1180750SdesThis documents OpenSSH's deviations and extensions to the published SSH 2180750Sdesprotocol. 3180750Sdes 4180750SdesNote that OpenSSH's sftp and sftp-server implement revision 3 of the SSH 5180750Sdesfilexfer protocol described in: 6180750Sdes 7180750Sdeshttp://www.openssh.com/txt/draft-ietf-secsh-filexfer-02.txt 8180750Sdes 9204917SdesNewer versions of the draft will not be supported, though some features 10204917Sdesare individually implemented as extensions described below. 11180750Sdes 12180750SdesThe protocol used by OpenSSH's ssh-agent is described in the file 13180750SdesPROTOCOL.agent 14180750Sdes 15221420Sdes1. Transport protocol changes 16180750Sdes 17221420Sdes1.1. transport: Protocol 2 MAC algorithm "umac-64@openssh.com" 18221420Sdes 19180750SdesThis is a new transport-layer MAC method using the UMAC algorithm 20180750Sdes(rfc4418). This method is identical to the "umac-64" method documented 21180750Sdesin: 22180750Sdes 23180750Sdeshttp://www.openssh.com/txt/draft-miller-secsh-umac-01.txt 24180750Sdes 25221420Sdes1.2. transport: Protocol 2 compression algorithm "zlib@openssh.com" 26180750Sdes 27180750SdesThis transport-layer compression method uses the zlib compression 28180750Sdesalgorithm (identical to the "zlib" method in rfc4253), but delays the 29180750Sdesstart of compression until after authentication has completed. This 30180750Sdesavoids exposing compression code to attacks from unauthenticated users. 31180750Sdes 32180750SdesThe method is documented in: 33180750Sdes 34180750Sdeshttp://www.openssh.com/txt/draft-miller-secsh-compression-delayed-00.txt 35180750Sdes 36221420Sdes1.3. transport: New public key algorithms "ssh-rsa-cert-v00@openssh.com", 37221420Sdes "ssh-dsa-cert-v00@openssh.com", 38221420Sdes "ecdsa-sha2-nistp256-cert-v01@openssh.com", 39221420Sdes "ecdsa-sha2-nistp384-cert-v01@openssh.com" and 40221420Sdes "ecdsa-sha2-nistp521-cert-v01@openssh.com" 41180750Sdes 42221420SdesOpenSSH introduces new public key algorithms to support certificate 43204917Sdesauthentication for users and hostkeys. These methods are documented in 44204917Sdesthe file PROTOCOL.certkeys 45204917Sdes 46221420Sdes1.4. transport: Elliptic Curve cryptography 47204917Sdes 48221420SdesOpenSSH supports ECC key exchange and public key authentication as 49221420Sdesspecified in RFC5656. Only the ecdsa-sha2-nistp256, ecdsa-sha2-nistp384 50221420Sdesand ecdsa-sha2-nistp521 curves over GF(p) are supported. Elliptic 51221420Sdescurve points encoded using point compression are NOT accepted or 52221420Sdesgenerated. 53221420Sdes 54248619Sdes1.5 transport: Protocol 2 Encrypt-then-MAC MAC algorithms 55248619Sdes 56248619SdesOpenSSH supports MAC algorithms, whose names contain "-etm", that 57248619Sdesperform the calculations in a different order to that defined in RFC 58248619Sdes4253. These variants use the so-called "encrypt then MAC" ordering, 59248619Sdescalculating the MAC over the packet ciphertext rather than the 60248619Sdesplaintext. This ordering closes a security flaw in the SSH transport 61248619Sdesprotocol, where decryption of unauthenticated ciphertext provided a 62248619Sdes"decryption oracle" that could, in conjunction with cipher flaws, reveal 63248619Sdessession plaintext. 64248619Sdes 65248619SdesSpecifically, the "-etm" MAC algorithms modify the transport protocol 66248619Sdesto calculate the MAC over the packet ciphertext and to send the packet 67248619Sdeslength unencrypted. This is necessary for the transport to obtain the 68248619Sdeslength of the packet and location of the MAC tag so that it may be 69248619Sdesverified without decrypting unauthenticated data. 70248619Sdes 71248619SdesAs such, the MAC covers: 72248619Sdes 73248619Sdes mac = MAC(key, sequence_number || packet_length || encrypted_packet) 74248619Sdes 75248619Sdeswhere "packet_length" is encoded as a uint32 and "encrypted_packet" 76248619Sdescontains: 77248619Sdes 78248619Sdes byte padding_length 79248619Sdes byte[n1] payload; n1 = packet_length - padding_length - 1 80248619Sdes byte[n2] random padding; n2 = padding_length 81248619Sdes 82248619Sdes1.6 transport: AES-GCM 83248619Sdes 84248619SdesOpenSSH supports the AES-GCM algorithm as specified in RFC 5647. 85248619SdesBecause of problems with the specification of the key exchange 86248619Sdesthe behaviour of OpenSSH differs from the RFC as follows: 87248619Sdes 88248619SdesAES-GCM is only negotiated as the cipher algorithms 89248619Sdes"aes128-gcm@openssh.com" or "aes256-gcm@openssh.com" and never as 90248619Sdesan MAC algorithm. Additionally, if AES-GCM is selected as the cipher 91248619Sdesthe exchanged MAC algorithms are ignored and there doesn't have to be 92248619Sdesa matching MAC. 93248619Sdes 94221420Sdes2. Connection protocol changes 95221420Sdes 96221420Sdes2.1. connection: Channel write close extension "eow@openssh.com" 97221420Sdes 98180750SdesThe SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF 99180750Sdesmessage to allow an endpoint to signal its peer that it will send no 100180750Sdesmore data over a channel. Unfortunately, there is no symmetric way for 101180750Sdesan endpoint to request that its peer should cease sending data to it 102180750Sdeswhile still keeping the channel open for the endpoint to send data to 103180750Sdesthe peer. 104180750Sdes 105180750SdesThis is desirable, since it saves the transmission of data that would 106180750Sdesotherwise need to be discarded and it allows an endpoint to signal local 107180750Sdesprocesses of the condition, e.g. by closing the corresponding file 108180750Sdesdescriptor. 109180750Sdes 110180750SdesOpenSSH implements a channel extension message to perform this 111180750Sdessignalling: "eow@openssh.com" (End Of Write). This message is sent by 112180750Sdesan endpoint when the local output of a session channel is closed or 113180750Sdesexperiences a write error. The message is formatted as follows: 114180750Sdes 115180750Sdes byte SSH_MSG_CHANNEL_REQUEST 116180750Sdes uint32 recipient channel 117180750Sdes string "eow@openssh.com" 118180750Sdes boolean FALSE 119180750Sdes 120180750SdesOn receiving this message, the peer SHOULD cease sending data of 121180750Sdesthe channel and MAY signal the process from which the channel data 122180750Sdesoriginates (e.g. by closing its read file descriptor). 123180750Sdes 124180750SdesAs with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does 125180750Sdesremain open after a "eow@openssh.com" has been sent and more data may 126180750Sdesstill be sent in the other direction. This message does not consume 127180750Sdeswindow space and may be sent even if no window space is available. 128180750Sdes 129192595SdesNB. due to certain broken SSH implementations aborting upon receipt 130192595Sdesof this message (in contravention of RFC4254 section 5.4), this 131192595Sdesmessage is only sent to OpenSSH peers (identified by banner). 132192595SdesOther SSH implementations may be whitelisted to receive this message 133192595Sdesupon request. 134192595Sdes 135221420Sdes2.2. connection: disallow additional sessions extension 136221420Sdes "no-more-sessions@openssh.com" 137180750Sdes 138180750SdesMost SSH connections will only ever request a single session, but a 139180750Sdesattacker may abuse a running ssh client to surreptitiously open 140180750Sdesadditional sessions under their control. OpenSSH provides a global 141180750Sdesrequest "no-more-sessions@openssh.com" to mitigate this attack. 142180750Sdes 143180750SdesWhen an OpenSSH client expects that it will never open another session 144180750Sdes(i.e. it has been started with connection multiplexing disabled), it 145180750Sdeswill send the following global request: 146180750Sdes 147180750Sdes byte SSH_MSG_GLOBAL_REQUEST 148180750Sdes string "no-more-sessions@openssh.com" 149180750Sdes char want-reply 150180750Sdes 151180750SdesOn receipt of such a message, an OpenSSH server will refuse to open 152180750Sdesfuture channels of type "session" and instead immediately abort the 153180750Sdesconnection. 154180750Sdes 155180750SdesNote that this is not a general defence against compromised clients 156180750Sdes(that is impossible), but it thwarts a simple attack. 157180750Sdes 158192595SdesNB. due to certain broken SSH implementations aborting upon receipt 159192595Sdesof this message, the no-more-sessions request is only sent to OpenSSH 160192595Sdesservers (identified by banner). Other SSH implementations may be 161192595Sdeswhitelisted to receive this message upon request. 162192595Sdes 163221420Sdes2.3. connection: Tunnel forward extension "tun@openssh.com" 164180750Sdes 165180750SdesOpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com" 166180750Sdeschannel type. This channel type supports forwarding of network packets 167180750Sdeswith datagram boundaries intact between endpoints equipped with 168180750Sdesinterfaces like the BSD tun(4) device. Tunnel forwarding channels are 169180750Sdesrequested by the client with the following packet: 170180750Sdes 171180750Sdes byte SSH_MSG_CHANNEL_OPEN 172180750Sdes string "tun@openssh.com" 173180750Sdes uint32 sender channel 174180750Sdes uint32 initial window size 175180750Sdes uint32 maximum packet size 176180750Sdes uint32 tunnel mode 177180750Sdes uint32 remote unit number 178180750Sdes 179180750SdesThe "tunnel mode" parameter specifies whether the tunnel should forward 180180750Sdeslayer 2 frames or layer 3 packets. It may take one of the following values: 181180750Sdes 182180750Sdes SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */ 183180750Sdes SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */ 184180750Sdes 185180750SdesThe "tunnel unit number" specifies the remote interface number, or may 186204917Sdesbe 0x7fffffff to allow the server to automatically chose an interface. A 187204917Sdesserver that is not willing to open a client-specified unit should refuse 188204917Sdesthe request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful 189204917Sdesopen, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS. 190180750Sdes 191180750SdesOnce established the client and server may exchange packet or frames 192180750Sdesover the tunnel channel by encapsulating them in SSH protocol strings 193180750Sdesand sending them as channel data. This ensures that packet boundaries 194180750Sdesare kept intact. Specifically, packets are transmitted using normal 195180750SdesSSH_MSG_CHANNEL_DATA packets: 196180750Sdes 197180750Sdes byte SSH_MSG_CHANNEL_DATA 198180750Sdes uint32 recipient channel 199180750Sdes string data 200180750Sdes 201180750SdesThe contents of the "data" field for layer 3 packets is: 202180750Sdes 203180750Sdes uint32 packet length 204180750Sdes uint32 address family 205180750Sdes byte[packet length - 4] packet data 206180750Sdes 207180750SdesThe "address family" field identifies the type of packet in the message. 208180750SdesIt may be one of: 209180750Sdes 210180750Sdes SSH_TUN_AF_INET 2 /* IPv4 */ 211180750Sdes SSH_TUN_AF_INET6 24 /* IPv6 */ 212180750Sdes 213180750SdesThe "packet data" field consists of the IPv4/IPv6 datagram itself 214180750Sdeswithout any link layer header. 215180750Sdes 216204917SdesThe contents of the "data" field for layer 2 packets is: 217180750Sdes 218180750Sdes uint32 packet length 219180750Sdes byte[packet length] frame 220180750Sdes 221180750SdesThe "frame" field contains an IEEE 802.3 Ethernet frame, including 222180750Sdesheader. 223180750Sdes 224221420Sdes3. SFTP protocol changes 225180750Sdes 226221420Sdes3.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK 227221420Sdes 228180750SdesWhen OpenSSH's sftp-server was implemented, the order of the arguments 229180750Sdesto the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately, 230180750Sdesthe reversal was not noticed until the server was widely deployed. Since 231180750Sdesfixing this to follow the specification would cause incompatibility, the 232180750Sdescurrent order was retained. For correct operation, clients should send 233180750SdesSSH_FXP_SYMLINK as follows: 234180750Sdes 235180750Sdes uint32 id 236180750Sdes string targetpath 237180750Sdes string linkpath 238180750Sdes 239221420Sdes3.2. sftp: Server extension announcement in SSH_FXP_VERSION 240180750Sdes 241180750SdesOpenSSH's sftp-server lists the extensions it supports using the 242180750Sdesstandard extension announcement mechanism in the SSH_FXP_VERSION server 243180750Sdeshello packet: 244180750Sdes 245180750Sdes uint32 3 /* protocol version */ 246180750Sdes string ext1-name 247180750Sdes string ext1-version 248180750Sdes string ext2-name 249180750Sdes string ext2-version 250180750Sdes ... 251180750Sdes string extN-name 252180750Sdes string extN-version 253180750Sdes 254180750SdesEach extension reports its integer version number as an ASCII encoded 255180750Sdesstring, e.g. "1". The version will be incremented if the extension is 256180750Sdesever changed in an incompatible way. The server MAY advertise the same 257180750Sdesextension with multiple versions (though this is unlikely). Clients MUST 258180750Sdescheck the version number before attempting to use the extension. 259180750Sdes 260221420Sdes3.3. sftp: Extension request "posix-rename@openssh.com" 261180750Sdes 262180750SdesThis operation provides a rename operation with POSIX semantics, which 263180750Sdesare different to those provided by the standard SSH_FXP_RENAME in 264180750Sdesdraft-ietf-secsh-filexfer-02.txt. This request is implemented as a 265180750SdesSSH_FXP_EXTENDED request with the following format: 266180750Sdes 267180750Sdes uint32 id 268180750Sdes string "posix-rename@openssh.com" 269180750Sdes string oldpath 270180750Sdes string newpath 271180750Sdes 272180750SdesOn receiving this request the server will perform the POSIX operation 273180750Sdesrename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message. 274180750SdesThis extension is advertised in the SSH_FXP_VERSION hello with version 275180750Sdes"1". 276180750Sdes 277221420Sdes3.4. sftp: Extension requests "statvfs@openssh.com" and 278180750Sdes "fstatvfs@openssh.com" 279180750Sdes 280180750SdesThese requests correspond to the statvfs and fstatvfs POSIX system 281180750Sdesinterfaces. The "statvfs@openssh.com" request operates on an explicit 282180750Sdespathname, and is formatted as follows: 283180750Sdes 284180750Sdes uint32 id 285180750Sdes string "statvfs@openssh.com" 286180750Sdes string path 287180750Sdes 288180750SdesThe "fstatvfs@openssh.com" operates on an open file handle: 289180750Sdes 290180750Sdes uint32 id 291180750Sdes string "fstatvfs@openssh.com" 292180750Sdes string handle 293180750Sdes 294180750SdesThese requests return a SSH_FXP_STATUS reply on failure. On success they 295180750Sdesreturn the following SSH_FXP_EXTENDED_REPLY reply: 296180750Sdes 297180750Sdes uint32 id 298180750Sdes uint64 f_bsize /* file system block size */ 299180750Sdes uint64 f_frsize /* fundamental fs block size */ 300180750Sdes uint64 f_blocks /* number of blocks (unit f_frsize) */ 301180750Sdes uint64 f_bfree /* free blocks in file system */ 302180750Sdes uint64 f_bavail /* free blocks for non-root */ 303180750Sdes uint64 f_files /* total file inodes */ 304180750Sdes uint64 f_ffree /* free file inodes */ 305180750Sdes uint64 f_favail /* free file inodes for to non-root */ 306180750Sdes uint64 f_fsid /* file system id */ 307180750Sdes uint64 f_flag /* bit mask of f_flag values */ 308180750Sdes uint64 f_namemax /* maximum filename length */ 309180750Sdes 310180750SdesThe values of the f_flag bitmask are as follows: 311180750Sdes 312180750Sdes #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */ 313180750Sdes #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */ 314180750Sdes 315180750SdesBoth the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are 316180750Sdesadvertised in the SSH_FXP_VERSION hello with version "2". 317180750Sdes 318221420Sdes10. sftp: Extension request "hardlink@openssh.com" 319221420Sdes 320221420SdesThis request is for creating a hard link to a regular file. This 321221420Sdesrequest is implemented as a SSH_FXP_EXTENDED request with the 322221420Sdesfollowing format: 323221420Sdes 324221420Sdes uint32 id 325221420Sdes string "hardlink@openssh.com" 326221420Sdes string oldpath 327221420Sdes string newpath 328221420Sdes 329221420SdesOn receiving this request the server will perform the operation 330221420Sdeslink(oldpath, newpath) and will respond with a SSH_FXP_STATUS message. 331221420SdesThis extension is advertised in the SSH_FXP_VERSION hello with version 332221420Sdes"1". 333221420Sdes 334248619Sdes$OpenBSD: PROTOCOL,v 1.20 2013/01/08 18:49:04 markus Exp $ 335