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 2. Connection protocol changes 55 56 2.1. connection: Channel write close extension "eow (a] openssh.com" 57 58 The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF 59 message to allow an endpoint to signal its peer that it will send no 60 more data over a channel. Unfortunately, there is no symmetric way for 61 an endpoint to request that its peer should cease sending data to it 62 while still keeping the channel open for the endpoint to send data to 63 the peer. 64 65 This is desirable, since it saves the transmission of data that would 66 otherwise need to be discarded and it allows an endpoint to signal local 67 processes of the condition, e.g. by closing the corresponding file 68 descriptor. 69 70 OpenSSH implements a channel extension message to perform this 71 signalling: "eow (a] openssh.com" (End Of Write). This message is sent by 72 an endpoint when the local output of a session channel is closed or 73 experiences a write error. The message is formatted as follows: 74 75 byte SSH_MSG_CHANNEL_REQUEST 76 uint32 recipient channel 77 string "eow (a] openssh.com" 78 boolean FALSE 79 80 On receiving this message, the peer SHOULD cease sending data of 81 the channel and MAY signal the process from which the channel data 82 originates (e.g. by closing its read file descriptor). 83 84 As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does 85 remain open after a "eow (a] openssh.com" has been sent and more data may 86 still be sent in the other direction. This message does not consume 87 window space and may be sent even if no window space is available. 88 89 NB. due to certain broken SSH implementations aborting upon receipt 90 of this message (in contravention of RFC4254 section 5.4), this 91 message is only sent to OpenSSH peers (identified by banner). 92 Other SSH implementations may be whitelisted to receive this message 93 upon request. 94 95 2.2. connection: disallow additional sessions extension 96 "no-more-sessions (a] openssh.com" 97 98 Most SSH connections will only ever request a single session, but a 99 attacker may abuse a running ssh client to surreptitiously open 100 additional sessions under their control. OpenSSH provides a global 101 request "no-more-sessions (a] openssh.com" to mitigate this attack. 102 103 When an OpenSSH client expects that it will never open another session 104 (i.e. it has been started with connection multiplexing disabled), it 105 will send the following global request: 106 107 byte SSH_MSG_GLOBAL_REQUEST 108 string "no-more-sessions (a] openssh.com" 109 char want-reply 110 111 On receipt of such a message, an OpenSSH server will refuse to open 112 future channels of type "session" and instead immediately abort the 113 connection. 114 115 Note that this is not a general defence against compromised clients 116 (that is impossible), but it thwarts a simple attack. 117 118 NB. due to certain broken SSH implementations aborting upon receipt 119 of this message, the no-more-sessions request is only sent to OpenSSH 120 servers (identified by banner). Other SSH implementations may be 121 whitelisted to receive this message upon request. 122 123 2.3. connection: Tunnel forward extension "tun (a] openssh.com" 124 125 OpenSSH supports layer 2 and layer 3 tunnelling via the "tun (a] openssh.com" 126 channel type. This channel type supports forwarding of network packets 127 with datagram boundaries intact between endpoints equipped with 128 interfaces like the BSD tun(4) device. Tunnel forwarding channels are 129 requested by the client with the following packet: 130 131 byte SSH_MSG_CHANNEL_OPEN 132 string "tun (a] openssh.com" 133 uint32 sender channel 134 uint32 initial window size 135 uint32 maximum packet size 136 uint32 tunnel mode 137 uint32 remote unit number 138 139 The "tunnel mode" parameter specifies whether the tunnel should forward 140 layer 2 frames or layer 3 packets. It may take one of the following values: 141 142 SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */ 143 SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */ 144 145 The "tunnel unit number" specifies the remote interface number, or may 146 be 0x7fffffff to allow the server to automatically chose an interface. A 147 server that is not willing to open a client-specified unit should refuse 148 the request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful 149 open, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS. 150 151 Once established the client and server may exchange packet or frames 152 over the tunnel channel by encapsulating them in SSH protocol strings 153 and sending them as channel data. This ensures that packet boundaries 154 are kept intact. Specifically, packets are transmitted using normal 155 SSH_MSG_CHANNEL_DATA packets: 156 157 byte SSH_MSG_CHANNEL_DATA 158 uint32 recipient channel 159 string data 160 161 The contents of the "data" field for layer 3 packets is: 162 163 uint32 packet length 164 uint32 address family 165 byte[packet length - 4] packet data 166 167 The "address family" field identifies the type of packet in the message. 168 It may be one of: 169 170 SSH_TUN_AF_INET 2 /* IPv4 */ 171 SSH_TUN_AF_INET6 24 /* IPv6 */ 172 173 The "packet data" field consists of the IPv4/IPv6 datagram itself 174 without any link layer header. 175 176 The contents of the "data" field for layer 2 packets is: 177 178 uint32 packet length 179 byte[packet length] frame 180 181 The "frame" field contains an IEEE 802.3 Ethernet frame, including 182 header. 183 184 3. SFTP protocol changes 185 186 3.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK 187 188 When OpenSSH's sftp-server was implemented, the order of the arguments 189 to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately, 190 the reversal was not noticed until the server was widely deployed. Since 191 fixing this to follow the specification would cause incompatibility, the 192 current order was retained. For correct operation, clients should send 193 SSH_FXP_SYMLINK as follows: 194 195 uint32 id 196 string targetpath 197 string linkpath 198 199 3.2. sftp: Server extension announcement in SSH_FXP_VERSION 200 201 OpenSSH's sftp-server lists the extensions it supports using the 202 standard extension announcement mechanism in the SSH_FXP_VERSION server 203 hello packet: 204 205 uint32 3 /* protocol version */ 206 string ext1-name 207 string ext1-version 208 string ext2-name 209 string ext2-version 210 ... 211 string extN-name 212 string extN-version 213 214 Each extension reports its integer version number as an ASCII encoded 215 string, e.g. "1". The version will be incremented if the extension is 216 ever changed in an incompatible way. The server MAY advertise the same 217 extension with multiple versions (though this is unlikely). Clients MUST 218 check the version number before attempting to use the extension. 219 220 3.3. sftp: Extension request "posix-rename (a] openssh.com" 221 222 This operation provides a rename operation with POSIX semantics, which 223 are different to those provided by the standard SSH_FXP_RENAME in 224 draft-ietf-secsh-filexfer-02.txt. This request is implemented as a 225 SSH_FXP_EXTENDED request with the following format: 226 227 uint32 id 228 string "posix-rename (a] openssh.com" 229 string oldpath 230 string newpath 231 232 On receiving this request the server will perform the POSIX operation 233 rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message. 234 This extension is advertised in the SSH_FXP_VERSION hello with version 235 "1". 236 237 3.4. sftp: Extension requests "statvfs (a] openssh.com" and 238 "fstatvfs (a] openssh.com" 239 240 These requests correspond to the statvfs and fstatvfs POSIX system 241 interfaces. The "statvfs (a] openssh.com" request operates on an explicit 242 pathname, and is formatted as follows: 243 244 uint32 id 245 string "statvfs (a] openssh.com" 246 string path 247 248 The "fstatvfs (a] openssh.com" operates on an open file handle: 249 250 uint32 id 251 string "fstatvfs (a] openssh.com" 252 string handle 253 254 These requests return a SSH_FXP_STATUS reply on failure. On success they 255 return the following SSH_FXP_EXTENDED_REPLY reply: 256 257 uint32 id 258 uint64 f_bsize /* file system block size */ 259 uint64 f_frsize /* fundamental fs block size */ 260 uint64 f_blocks /* number of blocks (unit f_frsize) */ 261 uint64 f_bfree /* free blocks in file system */ 262 uint64 f_bavail /* free blocks for non-root */ 263 uint64 f_files /* total file inodes */ 264 uint64 f_ffree /* free file inodes */ 265 uint64 f_favail /* free file inodes for to non-root */ 266 uint64 f_fsid /* file system id */ 267 uint64 f_flag /* bit mask of f_flag values */ 268 uint64 f_namemax /* maximum filename length */ 269 270 The values of the f_flag bitmask are as follows: 271 272 #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */ 273 #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */ 274 275 Both the "statvfs (a] openssh.com" and "fstatvfs (a] openssh.com" extensions are 276 advertised in the SSH_FXP_VERSION hello with version "2". 277 278 10. sftp: Extension request "hardlink (a] openssh.com" 279 280 This request is for creating a hard link to a regular file. This 281 request is implemented as a SSH_FXP_EXTENDED request with the 282 following format: 283 284 uint32 id 285 string "hardlink (a] openssh.com" 286 string oldpath 287 string newpath 288 289 On receiving this request the server will perform the operation 290 link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message. 291 This extension is advertised in the SSH_FXP_VERSION hello with version 292 "1". 293 294 $OpenBSD: PROTOCOL,v 1.17 2010/12/04 00:18:01 djm Exp $ 295
