1This documents OpenSSH's deviations and extensions to the published SSH 2protocol. 3 4Note that OpenSSH's sftp and sftp-server implement revision 3 of the SSH 5filexfer protocol described in: 6 7http://www.openssh.com/txt/draft-ietf-secsh-filexfer-02.txt 8 9Features from newer versions of the draft are not supported, unless 10explicitly implemented as extensions described below. 11 12The protocol used by OpenSSH's ssh-agent is described in the file 13PROTOCOL.agent 14 151. transport: Protocol 2 MAC algorithm "umac-64@openssh.com" 16 17This is a new transport-layer MAC method using the UMAC algorithm 18(rfc4418). This method is identical to the "umac-64" method documented 19in: 20 21http://www.openssh.com/txt/draft-miller-secsh-umac-01.txt 22 232. transport: Protocol 2 compression algorithm "zlib@openssh.com" 24 25This transport-layer compression method uses the zlib compression 26algorithm (identical to the "zlib" method in rfc4253), but delays the 27start of compression until after authentication has completed. This 28avoids exposing compression code to attacks from unauthenticated users. 29 30The method is documented in: 31 32http://www.openssh.com/txt/draft-miller-secsh-compression-delayed-00.txt 33 343. connection: Channel write close extension "eow@openssh.com" 35 36The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF 37message to allow an endpoint to signal its peer that it will send no 38more data over a channel. Unfortunately, there is no symmetric way for 39an endpoint to request that its peer should cease sending data to it 40while still keeping the channel open for the endpoint to send data to 41the peer. 42 43This is desirable, since it saves the transmission of data that would 44otherwise need to be discarded and it allows an endpoint to signal local 45processes of the condition, e.g. by closing the corresponding file 46descriptor. 47 48OpenSSH implements a channel extension message to perform this 49signalling: "eow@openssh.com" (End Of Write). This message is sent by 50an endpoint when the local output of a session channel is closed or 51experiences a write error. The message is formatted as follows: 52 53 byte SSH_MSG_CHANNEL_REQUEST 54 uint32 recipient channel 55 string "eow@openssh.com" 56 boolean FALSE 57 58On receiving this message, the peer SHOULD cease sending data of 59the channel and MAY signal the process from which the channel data 60originates (e.g. by closing its read file descriptor). 61 62As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does 63remain open after a "eow@openssh.com" has been sent and more data may 64still be sent in the other direction. This message does not consume 65window space and may be sent even if no window space is available. 66 67NB. due to certain broken SSH implementations aborting upon receipt 68of this message (in contravention of RFC4254 section 5.4), this 69message is only sent to OpenSSH peers (identified by banner). 70Other SSH implementations may be whitelisted to receive this message 71upon request. 72 734. connection: disallow additional sessions extension 74 "no-more-sessions@openssh.com" 75 76Most SSH connections will only ever request a single session, but a 77attacker may abuse a running ssh client to surreptitiously open 78additional sessions under their control. OpenSSH provides a global 79request "no-more-sessions@openssh.com" to mitigate this attack. 80 81When an OpenSSH client expects that it will never open another session 82(i.e. it has been started with connection multiplexing disabled), it 83will send the following global request: 84 85 byte SSH_MSG_GLOBAL_REQUEST 86 string "no-more-sessions@openssh.com" 87 char want-reply 88 89On receipt of such a message, an OpenSSH server will refuse to open 90future channels of type "session" and instead immediately abort the 91connection. 92 93Note that this is not a general defence against compromised clients 94(that is impossible), but it thwarts a simple attack. 95 96NB. due to certain broken SSH implementations aborting upon receipt 97of this message, the no-more-sessions request is only sent to OpenSSH 98servers (identified by banner). Other SSH implementations may be 99whitelisted to receive this message upon request. 100 1015. connection: Tunnel forward extension "tun@openssh.com" 102 103OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com" 104channel type. This channel type supports forwarding of network packets 105with datagram boundaries intact between endpoints equipped with 106interfaces like the BSD tun(4) device. Tunnel forwarding channels are 107requested by the client with the following packet: 108 109 byte SSH_MSG_CHANNEL_OPEN 110 string "tun@openssh.com" 111 uint32 sender channel 112 uint32 initial window size 113 uint32 maximum packet size 114 uint32 tunnel mode 115 uint32 remote unit number 116 117The "tunnel mode" parameter specifies whether the tunnel should forward 118layer 2 frames or layer 3 packets. It may take one of the following values: 119 120 SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */ 121 SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */ 122 123The "tunnel unit number" specifies the remote interface number, or may 124be zero to allow the server to automatically chose an interface. A server 125that is not willing to open a client-specified unit should refuse the 126request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful open, 127the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS. 128 129Once established the client and server may exchange packet or frames 130over the tunnel channel by encapsulating them in SSH protocol strings 131and sending them as channel data. This ensures that packet boundaries 132are kept intact. Specifically, packets are transmitted using normal 133SSH_MSG_CHANNEL_DATA packets: 134 135 byte SSH_MSG_CHANNEL_DATA 136 uint32 recipient channel 137 string data 138 139The contents of the "data" field for layer 3 packets is: 140 141 uint32 packet length 142 uint32 address family 143 byte[packet length - 4] packet data 144 145The "address family" field identifies the type of packet in the message. 146It may be one of: 147 148 SSH_TUN_AF_INET 2 /* IPv4 */ 149 SSH_TUN_AF_INET6 24 /* IPv6 */ 150 151The "packet data" field consists of the IPv4/IPv6 datagram itself 152without any link layer header. 153 154The contents of the "data" field for layer 3 packets is: 155 156 uint32 packet length 157 byte[packet length] frame 158 159The "frame" field contains an IEEE 802.3 Ethernet frame, including 160header. 161 1626. sftp: Reversal of arguments to SSH_FXP_SYMLINK 163 164When OpenSSH's sftp-server was implemented, the order of the arguments 165to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately, 166the reversal was not noticed until the server was widely deployed. Since 167fixing this to follow the specification would cause incompatibility, the 168current order was retained. For correct operation, clients should send 169SSH_FXP_SYMLINK as follows: 170 171 uint32 id 172 string targetpath 173 string linkpath 174 1757. sftp: Server extension announcement in SSH_FXP_VERSION 176 177OpenSSH's sftp-server lists the extensions it supports using the 178standard extension announcement mechanism in the SSH_FXP_VERSION server 179hello packet: 180 181 uint32 3 /* protocol version */ 182 string ext1-name 183 string ext1-version 184 string ext2-name 185 string ext2-version 186 ... 187 string extN-name 188 string extN-version 189 190Each extension reports its integer version number as an ASCII encoded 191string, e.g. "1". The version will be incremented if the extension is 192ever changed in an incompatible way. The server MAY advertise the same 193extension with multiple versions (though this is unlikely). Clients MUST 194check the version number before attempting to use the extension. 195 1968. sftp: Extension request "posix-rename@openssh.com" 197 198This operation provides a rename operation with POSIX semantics, which 199are different to those provided by the standard SSH_FXP_RENAME in 200draft-ietf-secsh-filexfer-02.txt. This request is implemented as a 201SSH_FXP_EXTENDED request with the following format: 202 203 uint32 id 204 string "posix-rename@openssh.com" 205 string oldpath 206 string newpath 207 208On receiving this request the server will perform the POSIX operation 209rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message. 210This extension is advertised in the SSH_FXP_VERSION hello with version 211"1". 212 2139. sftp: Extension requests "statvfs@openssh.com" and 214 "fstatvfs@openssh.com" 215 216These requests correspond to the statvfs and fstatvfs POSIX system 217interfaces. The "statvfs@openssh.com" request operates on an explicit 218pathname, and is formatted as follows: 219 220 uint32 id 221 string "statvfs@openssh.com" 222 string path 223 224The "fstatvfs@openssh.com" operates on an open file handle: 225 226 uint32 id 227 string "fstatvfs@openssh.com" 228 string handle 229 230These requests return a SSH_FXP_STATUS reply on failure. On success they 231return the following SSH_FXP_EXTENDED_REPLY reply: 232 233 uint32 id 234 uint64 f_bsize /* file system block size */ 235 uint64 f_frsize /* fundamental fs block size */ 236 uint64 f_blocks /* number of blocks (unit f_frsize) */ 237 uint64 f_bfree /* free blocks in file system */ 238 uint64 f_bavail /* free blocks for non-root */ 239 uint64 f_files /* total file inodes */ 240 uint64 f_ffree /* free file inodes */ 241 uint64 f_favail /* free file inodes for to non-root */ 242 uint64 f_fsid /* file system id */ 243 uint64 f_flag /* bit mask of f_flag values */ 244 uint64 f_namemax /* maximum filename length */ 245 246The values of the f_flag bitmask are as follows: 247 248 #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */ 249 #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */ 250 251Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are 252advertised in the SSH_FXP_VERSION hello with version "2". 253 254$OpenBSD: PROTOCOL,v 1.12 2009/02/14 06:35:49 djm Exp $ 255