PROTOCOL revision 1.2.2.1
1 This documents OpenSSH's deviations and extensions to the published SSH 2 protocol. 3 4 Note that OpenSSH's sftp and sftp-server implement revision 3 of the SSH 5 filexfer protocol described in: 6 7 http://www.openssh.com/txt/draft-ietf-secsh-filexfer-02.txt 8 9 Newer versions of the draft will not be supported, though some features 10 are individually implemented as extensions described below. 11 12 The protocol used by OpenSSH's ssh-agent is described in the file 13 PROTOCOL.agent 14 15 1. Transport protocol changes 16 17 1.1. transport: Protocol 2 MAC algorithm "umac-64 (a] openssh.com" 18 19 This is a new transport-layer MAC method using the UMAC algorithm 20 (rfc4418). This method is identical to the "umac-64" method documented 21 in: 22 23 http://www.openssh.com/txt/draft-miller-secsh-umac-01.txt 24 25 1.2. transport: Protocol 2 compression algorithm "zlib (a] openssh.com" 26 27 This transport-layer compression method uses the zlib compression 28 algorithm (identical to the "zlib" method in rfc4253), but delays the 29 start of compression until after authentication has completed. This 30 avoids exposing compression code to attacks from unauthenticated users. 31 32 The method is documented in: 33 34 http://www.openssh.com/txt/draft-miller-secsh-compression-delayed-00.txt 35 36 1.3. transport: New public key algorithms "ssh-rsa-cert-v00 (a] openssh.com", 37 "ssh-dsa-cert-v00 (a] openssh.com", 38 "ecdsa-sha2-nistp256-cert-v01 (a] openssh.com", 39 "ecdsa-sha2-nistp384-cert-v01 (a] openssh.com" and 40 "ecdsa-sha2-nistp521-cert-v01 (a] openssh.com" 41 42 OpenSSH introduces new public key algorithms to support certificate 43 authentication for users and hostkeys. These methods are documented in 44 the file PROTOCOL.certkeys 45 46 1.4. transport: Elliptic Curve cryptography 47 48 OpenSSH supports ECC key exchange and public key authentication as 49 specified in RFC5656. Only the ecdsa-sha2-nistp256, ecdsa-sha2-nistp384 50 and ecdsa-sha2-nistp521 curves over GF(p) are supported. Elliptic 51 curve points encoded using point compression are NOT accepted or 52 generated. 53 54 1.5 transport: Protocol 2 Encrypt-then-MAC MAC algorithms 55 56 OpenSSH supports MAC algorithms, whose names contain "-etm", that 57 perform the calculations in a different order to that defined in RFC 58 4253. These variants use the so-called "encrypt then MAC" ordering, 59 calculating the MAC over the packet ciphertext rather than the 60 plaintext. This ordering closes a security flaw in the SSH transport 61 protocol, where decryption of unauthenticated ciphertext provided a 62 "decryption oracle" that could, in conjunction with cipher flaws, reveal 63 session plaintext. 64 65 Specifically, the "-etm" MAC algorithms modify the transport protocol 66 to calculate the MAC over the packet ciphertext and to send the packet 67 length unencrypted. This is necessary for the transport to obtain the 68 length of the packet and location of the MAC tag so that it may be 69 verified without decrypting unauthenticated data. 70 71 As such, the MAC covers: 72 73 mac = MAC(key, sequence_number || packet_length || encrypted_packet) 74 75 where "packet_length" is encoded as a uint32 and "encrypted_packet" 76 contains: 77 78 byte padding_length 79 byte[n1] payload; n1 = packet_length - padding_length - 1 80 byte[n2] random padding; n2 = padding_length 81 82 1.6 transport: AES-GCM 83 84 OpenSSH supports the AES-GCM algorithm as specified in RFC 5647. 85 Because of problems with the specification of the key exchange 86 the behaviour of OpenSSH differs from the RFC as follows: 87 88 AES-GCM is only negotiated as the cipher algorithms 89 "aes128-gcm (a] openssh.com" or "aes256-gcm (a] openssh.com" and never as 90 an MAC algorithm. Additionally, if AES-GCM is selected as the cipher 91 the exchanged MAC algorithms are ignored and there doesn't have to be 92 a matching MAC. 93 94 2. Connection protocol changes 95 96 2.1. connection: Channel write close extension "eow (a] openssh.com" 97 98 The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF 99 message to allow an endpoint to signal its peer that it will send no 100 more data over a channel. Unfortunately, there is no symmetric way for 101 an endpoint to request that its peer should cease sending data to it 102 while still keeping the channel open for the endpoint to send data to 103 the peer. 104 105 This is desirable, since it saves the transmission of data that would 106 otherwise need to be discarded and it allows an endpoint to signal local 107 processes of the condition, e.g. by closing the corresponding file 108 descriptor. 109 110 OpenSSH implements a channel extension message to perform this 111 signalling: "eow (a] openssh.com" (End Of Write). This message is sent by 112 an endpoint when the local output of a session channel is closed or 113 experiences a write error. The message is formatted as follows: 114 115 byte SSH_MSG_CHANNEL_REQUEST 116 uint32 recipient channel 117 string "eow (a] openssh.com" 118 boolean FALSE 119 120 On receiving this message, the peer SHOULD cease sending data of 121 the channel and MAY signal the process from which the channel data 122 originates (e.g. by closing its read file descriptor). 123 124 As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does 125 remain open after a "eow (a] openssh.com" has been sent and more data may 126 still be sent in the other direction. This message does not consume 127 window space and may be sent even if no window space is available. 128 129 NB. due to certain broken SSH implementations aborting upon receipt 130 of this message (in contravention of RFC4254 section 5.4), this 131 message is only sent to OpenSSH peers (identified by banner). 132 Other SSH implementations may be whitelisted to receive this message 133 upon request. 134 135 2.2. connection: disallow additional sessions extension 136 "no-more-sessions (a] openssh.com" 137 138 Most SSH connections will only ever request a single session, but a 139 attacker may abuse a running ssh client to surreptitiously open 140 additional sessions under their control. OpenSSH provides a global 141 request "no-more-sessions (a] openssh.com" to mitigate this attack. 142 143 When an OpenSSH client expects that it will never open another session 144 (i.e. it has been started with connection multiplexing disabled), it 145 will send the following global request: 146 147 byte SSH_MSG_GLOBAL_REQUEST 148 string "no-more-sessions (a] openssh.com" 149 char want-reply 150 151 On receipt of such a message, an OpenSSH server will refuse to open 152 future channels of type "session" and instead immediately abort the 153 connection. 154 155 Note that this is not a general defence against compromised clients 156 (that is impossible), but it thwarts a simple attack. 157 158 NB. due to certain broken SSH implementations aborting upon receipt 159 of this message, the no-more-sessions request is only sent to OpenSSH 160 servers (identified by banner). Other SSH implementations may be 161 whitelisted to receive this message upon request. 162 163 2.3. connection: Tunnel forward extension "tun (a] openssh.com" 164 165 OpenSSH supports layer 2 and layer 3 tunnelling via the "tun (a] openssh.com" 166 channel type. This channel type supports forwarding of network packets 167 with datagram boundaries intact between endpoints equipped with 168 interfaces like the BSD tun(4) device. Tunnel forwarding channels are 169 requested by the client with the following packet: 170 171 byte SSH_MSG_CHANNEL_OPEN 172 string "tun (a] openssh.com" 173 uint32 sender channel 174 uint32 initial window size 175 uint32 maximum packet size 176 uint32 tunnel mode 177 uint32 remote unit number 178 179 The "tunnel mode" parameter specifies whether the tunnel should forward 180 layer 2 frames or layer 3 packets. It may take one of the following values: 181 182 SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */ 183 SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */ 184 185 The "tunnel unit number" specifies the remote interface number, or may 186 be 0x7fffffff to allow the server to automatically chose an interface. A 187 server that is not willing to open a client-specified unit should refuse 188 the request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful 189 open, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS. 190 191 Once established the client and server may exchange packet or frames 192 over the tunnel channel by encapsulating them in SSH protocol strings 193 and sending them as channel data. This ensures that packet boundaries 194 are kept intact. Specifically, packets are transmitted using normal 195 SSH_MSG_CHANNEL_DATA packets: 196 197 byte SSH_MSG_CHANNEL_DATA 198 uint32 recipient channel 199 string data 200 201 The contents of the "data" field for layer 3 packets is: 202 203 uint32 packet length 204 uint32 address family 205 byte[packet length - 4] packet data 206 207 The "address family" field identifies the type of packet in the message. 208 It may be one of: 209 210 SSH_TUN_AF_INET 2 /* IPv4 */ 211 SSH_TUN_AF_INET6 24 /* IPv6 */ 212 213 The "packet data" field consists of the IPv4/IPv6 datagram itself 214 without any link layer header. 215 216 The contents of the "data" field for layer 2 packets is: 217 218 uint32 packet length 219 byte[packet length] frame 220 221 The "frame" field contains an IEEE 802.3 Ethernet frame, including 222 header. 223 224 3. SFTP protocol changes 225 226 3.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK 227 228 When OpenSSH's sftp-server was implemented, the order of the arguments 229 to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately, 230 the reversal was not noticed until the server was widely deployed. Since 231 fixing this to follow the specification would cause incompatibility, the 232 current order was retained. For correct operation, clients should send 233 SSH_FXP_SYMLINK as follows: 234 235 uint32 id 236 string targetpath 237 string linkpath 238 239 3.2. sftp: Server extension announcement in SSH_FXP_VERSION 240 241 OpenSSH's sftp-server lists the extensions it supports using the 242 standard extension announcement mechanism in the SSH_FXP_VERSION server 243 hello packet: 244 245 uint32 3 /* protocol version */ 246 string ext1-name 247 string ext1-version 248 string ext2-name 249 string ext2-version 250 ... 251 string extN-name 252 string extN-version 253 254 Each extension reports its integer version number as an ASCII encoded 255 string, e.g. "1". The version will be incremented if the extension is 256 ever changed in an incompatible way. The server MAY advertise the same 257 extension with multiple versions (though this is unlikely). Clients MUST 258 check the version number before attempting to use the extension. 259 260 3.3. sftp: Extension request "posix-rename (a] openssh.com" 261 262 This operation provides a rename operation with POSIX semantics, which 263 are different to those provided by the standard SSH_FXP_RENAME in 264 draft-ietf-secsh-filexfer-02.txt. This request is implemented as a 265 SSH_FXP_EXTENDED request with the following format: 266 267 uint32 id 268 string "posix-rename (a] openssh.com" 269 string oldpath 270 string newpath 271 272 On receiving this request the server will perform the POSIX operation 273 rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message. 274 This extension is advertised in the SSH_FXP_VERSION hello with version 275 "1". 276 277 3.4. sftp: Extension requests "statvfs (a] openssh.com" and 278 "fstatvfs (a] openssh.com" 279 280 These requests correspond to the statvfs and fstatvfs POSIX system 281 interfaces. The "statvfs (a] openssh.com" request operates on an explicit 282 pathname, and is formatted as follows: 283 284 uint32 id 285 string "statvfs (a] openssh.com" 286 string path 287 288 The "fstatvfs (a] openssh.com" operates on an open file handle: 289 290 uint32 id 291 string "fstatvfs (a] openssh.com" 292 string handle 293 294 These requests return a SSH_FXP_STATUS reply on failure. On success they 295 return the following SSH_FXP_EXTENDED_REPLY reply: 296 297 uint32 id 298 uint64 f_bsize /* file system block size */ 299 uint64 f_frsize /* fundamental fs block size */ 300 uint64 f_blocks /* number of blocks (unit f_frsize) */ 301 uint64 f_bfree /* free blocks in file system */ 302 uint64 f_bavail /* free blocks for non-root */ 303 uint64 f_files /* total file inodes */ 304 uint64 f_ffree /* free file inodes */ 305 uint64 f_favail /* free file inodes for to non-root */ 306 uint64 f_fsid /* file system id */ 307 uint64 f_flag /* bit mask of f_flag values */ 308 uint64 f_namemax /* maximum filename length */ 309 310 The values of the f_flag bitmask are as follows: 311 312 #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */ 313 #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */ 314 315 Both the "statvfs (a] openssh.com" and "fstatvfs (a] openssh.com" extensions are 316 advertised in the SSH_FXP_VERSION hello with version "2". 317 318 10. sftp: Extension request "hardlink (a] openssh.com" 319 320 This request is for creating a hard link to a regular file. This 321 request is implemented as a SSH_FXP_EXTENDED request with the 322 following format: 323 324 uint32 id 325 string "hardlink (a] openssh.com" 326 string oldpath 327 string newpath 328 329 On receiving this request the server will perform the operation 330 link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message. 331 This extension is advertised in the SSH_FXP_VERSION hello with version 332 "1". 333 334 $OpenBSD: PROTOCOL,v 1.20 2013/01/08 18:49:04 markus Exp $ 335 $NetBSD: PROTOCOL,v 1.2.2.1 2014/05/22 13:21:34 yamt Exp $ 336
Indexes created Fri Jul 10 00:26:32 UTC 2026