1 This documents OpenSSH's deviations and extensions to the published SSH
4 Note that OpenSSH's sftp and sftp-server implement revision 3 of the SSH
5 filexfer protocol described in:
7 http://www.openssh.com/txt/draft-ietf-secsh-filexfer-02.txt
9 Newer versions of the draft will not be supported, though some features
10 are individually implemented as extensions described below.
12 The protocol used by OpenSSH's ssh-agent is described in the file
15 1. Transport protocol changes
17 1.1. transport: Protocol 2 MAC algorithm "umac-64@openssh.com"
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
23 http://www.openssh.com/txt/draft-miller-secsh-umac-01.txt
25 1.2. transport: Protocol 2 compression algorithm "zlib@openssh.com"
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.
32 The method is documented in:
34 http://www.openssh.com/txt/draft-miller-secsh-compression-delayed-00.txt
36 1.3. transport: New public key algorithms "ssh-rsa-cert-v00@openssh.com",
37 "ssh-dsa-cert-v00@openssh.com",
38 "ecdsa-sha2-nistp256-cert-v01@openssh.com",
39 "ecdsa-sha2-nistp384-cert-v01@openssh.com" and
40 "ecdsa-sha2-nistp521-cert-v01@openssh.com"
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
46 1.4. transport: Elliptic Curve cryptography
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
54 1.5 transport: Protocol 2 Encrypt-then-MAC MAC algorithms
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
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.
71 As such, the MAC covers:
73 mac = MAC(key, sequence_number || packet_length || encrypted_packet)
75 where "packet_length" is encoded as a uint32 and "encrypted_packet"
79 byte[n1] payload; n1 = packet_length - padding_length - 1
80 byte[n2] random padding; n2 = padding_length
82 1.6 transport: AES-GCM
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:
88 AES-GCM is only negotiated as the cipher algorithms
89 "aes128-gcm@openssh.com" or "aes256-gcm@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
94 2. Connection protocol changes
96 2.1. connection: Channel write close extension "eow@openssh.com"
98 The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF
99 message to allow an endpoint to signal its peer that it will send no
100 more data over a channel. Unfortunately, there is no symmetric way for
101 an endpoint to request that its peer should cease sending data to it
102 while still keeping the channel open for the endpoint to send data to
105 This is desirable, since it saves the transmission of data that would
106 otherwise need to be discarded and it allows an endpoint to signal local
107 processes of the condition, e.g. by closing the corresponding file
110 OpenSSH implements a channel extension message to perform this
111 signalling: "eow@openssh.com" (End Of Write). This message is sent by
112 an endpoint when the local output of a session channel is closed or
113 experiences a write error. The message is formatted as follows:
115 byte SSH_MSG_CHANNEL_REQUEST
116 uint32 recipient channel
117 string "eow@openssh.com"
120 On receiving this message, the peer SHOULD cease sending data of
121 the channel and MAY signal the process from which the channel data
122 originates (e.g. by closing its read file descriptor).
124 As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does
125 remain open after a "eow@openssh.com" has been sent and more data may
126 still be sent in the other direction. This message does not consume
127 window space and may be sent even if no window space is available.
129 NB. due to certain broken SSH implementations aborting upon receipt
130 of this message (in contravention of RFC4254 section 5.4), this
131 message is only sent to OpenSSH peers (identified by banner).
132 Other SSH implementations may be whitelisted to receive this message
135 2.2. connection: disallow additional sessions extension
136 "no-more-sessions@openssh.com"
138 Most SSH connections will only ever request a single session, but a
139 attacker may abuse a running ssh client to surreptitiously open
140 additional sessions under their control. OpenSSH provides a global
141 request "no-more-sessions@openssh.com" to mitigate this attack.
143 When an OpenSSH client expects that it will never open another session
144 (i.e. it has been started with connection multiplexing disabled), it
145 will send the following global request:
147 byte SSH_MSG_GLOBAL_REQUEST
148 string "no-more-sessions@openssh.com"
151 On receipt of such a message, an OpenSSH server will refuse to open
152 future channels of type "session" and instead immediately abort the
155 Note that this is not a general defence against compromised clients
156 (that is impossible), but it thwarts a simple attack.
158 NB. due to certain broken SSH implementations aborting upon receipt
159 of this message, the no-more-sessions request is only sent to OpenSSH
160 servers (identified by banner). Other SSH implementations may be
161 whitelisted to receive this message upon request.
163 2.3. connection: Tunnel forward extension "tun@openssh.com"
165 OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com"
166 channel type. This channel type supports forwarding of network packets
167 with datagram boundaries intact between endpoints equipped with
168 interfaces like the BSD tun(4) device. Tunnel forwarding channels are
169 requested by the client with the following packet:
171 byte SSH_MSG_CHANNEL_OPEN
172 string "tun@openssh.com"
173 uint32 sender channel
174 uint32 initial window size
175 uint32 maximum packet size
177 uint32 remote unit number
179 The "tunnel mode" parameter specifies whether the tunnel should forward
180 layer 2 frames or layer 3 packets. It may take one of the following values:
182 SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */
183 SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */
185 The "tunnel unit number" specifies the remote interface number, or may
186 be 0x7fffffff to allow the server to automatically chose an interface. A
187 server that is not willing to open a client-specified unit should refuse
188 the request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful
189 open, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS.
191 Once established the client and server may exchange packet or frames
192 over the tunnel channel by encapsulating them in SSH protocol strings
193 and sending them as channel data. This ensures that packet boundaries
194 are kept intact. Specifically, packets are transmitted using normal
195 SSH_MSG_CHANNEL_DATA packets:
197 byte SSH_MSG_CHANNEL_DATA
198 uint32 recipient channel
201 The contents of the "data" field for layer 3 packets is:
204 uint32 address family
205 byte[packet length - 4] packet data
207 The "address family" field identifies the type of packet in the message.
210 SSH_TUN_AF_INET 2 /* IPv4 */
211 SSH_TUN_AF_INET6 24 /* IPv6 */
213 The "packet data" field consists of the IPv4/IPv6 datagram itself
214 without any link layer header.
216 The contents of the "data" field for layer 2 packets is:
219 byte[packet length] frame
221 The "frame" field contains an IEEE 802.3 Ethernet frame, including
224 3. SFTP protocol changes
226 3.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK
228 When OpenSSH's sftp-server was implemented, the order of the arguments
229 to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
230 the reversal was not noticed until the server was widely deployed. Since
231 fixing this to follow the specification would cause incompatibility, the
232 current order was retained. For correct operation, clients should send
233 SSH_FXP_SYMLINK as follows:
239 3.2. sftp: Server extension announcement in SSH_FXP_VERSION
241 OpenSSH's sftp-server lists the extensions it supports using the
242 standard extension announcement mechanism in the SSH_FXP_VERSION server
245 uint32 3 /* protocol version */
254 Each extension reports its integer version number as an ASCII encoded
255 string, e.g. "1". The version will be incremented if the extension is
256 ever changed in an incompatible way. The server MAY advertise the same
257 extension with multiple versions (though this is unlikely). Clients MUST
258 check the version number before attempting to use the extension.
260 3.3. sftp: Extension request "posix-rename@openssh.com"
262 This operation provides a rename operation with POSIX semantics, which
263 are different to those provided by the standard SSH_FXP_RENAME in
264 draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
265 SSH_FXP_EXTENDED request with the following format:
268 string "posix-rename@openssh.com"
272 On receiving this request the server will perform the POSIX operation
273 rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
274 This extension is advertised in the SSH_FXP_VERSION hello with version
277 3.4. sftp: Extension requests "statvfs@openssh.com" and
278 "fstatvfs@openssh.com"
280 These requests correspond to the statvfs and fstatvfs POSIX system
281 interfaces. The "statvfs@openssh.com" request operates on an explicit
282 pathname, and is formatted as follows:
285 string "statvfs@openssh.com"
288 The "fstatvfs@openssh.com" operates on an open file handle:
291 string "fstatvfs@openssh.com"
294 These requests return a SSH_FXP_STATUS reply on failure. On success they
295 return the following SSH_FXP_EXTENDED_REPLY reply:
298 uint64 f_bsize /* file system block size */
299 uint64 f_frsize /* fundamental fs block size */
300 uint64 f_blocks /* number of blocks (unit f_frsize) */
301 uint64 f_bfree /* free blocks in file system */
302 uint64 f_bavail /* free blocks for non-root */
303 uint64 f_files /* total file inodes */
304 uint64 f_ffree /* free file inodes */
305 uint64 f_favail /* free file inodes for to non-root */
306 uint64 f_fsid /* file system id */
307 uint64 f_flag /* bit mask of f_flag values */
308 uint64 f_namemax /* maximum filename length */
310 The values of the f_flag bitmask are as follows:
312 #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
313 #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
315 Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are
316 advertised in the SSH_FXP_VERSION hello with version "2".
318 10. sftp: Extension request "hardlink@openssh.com"
320 This request is for creating a hard link to a regular file. This
321 request is implemented as a SSH_FXP_EXTENDED request with the
325 string "hardlink@openssh.com"
329 On receiving this request the server will perform the operation
330 link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
331 This extension is advertised in the SSH_FXP_VERSION hello with version
334 $OpenBSD: PROTOCOL,v 1.20 2013/01/08 18:49:04 markus Exp $