC algorithm "umac-64@openssh.com" + +This is a new transport-layer MAC method using the UMAC algorithm +(rfc4418). This method is identical to the "umac-64" method documented +in: + +http://www.openssh.com/txt/draft-miller-secsh-umac-01.txt + +2. transport: Protocol 2 compression algorithm "zlib@openssh.com" + +This transport-layer compression method uses the zlib compression +algorithm (identical to the "zlib" method in rfc4253), but delays the +start of compression until after authentication has completed. This +avoids exposing compression code to attacks from unauthenticated users. + +The method is documented in: + +http://www.openssh.com/txt/draft-miller-secsh-compression-delayed-00.txt + +3. connection: Channel write close extension "eow@openssh.com" + +The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF +message to allow an endpoint to signal its peer that it will send no +more data over a channel. Unfortunately, there is no symmetric way for +an endpoint to request that its peer should cease sending data to it +while still keeping the channel open for the endpoint to send data to +the peer. + +This is desirable, since it saves the transmission of data that would +otherwise need to be discarded and it allows an endpoint to signal local +processes of the condition, e.g. by closing the corresponding file +descriptor. + +OpenSSH implements a channel extension message to perform this +signalling: "eow@openssh.com" (End Of Write). This message is sent by +an endpoint when the local output of a session channel is closed or +experiences a write error. The message is formatted as follows: + + byte SSH_MSG_CHANNEL_REQUEST + uint32 recipient channel + string "eow@openssh.com" + boolean FALSE + +On receiving this message, the peer SHOULD cease sending data of +the channel and MAY signal the process from which the channel data +originates (e.g. by closing its read file descriptor). + +As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does +remain open after a "eow@openssh.com" has been sent and more data may +still be sent in the other direction. This message does not consume +window space and may be sent even if no window space is available. + +NB. due to certain broken SSH implementations aborting upon receipt +of this message (in contravention of RFC4254 section 5.4), this +message is only sent to OpenSSH peers (identified by banner). +Other SSH implementations may be whitelisted to receive this message +upon request. + +4. connection: disallow additional sessions extension + "no-more-sessions@openssh.com" + +Most SSH connections will only ever request a single session, but a +attacker may abuse a running ssh client to surreptitiously open +additional sessions under their control. OpenSSH provides a global +request "no-more-sessions@openssh.com" to mitigate this attack. + +When an OpenSSH client expects that it will never open another session +(i.e. it has been started with connection multiplexing disabled), it +will send the following global request: + + byte SSH_MSG_GLOBAL_REQUEST + string "no-more-sessions@openssh.com" + char want-reply + +On receipt of such a message, an OpenSSH server will refuse to open +future channels of type "session" and instead immediately abort the +connection. + +Note that this is not a general defence against compromised clients +(that is impossible), but it thwarts a simple attack. + +NB. due to certain broken SSH implementations aborting upon receipt +of this message, the no-more-sessions request is only sent to OpenSSH +servers (identified by banner). Other SSH implementations may be +whitelisted to receive this message upon request. + +5. connection: Tunnel forward extension "tun@openssh.com" + +OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com" +channel type. This channel type supports forwarding of network packets +with datagram boundaries intact between endpoints equipped with +interfaces like the BSD tun(4) device. Tunnel forwarding channels are +requested by the client with the following packet: + + byte SSH_MSG_CHANNEL_OPEN + string "tun@openssh.com" + uint32 sender channel + uint32 initial window size + uint32 maximum packet size + uint32 tunnel mode + uint32 remote unit number + +The "tunnel mode" parameter specifies whether the tunnel should forward +layer 2 frames or layer 3 packets. It may take one of the following values: + + SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */ + SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */ + +The "tunnel unit number" specifies the remote interface number, or may +be zero to allow the server to automatically chose an interface. A server +that is not willing to open a client-specified unit should refuse the +request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful open, +the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS. + +Once established the client and server may exchange packet or frames +over the tunnel channel by encapsulating them in SSH protocol strings +and sending them as channel data. This ensures that packet boundaries +are kept intact. Specifically, packets are transmitted using normal +SSH_MSG_CHANNEL_DATA packets: + + byte SSH_MSG_CHANNEL_DATA + uint32 recipient channel + string data + +The contents of the "data" field for layer 3 packets is: + + uint32 packet length + uint32 address family + byte[packet length - 4] packet data + +The "address family" field identifies the type of packet in the message. +It may be one of: + + SSH_TUN_AF_INET 2 /* IPv4 */ + SSH_TUN_AF_INET6 24 /* IPv6 */ + +The "packet data" field consists of the IPv4/IPv6 datagram itself +without any link layer header. + +The contents of the "data" field for layer 3 packets is: + + uint32 packet length + byte[packet length] frame + +The "frame" field contains an IEEE 802.3 Ethernet frame, including +header. + +6. sftp: Reversal of arguments to SSH_FXP_SYMLINK + +When OpenSSH's sftp-server was implemented, the order of the arguments +to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately, +the reversal was not noticed until the server was widely deployed. Since +fixing this to follow the specification would cause incompatibility, the +current order was retained. For correct operation, clients should send +SSH_FXP_SYMLINK as follows: + + uint32 id + string targetpath + string linkpath + +7. sftp: Server extension announcement in SSH_FXP_VERSION + +OpenSSH's sftp-server lists the extensions it supports using the +standard extension announcement mechanism in the SSH_FXP_VERSION server +hello packet: + + uint32 3 /* protocol version */ + string ext1-name + string ext1-version + string ext2-name + string ext2-version + ... + string extN-name + string extN-version + +Each extension reports its integer version number as an ASCII encoded +string, e.g. "1". The version will be incremented if the extension is +ever changed in an incompatible way. The server MAY advertise the same +extension with multiple versions (though this is unlikely). Clients MUST +check the version number before attempting to use the extension. + +8. sftp: Extension request "posix-rename@openssh.com" + +This operation provides a rename operation with POSIX semantics, which +are different to those provided by the standard SSH_FXP_RENAME in +draft-ietf-secsh-filexfer-02.txt. This request is implemented as a +SSH_FXP_EXTENDED request with the following format: + + uint32 id + string "posix-rename@openssh.com" + string oldpath + string newpath + +On receiving this request the server will perform the POSIX operation +rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message. +This extension is advertised in the SSH_FXP_VERSION hello with version +"1". + +9. sftp: Extension requests "statvfs@openssh.com" and + "fstatvfs@openssh.com" + +These requests correspond to the statvfs and fstatvfs POSIX system +interfaces. The "statvfs@openssh.com" request operates on an explicit +pathname, and is formatted as follows: + + uint32 id + string "statvfs@openssh.com" + string path + +The "fstatvfs@openssh.com" operates on an open file handle: + + uint32 id + string "fstatvfs@openssh.com" + string handle + +These requests return a SSH_FXP_STATUS reply on failure. On success they +return the following SSH_FXP_EXTENDED_REPLY reply: + + uint32 id + uint64 f_bsize /* file system block size */ + uint64 f_frsize /* fundamental fs block size */ + uint64 f_blocks /* number of blocks (unit f_frsize) */ + uint64 f_bfree /* free blocks in file system */ + uint64 f_bavail /* free blocks for non-root */ + uint64 f_files /* total file inodes */ + uint64 f_ffree /* free file inodes */ + uint64 f_favail /* free file inodes for to non-root */ + uint64 f_fsid /* file system id */ + uint64 f_flag /* bit mask of f_flag values */ + uint64 f_namemax /* maximum filename length */ + +The values of the f_flag bitmask are as follows: + + #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */ + #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */ + +Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are +advertised in the SSH_FXP_VERSION hello with version "2". + +$OpenBSD: PROTOCOL,v 1.12 2009/02/14 06:35:49 djm Exp $ |
