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 250 251 Similar to forwarded-tcpip, forwarded-streamlocal is sent by the 252 server when the client has previously send the server a streamlocal-forward 253 GLOBAL_REQUEST. 254 255 byte SSH_MSG_CHANNEL_OPEN 256 string "forwarded-streamlocal (a] openssh.com" 257 uint32 sender channel 258 uint32 initial window size 259 uint32 maximum packet size 260 string socket path 261 string reserved for future use 262 263 The reserved field is not currently defined and is ignored on the 264 remote end. It is intended to be used in the future to pass 265 information about the socket file, such as ownership and mode. 266 The client currently sends the empty string for this field. 267 268 Similar to tcpip-forward, streamlocal-forward is sent by the client 269 to request remote forwarding of a Unix domain socket. 270 271 byte SSH2_MSG_GLOBAL_REQUEST 272 string "streamlocal-forward (a] openssh.com" 273 boolean TRUE 274 string socket path 275 276 Similar to cancel-tcpip-forward, cancel-streamlocal-forward is sent 277 by the client cancel the forwarding of a Unix domain socket. 278 279 byte SSH2_MSG_GLOBAL_REQUEST 280 string "cancel-streamlocal-forward (a] openssh.com" 281 boolean FALSE 282 string socket path 283 284 2.5. connection: hostkey update and rotation "hostkeys-00 (a] openssh.com" 285 and "hostkeys-prove-00 (a] openssh.com" 286 287 OpenSSH supports a protocol extension allowing a server to inform 288 a client of all its protocol v.2 host keys after user-authentication 289 has completed. 290 291 byte SSH_MSG_GLOBAL_REQUEST 292 string "hostkeys-00 (a] openssh.com" 293 string[] hostkeys 294 295 Upon receiving this message, a client should check which of the 296 supplied host keys are present in known_hosts. For keys that are 297 not present, it should send a "hostkeys-prove (a] openssh.com" message 298 to request the server prove ownership of the private half of the 299 key. 300 301 byte SSH_MSG_GLOBAL_REQUEST 302 string "hostkeys-prove-00 (a] openssh.com" 303 char 1 /* want-reply */ 304 string[] hostkeys 305 306 When a server receives this message, it should generate a signature 307 using each requested key over the following: 308 309 string "hostkeys-prove-00 (a] openssh.com" 310 string session identifier 311 string hostkey 312 313 These signatures should be included in the reply, in the order matching 314 the hostkeys in the request: 315 316 byte SSH_MSG_REQUEST_SUCCESS 317 string[] signatures 318 319 When the client receives this reply (and not a failure), it should 320 validate the signatures and may update its known_hosts file, adding keys 321 that it has not seen before and deleting keys for the server host that 322 are no longer offered. 323 324 These extensions let a client learn key types that it had not previously 325 encountered, thereby allowing it to potentially upgrade from weaker 326 key algorithms to better ones. It also supports graceful key rotation: 327 a server may offer multiple keys of the same type for a period (to 328 give clients an opportunity to learn them using this extension) before 329 removing the deprecated key from those offered. 330 331 3. SFTP protocol changes 332 333 3.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK 334 335 When OpenSSH's sftp-server was implemented, the order of the arguments 336 to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately, 337 the reversal was not noticed until the server was widely deployed. Since 338 fixing this to follow the specification would cause incompatibility, the 339 current order was retained. For correct operation, clients should send 340 SSH_FXP_SYMLINK as follows: 341 342 uint32 id 343 string targetpath 344 string linkpath 345 346 3.2. sftp: Server extension announcement in SSH_FXP_VERSION 347 348 OpenSSH's sftp-server lists the extensions it supports using the 349 standard extension announcement mechanism in the SSH_FXP_VERSION server 350 hello packet: 351 352 uint32 3 /* protocol version */ 353 string ext1-name 354 string ext1-version 355 string ext2-name 356 string ext2-version 357 ... 358 string extN-name 359 string extN-version 360 361 Each extension reports its integer version number as an ASCII encoded 362 string, e.g. "1". The version will be incremented if the extension is 363 ever changed in an incompatible way. The server MAY advertise the same 364 extension with multiple versions (though this is unlikely). Clients MUST 365 check the version number before attempting to use the extension. 366 367 3.3. sftp: Extension request "posix-rename (a] openssh.com" 368 369 This operation provides a rename operation with POSIX semantics, which 370 are different to those provided by the standard SSH_FXP_RENAME in 371 draft-ietf-secsh-filexfer-02.txt. This request is implemented as a 372 SSH_FXP_EXTENDED request with the following format: 373 374 uint32 id 375 string "posix-rename (a] openssh.com" 376 string oldpath 377 string newpath 378 379 On receiving this request the server will perform the POSIX operation 380 rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message. 381 This extension is advertised in the SSH_FXP_VERSION hello with version 382 "1". 383 384 3.4. sftp: Extension requests "statvfs (a] openssh.com" and 385 "fstatvfs (a] openssh.com" 386 387 These requests correspond to the statvfs and fstatvfs POSIX system 388 interfaces. The "statvfs (a] openssh.com" request operates on an explicit 389 pathname, and is formatted as follows: 390 391 uint32 id 392 string "statvfs (a] openssh.com" 393 string path 394 395 The "fstatvfs (a] openssh.com" operates on an open file handle: 396 397 uint32 id 398 string "fstatvfs (a] openssh.com" 399 string handle 400 401 These requests return a SSH_FXP_STATUS reply on failure. On success they 402 return the following SSH_FXP_EXTENDED_REPLY reply: 403 404 uint32 id 405 uint64 f_bsize /* file system block size */ 406 uint64 f_frsize /* fundamental fs block size */ 407 uint64 f_blocks /* number of blocks (unit f_frsize) */ 408 uint64 f_bfree /* free blocks in file system */ 409 uint64 f_bavail /* free blocks for non-root */ 410 uint64 f_files /* total file inodes */ 411 uint64 f_ffree /* free file inodes */ 412 uint64 f_favail /* free file inodes for to non-root */ 413 uint64 f_fsid /* file system id */ 414 uint64 f_flag /* bit mask of f_flag values */ 415 uint64 f_namemax /* maximum filename length */ 416 417 The values of the f_flag bitmask are as follows: 418 419 #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */ 420 #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */ 421 422 Both the "statvfs (a] openssh.com" and "fstatvfs (a] openssh.com" extensions are 423 advertised in the SSH_FXP_VERSION hello with version "2". 424 425 10. sftp: Extension request "hardlink (a] openssh.com" 426 427 This request is for creating a hard link to a regular file. This 428 request is implemented as a SSH_FXP_EXTENDED request with the 429 following format: 430 431 uint32 id 432 string "hardlink (a] openssh.com" 433 string oldpath 434 string newpath 435 436 On receiving this request the server will perform the operation 437 link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message. 438 This extension is advertised in the SSH_FXP_VERSION hello with version 439 "1". 440 441 10. sftp: Extension request "fsync (a] openssh.com" 442 443 This request asks the server to call fsync(2) on an open file handle. 444 445 uint32 id 446 string "fsync (a] openssh.com" 447 string handle 448 449 One receiving this request, a server will call fsync(handle_fd) and will 450 respond with a SSH_FXP_STATUS message. 451 452 This extension is advertised in the SSH_FXP_VERSION hello with version 453 "1". 454 455 $OpenBSD: PROTOCOL,v 1.29 2015/07/17 03:09:19 djm Exp $ 456
