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 https://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 https://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 https://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 host keys. These methods are documented 44 in 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 1.7 transport: chacha20-poly1305 (a] openssh.com authenticated encryption 95 96 OpenSSH supports authenticated encryption using ChaCha20 and Poly1305 97 as described in PROTOCOL.chacha20poly1305. 98 99 1.8 transport: curve25519-sha256 (a] libssh.org key exchange algorithm 100 101 OpenSSH supports the use of ECDH in Curve25519 for key exchange as 102 described at: 103 http://git.libssh.org/users/aris/libssh.git/plain/doc/curve25519-sha256@libssh.org.txt?h=curve25519 104 105 2. Connection protocol changes 106 107 2.1. connection: Channel write close extension "eow (a] openssh.com" 108 109 The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF 110 message to allow an endpoint to signal its peer that it will send no 111 more data over a channel. Unfortunately, there is no symmetric way for 112 an endpoint to request that its peer should cease sending data to it 113 while still keeping the channel open for the endpoint to send data to 114 the peer. 115 116 This is desirable, since it saves the transmission of data that would 117 otherwise need to be discarded and it allows an endpoint to signal local 118 processes of the condition, e.g. by closing the corresponding file 119 descriptor. 120 121 OpenSSH implements a channel extension message to perform this 122 signalling: "eow (a] openssh.com" (End Of Write). This message is sent by 123 an endpoint when the local output of a session channel is closed or 124 experiences a write error. The message is formatted as follows: 125 126 byte SSH_MSG_CHANNEL_REQUEST 127 uint32 recipient channel 128 string "eow (a] openssh.com" 129 boolean FALSE 130 131 On receiving this message, the peer SHOULD cease sending data of 132 the channel and MAY signal the process from which the channel data 133 originates (e.g. by closing its read file descriptor). 134 135 As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does 136 remain open after a "eow (a] openssh.com" has been sent and more data may 137 still be sent in the other direction. This message does not consume 138 window space and may be sent even if no window space is available. 139 140 NB. due to certain broken SSH implementations aborting upon receipt 141 of this message (in contravention of RFC4254 section 5.4), this 142 message is only sent to OpenSSH peers (identified by banner). 143 Other SSH implementations may be whitelisted to receive this message 144 upon request. 145 146 2.2. connection: disallow additional sessions extension 147 "no-more-sessions (a] openssh.com" 148 149 Most SSH connections will only ever request a single session, but a 150 attacker may abuse a running ssh client to surreptitiously open 151 additional sessions under their control. OpenSSH provides a global 152 request "no-more-sessions (a] openssh.com" to mitigate this attack. 153 154 When an OpenSSH client expects that it will never open another session 155 (i.e. it has been started with connection multiplexing disabled), it 156 will send the following global request: 157 158 byte SSH_MSG_GLOBAL_REQUEST 159 string "no-more-sessions (a] openssh.com" 160 char want-reply 161 162 On receipt of such a message, an OpenSSH server will refuse to open 163 future channels of type "session" and instead immediately abort the 164 connection. 165 166 Note that this is not a general defence against compromised clients 167 (that is impossible), but it thwarts a simple attack. 168 169 NB. due to certain broken SSH implementations aborting upon receipt 170 of this message, the no-more-sessions request is only sent to OpenSSH 171 servers (identified by banner). Other SSH implementations may be 172 whitelisted to receive this message upon request. 173 174 2.3. connection: Tunnel forward extension "tun (a] openssh.com" 175 176 OpenSSH supports layer 2 and layer 3 tunnelling via the "tun (a] openssh.com" 177 channel type. This channel type supports forwarding of network packets 178 with datagram boundaries intact between endpoints equipped with 179 interfaces like the BSD tun(4) device. Tunnel forwarding channels are 180 requested by the client with the following packet: 181 182 byte SSH_MSG_CHANNEL_OPEN 183 string "tun (a] openssh.com" 184 uint32 sender channel 185 uint32 initial window size 186 uint32 maximum packet size 187 uint32 tunnel mode 188 uint32 remote unit number 189 190 The "tunnel mode" parameter specifies whether the tunnel should forward 191 layer 2 frames or layer 3 packets. It may take one of the following values: 192 193 SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */ 194 SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */ 195 196 The "tunnel unit number" specifies the remote interface number, or may 197 be 0x7fffffff to allow the server to automatically chose an interface. A 198 server that is not willing to open a client-specified unit should refuse 199 the request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful 200 open, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS. 201 202 Once established the client and server may exchange packet or frames 203 over the tunnel channel by encapsulating them in SSH protocol strings 204 and sending them as channel data. This ensures that packet boundaries 205 are kept intact. Specifically, packets are transmitted using normal 206 SSH_MSG_CHANNEL_DATA packets: 207 208 byte SSH_MSG_CHANNEL_DATA 209 uint32 recipient channel 210 string data 211 212 The contents of the "data" field for layer 3 packets is: 213 214 uint32 packet length 215 uint32 address family 216 byte[packet length - 4] packet data 217 218 The "address family" field identifies the type of packet in the message. 219 It may be one of: 220 221 SSH_TUN_AF_INET 2 /* IPv4 */ 222 SSH_TUN_AF_INET6 24 /* IPv6 */ 223 224 The "packet data" field consists of the IPv4/IPv6 datagram itself 225 without any link layer header. 226 227 The contents of the "data" field for layer 2 packets is: 228 229 uint32 packet length 230 byte[packet length] frame 231 232 The "frame" field contains an IEEE 802.3 Ethernet frame, including 233 header. 234 235 2.4. connection: Unix domain socket forwarding 236 237 OpenSSH supports local and remote Unix domain socket forwarding 238 using the "streamlocal" extension. Forwarding is initiated as per 239 TCP sockets but with a single path instead of a host and port. 240 241 Similar to direct-tcpip, direct-streamlocal is sent by the client 242 to request that the server make a connection to a Unix domain socket. 243 244 byte SSH_MSG_CHANNEL_OPEN 245 string "direct-streamlocal (a] openssh.com" 246 uint32 sender channel 247 uint32 initial window size 248 uint32 maximum packet size 249 string socket path 250 string reserved 251 uint32 reserved 252 253 Similar to forwarded-tcpip, forwarded-streamlocal is sent by the 254 server when the client has previously send the server a streamlocal-forward 255 GLOBAL_REQUEST. 256 257 byte SSH_MSG_CHANNEL_OPEN 258 string "forwarded-streamlocal (a] openssh.com" 259 uint32 sender channel 260 uint32 initial window size 261 uint32 maximum packet size 262 string socket path 263 string reserved for future use 264 265 The reserved field is not currently defined and is ignored on the 266 remote end. It is intended to be used in the future to pass 267 information about the socket file, such as ownership and mode. 268 The client currently sends the empty string for this field. 269 270 Similar to tcpip-forward, streamlocal-forward is sent by the client 271 to request remote forwarding of a Unix domain socket. 272 273 byte SSH2_MSG_GLOBAL_REQUEST 274 string "streamlocal-forward (a] openssh.com" 275 boolean TRUE 276 string socket path 277 278 Similar to cancel-tcpip-forward, cancel-streamlocal-forward is sent 279 by the client cancel the forwarding of a Unix domain socket. 280 281 byte SSH2_MSG_GLOBAL_REQUEST 282 string "cancel-streamlocal-forward (a] openssh.com" 283 boolean FALSE 284 string socket path 285 286 2.5. connection: hostkey update and rotation "hostkeys-00 (a] openssh.com" 287 and "hostkeys-prove-00 (a] openssh.com" 288 289 OpenSSH supports a protocol extension allowing a server to inform 290 a client of all its protocol v.2 host keys after user-authentication 291 has completed. 292 293 byte SSH_MSG_GLOBAL_REQUEST 294 string "hostkeys-00 (a] openssh.com" 295 string[] hostkeys 296 297 Upon receiving this message, a client should check which of the 298 supplied host keys are present in known_hosts. For keys that are 299 not present, it should send a "hostkeys-prove (a] openssh.com" message 300 to request the server prove ownership of the private half of the 301 key. 302 303 byte SSH_MSG_GLOBAL_REQUEST 304 string "hostkeys-prove-00 (a] openssh.com" 305 char 1 /* want-reply */ 306 string[] hostkeys 307 308 When a server receives this message, it should generate a signature 309 using each requested key over the following: 310 311 string "hostkeys-prove-00 (a] openssh.com" 312 string session identifier 313 string hostkey 314 315 These signatures should be included in the reply, in the order matching 316 the hostkeys in the request: 317 318 byte SSH_MSG_REQUEST_SUCCESS 319 string[] signatures 320 321 When the client receives this reply (and not a failure), it should 322 validate the signatures and may update its known_hosts file, adding keys 323 that it has not seen before and deleting keys for the server host that 324 are no longer offered. 325 326 These extensions let a client learn key types that it had not previously 327 encountered, thereby allowing it to potentially upgrade from weaker 328 key algorithms to better ones. It also supports graceful key rotation: 329 a server may offer multiple keys of the same type for a period (to 330 give clients an opportunity to learn them using this extension) before 331 removing the deprecated key from those offered. 332 333 3. SFTP protocol changes 334 335 3.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK 336 337 When OpenSSH's sftp-server was implemented, the order of the arguments 338 to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately, 339 the reversal was not noticed until the server was widely deployed. Since 340 fixing this to follow the specification would cause incompatibility, the 341 current order was retained. For correct operation, clients should send 342 SSH_FXP_SYMLINK as follows: 343 344 uint32 id 345 string targetpath 346 string linkpath 347 348 3.2. sftp: Server extension announcement in SSH_FXP_VERSION 349 350 OpenSSH's sftp-server lists the extensions it supports using the 351 standard extension announcement mechanism in the SSH_FXP_VERSION server 352 hello packet: 353 354 uint32 3 /* protocol version */ 355 string ext1-name 356 string ext1-version 357 string ext2-name 358 string ext2-version 359 ... 360 string extN-name 361 string extN-version 362 363 Each extension reports its integer version number as an ASCII encoded 364 string, e.g. "1". The version will be incremented if the extension is 365 ever changed in an incompatible way. The server MAY advertise the same 366 extension with multiple versions (though this is unlikely). Clients MUST 367 check the version number before attempting to use the extension. 368 369 3.3. sftp: Extension request "posix-rename (a] openssh.com" 370 371 This operation provides a rename operation with POSIX semantics, which 372 are different to those provided by the standard SSH_FXP_RENAME in 373 draft-ietf-secsh-filexfer-02.txt. This request is implemented as a 374 SSH_FXP_EXTENDED request with the following format: 375 376 uint32 id 377 string "posix-rename (a] openssh.com" 378 string oldpath 379 string newpath 380 381 On receiving this request the server will perform the POSIX operation 382 rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message. 383 This extension is advertised in the SSH_FXP_VERSION hello with version 384 "1". 385 386 3.4. sftp: Extension requests "statvfs (a] openssh.com" and 387 "fstatvfs (a] openssh.com" 388 389 These requests correspond to the statvfs and fstatvfs POSIX system 390 interfaces. The "statvfs (a] openssh.com" request operates on an explicit 391 pathname, and is formatted as follows: 392 393 uint32 id 394 string "statvfs (a] openssh.com" 395 string path 396 397 The "fstatvfs (a] openssh.com" operates on an open file handle: 398 399 uint32 id 400 string "fstatvfs (a] openssh.com" 401 string handle 402 403 These requests return a SSH_FXP_STATUS reply on failure. On success they 404 return the following SSH_FXP_EXTENDED_REPLY reply: 405 406 uint32 id 407 uint64 f_bsize /* file system block size */ 408 uint64 f_frsize /* fundamental fs block size */ 409 uint64 f_blocks /* number of blocks (unit f_frsize) */ 410 uint64 f_bfree /* free blocks in file system */ 411 uint64 f_bavail /* free blocks for non-root */ 412 uint64 f_files /* total file inodes */ 413 uint64 f_ffree /* free file inodes */ 414 uint64 f_favail /* free file inodes for to non-root */ 415 uint64 f_fsid /* file system id */ 416 uint64 f_flag /* bit mask of f_flag values */ 417 uint64 f_namemax /* maximum filename length */ 418 419 The values of the f_flag bitmask are as follows: 420 421 #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */ 422 #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */ 423 424 Both the "statvfs (a] openssh.com" and "fstatvfs (a] openssh.com" extensions are 425 advertised in the SSH_FXP_VERSION hello with version "2". 426 427 10. sftp: Extension request "hardlink (a] openssh.com" 428 429 This request is for creating a hard link to a regular file. This 430 request is implemented as a SSH_FXP_EXTENDED request with the 431 following format: 432 433 uint32 id 434 string "hardlink (a] openssh.com" 435 string oldpath 436 string newpath 437 438 On receiving this request the server will perform the operation 439 link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message. 440 This extension is advertised in the SSH_FXP_VERSION hello with version 441 "1". 442 443 10. sftp: Extension request "fsync (a] openssh.com" 444 445 This request asks the server to call fsync(2) on an open file handle. 446 447 uint32 id 448 string "fsync (a] openssh.com" 449 string handle 450 451 One receiving this request, a server will call fsync(handle_fd) and will 452 respond with a SSH_FXP_STATUS message. 453 454 This extension is advertised in the SSH_FXP_VERSION hello with version 455 "1". 456 457 $OpenBSD: PROTOCOL,v 1.30 2016/04/08 06:35:54 djm Exp $ 458
