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 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 for future use 251 252 Similar to forwarded-tcpip, forwarded-streamlocal is sent by the 253 server when the client has previously send the server a streamlocal-forward 254 GLOBAL_REQUEST. 255 256 byte SSH_MSG_CHANNEL_OPEN 257 string "forwarded-streamlocal (a] openssh.com" 258 uint32 sender channel 259 uint32 initial window size 260 uint32 maximum packet size 261 string socket path 262 string reserved for future use 263 264 The reserved field is not currently defined and is ignored on the 265 remote end. It is intended to be used in the future to pass 266 information about the socket file, such as ownership and mode. 267 The client currently sends the empty string for this field. 268 269 Similar to tcpip-forward, streamlocal-forward is sent by the client 270 to request remote forwarding of a Unix domain socket. 271 272 byte SSH2_MSG_GLOBAL_REQUEST 273 string "streamlocal-forward (a] openssh.com" 274 boolean TRUE 275 string socket path 276 277 Similar to cancel-tcpip-forward, cancel-streamlocal-forward is sent 278 by the client cancel the forwarding of a Unix domain socket. 279 280 byte SSH2_MSG_GLOBAL_REQUEST 281 string "cancel-streamlocal-forward (a] openssh.com" 282 boolean FALSE 283 string socket path 284 285 2.5. connection: hostkey update and rotation "hostkeys-00 (a] openssh.com" 286 and "hostkeys-prove-00 (a] openssh.com" 287 288 OpenSSH supports a protocol extension allowing a server to inform 289 a client of all its protocol v.2 host keys after user-authentication 290 has completed. 291 292 byte SSH_MSG_GLOBAL_REQUEST 293 string "hostkeys-00 (a] openssh.com" 294 string[] hostkeys 295 296 Upon receiving this message, a client should check which of the 297 supplied host keys are present in known_hosts. For keys that are 298 not present, it should send a "hostkeys-prove (a] openssh.com" message 299 to request the server prove ownership of the private half of the 300 key. 301 302 byte SSH_MSG_GLOBAL_REQUEST 303 string "hostkeys-prove-00 (a] openssh.com" 304 char 1 /* want-reply */ 305 string[] hostkeys 306 307 When a server receives this message, it should generate a signature 308 using each requested key over the following: 309 310 string "hostkeys-prove-00 (a] openssh.com" 311 string session identifier 312 string hostkey 313 314 These signatures should be included in the reply, in the order matching 315 the hostkeys in the request: 316 317 byte SSH_MSG_REQUEST_SUCCESS 318 string[] signatures 319 320 When the client receives this reply (and not a failure), it should 321 validate the signatures and may update its known_hosts file, adding keys 322 that it has not seen before and deleting keys for the server host that 323 are no longer offered. 324 325 These extensions let a client learn key types that it had not previously 326 encountered, thereby allowing it to potentially upgrade from weaker 327 key algorithms to better ones. It also supports graceful key rotation: 328 a server may offer multiple keys of the same type for a period (to 329 give clients an opportunity to learn them using this extension) before 330 removing the deprecated key from those offered. 331 332 3. SFTP protocol changes 333 334 3.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK 335 336 When OpenSSH's sftp-server was implemented, the order of the arguments 337 to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately, 338 the reversal was not noticed until the server was widely deployed. Since 339 fixing this to follow the specification would cause incompatibility, the 340 current order was retained. For correct operation, clients should send 341 SSH_FXP_SYMLINK as follows: 342 343 uint32 id 344 string targetpath 345 string linkpath 346 347 3.2. sftp: Server extension announcement in SSH_FXP_VERSION 348 349 OpenSSH's sftp-server lists the extensions it supports using the 350 standard extension announcement mechanism in the SSH_FXP_VERSION server 351 hello packet: 352 353 uint32 3 /* protocol version */ 354 string ext1-name 355 string ext1-version 356 string ext2-name 357 string ext2-version 358 ... 359 string extN-name 360 string extN-version 361 362 Each extension reports its integer version number as an ASCII encoded 363 string, e.g. "1". The version will be incremented if the extension is 364 ever changed in an incompatible way. The server MAY advertise the same 365 extension with multiple versions (though this is unlikely). Clients MUST 366 check the version number before attempting to use the extension. 367 368 3.3. sftp: Extension request "posix-rename (a] openssh.com" 369 370 This operation provides a rename operation with POSIX semantics, which 371 are different to those provided by the standard SSH_FXP_RENAME in 372 draft-ietf-secsh-filexfer-02.txt. This request is implemented as a 373 SSH_FXP_EXTENDED request with the following format: 374 375 uint32 id 376 string "posix-rename (a] openssh.com" 377 string oldpath 378 string newpath 379 380 On receiving this request the server will perform the POSIX operation 381 rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message. 382 This extension is advertised in the SSH_FXP_VERSION hello with version 383 "1". 384 385 3.4. sftp: Extension requests "statvfs (a] openssh.com" and 386 "fstatvfs (a] openssh.com" 387 388 These requests correspond to the statvfs and fstatvfs POSIX system 389 interfaces. The "statvfs (a] openssh.com" request operates on an explicit 390 pathname, and is formatted as follows: 391 392 uint32 id 393 string "statvfs (a] openssh.com" 394 string path 395 396 The "fstatvfs (a] openssh.com" operates on an open file handle: 397 398 uint32 id 399 string "fstatvfs (a] openssh.com" 400 string handle 401 402 These requests return a SSH_FXP_STATUS reply on failure. On success they 403 return the following SSH_FXP_EXTENDED_REPLY reply: 404 405 uint32 id 406 uint64 f_bsize /* file system block size */ 407 uint64 f_frsize /* fundamental fs block size */ 408 uint64 f_blocks /* number of blocks (unit f_frsize) */ 409 uint64 f_bfree /* free blocks in file system */ 410 uint64 f_bavail /* free blocks for non-root */ 411 uint64 f_files /* total file inodes */ 412 uint64 f_ffree /* free file inodes */ 413 uint64 f_favail /* free file inodes for to non-root */ 414 uint64 f_fsid /* file system id */ 415 uint64 f_flag /* bit mask of f_flag values */ 416 uint64 f_namemax /* maximum filename length */ 417 418 The values of the f_flag bitmask are as follows: 419 420 #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */ 421 #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */ 422 423 Both the "statvfs (a] openssh.com" and "fstatvfs (a] openssh.com" extensions are 424 advertised in the SSH_FXP_VERSION hello with version "2". 425 426 10. sftp: Extension request "hardlink (a] openssh.com" 427 428 This request is for creating a hard link to a regular file. This 429 request is implemented as a SSH_FXP_EXTENDED request with the 430 following format: 431 432 uint32 id 433 string "hardlink (a] openssh.com" 434 string oldpath 435 string newpath 436 437 On receiving this request the server will perform the operation 438 link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message. 439 This extension is advertised in the SSH_FXP_VERSION hello with version 440 "1". 441 442 10. sftp: Extension request "fsync (a] openssh.com" 443 444 This request asks the server to call fsync(2) on an open file handle. 445 446 uint32 id 447 string "fsync (a] openssh.com" 448 string handle 449 450 One receiving this request, a server will call fsync(handle_fd) and will 451 respond with a SSH_FXP_STATUS message. 452 453 This extension is advertised in the SSH_FXP_VERSION hello with version 454 "1". 455 456 $OpenBSD: PROTOCOL,v 1.27 2015/02/20 22:17:21 djm Exp $ 457
