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 https://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 https://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 https://www.openssh.com/txt/draft-miller-secsh-compression-delayed-00.txt
36 1.3. transport: New public key algorithms "ssh-rsa-cert-v01@openssh.com",
37 "ssh-dsa-cert-v01@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 host keys. These methods are documented
44 in 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 1.7 transport: chacha20-poly1305@openssh.com authenticated encryption
96 OpenSSH supports authenticated encryption using ChaCha20 and Poly1305
97 as described in PROTOCOL.chacha20poly1305.
99 1.8 transport: curve25519-sha256@libssh.org key exchange algorithm
101 OpenSSH supports the use of ECDH in Curve25519 for key exchange as
103 http://git.libssh.org/users/aris/libssh.git/plain/doc/curve25519-sha256@libssh.org.txt?h=curve25519
105 2. Connection protocol changes
107 2.1. connection: Channel write close extension "eow@openssh.com"
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
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
121 OpenSSH implements a channel extension message to perform this
122 signalling: "eow@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:
126 byte SSH_MSG_CHANNEL_REQUEST
127 uint32 recipient channel
128 string "eow@openssh.com"
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).
135 As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does
136 remain open after a "eow@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.
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 listed to receive this message
146 2.2. connection: disallow additional sessions extension
147 "no-more-sessions@openssh.com"
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@openssh.com" to mitigate this attack.
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:
158 byte SSH_MSG_GLOBAL_REQUEST
159 string "no-more-sessions@openssh.com"
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
166 Note that this is not a general defence against compromised clients
167 (that is impossible), but it thwarts a simple attack.
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 listed to receive this message upon request.
174 2.3. connection: Tunnel forward extension "tun@openssh.com"
176 OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@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:
182 byte SSH_MSG_CHANNEL_OPEN
183 string "tun@openssh.com"
184 uint32 sender channel
185 uint32 initial window size
186 uint32 maximum packet size
188 uint32 remote unit number
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:
193 SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */
194 SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */
196 The "tunnel unit number" specifies the remote interface number, or may
197 be 0x7fffffff to allow the server to automatically choose 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.
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:
208 byte SSH_MSG_CHANNEL_DATA
209 uint32 recipient channel
212 The contents of the "data" field for layer 3 packets is:
215 uint32 address family
216 byte[packet length - 4] packet data
218 The "address family" field identifies the type of packet in the message.
221 SSH_TUN_AF_INET 2 /* IPv4 */
222 SSH_TUN_AF_INET6 24 /* IPv6 */
224 The "packet data" field consists of the IPv4/IPv6 datagram itself
225 without any link layer header.
227 The contents of the "data" field for layer 2 packets is:
230 byte[packet length] frame
232 The "frame" field contains an IEEE 802.3 Ethernet frame, including
235 2.4. connection: Unix domain socket forwarding
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.
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.
244 byte SSH_MSG_CHANNEL_OPEN
245 string "direct-streamlocal@openssh.com"
246 uint32 sender channel
247 uint32 initial window size
248 uint32 maximum packet size
253 Similar to forwarded-tcpip, forwarded-streamlocal is sent by the
254 server when the client has previously send the server a streamlocal-forward
257 byte SSH_MSG_CHANNEL_OPEN
258 string "forwarded-streamlocal@openssh.com"
259 uint32 sender channel
260 uint32 initial window size
261 uint32 maximum packet size
263 string reserved for future use
265 The reserved field is not currently defined and is ignored on the
266 remote end. It is intended to be used in the future to pass
267 information about the socket file, such as ownership and mode.
268 The client currently sends the empty string for this field.
270 Similar to tcpip-forward, streamlocal-forward is sent by the client
271 to request remote forwarding of a Unix domain socket.
273 byte SSH2_MSG_GLOBAL_REQUEST
274 string "streamlocal-forward@openssh.com"
278 Similar to cancel-tcpip-forward, cancel-streamlocal-forward is sent
279 by the client cancel the forwarding of a Unix domain socket.
281 byte SSH2_MSG_GLOBAL_REQUEST
282 string "cancel-streamlocal-forward@openssh.com"
286 2.5. connection: hostkey update and rotation "hostkeys-00@openssh.com"
287 and "hostkeys-prove-00@openssh.com"
289 OpenSSH supports a protocol extension allowing a server to inform
290 a client of all its protocol v.2 host keys after user-authentication
293 byte SSH_MSG_GLOBAL_REQUEST
294 string "hostkeys-00@openssh.com"
295 char 0 /* want-reply */
298 Upon receiving this message, a client should check which of the
299 supplied host keys are present in known_hosts.
301 Note that the server may send key types that the client does not
302 support. The client should disregard such keys if they are received.
304 If the client identifies any keys that are not present for the host,
305 it should send a "hostkeys-prove@openssh.com" message to request the
306 server prove ownership of the private half of the key.
308 byte SSH_MSG_GLOBAL_REQUEST
309 string "hostkeys-prove-00@openssh.com"
310 char 1 /* want-reply */
313 When a server receives this message, it should generate a signature
314 using each requested key over the following:
316 string "hostkeys-prove-00@openssh.com"
317 string session identifier
320 These signatures should be included in the reply, in the order matching
321 the hostkeys in the request:
323 byte SSH_MSG_REQUEST_SUCCESS
326 When the client receives this reply (and not a failure), it should
327 validate the signatures and may update its known_hosts file, adding keys
328 that it has not seen before and deleting keys for the server host that
329 are no longer offered.
331 These extensions let a client learn key types that it had not previously
332 encountered, thereby allowing it to potentially upgrade from weaker
333 key algorithms to better ones. It also supports graceful key rotation:
334 a server may offer multiple keys of the same type for a period (to
335 give clients an opportunity to learn them using this extension) before
336 removing the deprecated key from those offered.
338 2.6. connection: SIGINFO support for "signal" channel request
340 The SSH channels protocol (RFC4254 section 6.9) supports sending a
341 signal to a session attached to a channel. OpenSSH supports one
342 extension signal "INFO@openssh.com" that allows sending SIGINFO on
345 3. Authentication protocol changes
347 3.1. Host-bound public key authentication
349 This is trivial change to the traditional "publickey" authentication
350 method. The authentication request is identical to the original method
351 but for the name and one additional field:
353 byte SSH2_MSG_USERAUTH_REQUEST
355 string "ssh-connection"
356 string "publickey-hostbound-v00@openssh.com"
360 string server host key
362 Because the entire SSH2_MSG_USERAUTH_REQUEST message is included in
363 the signed data, this ensures that a binding between the destination
364 user, the server identity and the session identifier is visible to the
365 signer. OpenSSH uses this binding via signed data to implement per-key
366 restrictions in ssh-agent.
368 A server may advertise this method using the SSH2_MSG_EXT_INFO
369 mechanism (RFC8308), with the following message:
371 string "publickey-hostbound@openssh.com"
374 Clients should prefer host-bound authentication when advertised by
377 4. SFTP protocol changes
379 4.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK
381 When OpenSSH's sftp-server was implemented, the order of the arguments
382 to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
383 the reversal was not noticed until the server was widely deployed. Since
384 fixing this to follow the specification would cause incompatibility, the
385 current order was retained. For correct operation, clients should send
386 SSH_FXP_SYMLINK as follows:
392 4.2. sftp: Server extension announcement in SSH_FXP_VERSION
394 OpenSSH's sftp-server lists the extensions it supports using the
395 standard extension announcement mechanism in the SSH_FXP_VERSION server
398 uint32 3 /* protocol version */
407 Each extension reports its integer version number as an ASCII encoded
408 string, e.g. "1". The version will be incremented if the extension is
409 ever changed in an incompatible way. The server MAY advertise the same
410 extension with multiple versions (though this is unlikely). Clients MUST
411 check the version number before attempting to use the extension.
413 4.3. sftp: Extension request "posix-rename@openssh.com"
415 This operation provides a rename operation with POSIX semantics, which
416 are different to those provided by the standard SSH_FXP_RENAME in
417 draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
418 SSH_FXP_EXTENDED request with the following format:
421 string "posix-rename@openssh.com"
425 On receiving this request the server will perform the POSIX operation
426 rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
427 This extension is advertised in the SSH_FXP_VERSION hello with version
430 4.4. sftp: Extension requests "statvfs@openssh.com" and
431 "fstatvfs@openssh.com"
433 These requests correspond to the statvfs and fstatvfs POSIX system
434 interfaces. The "statvfs@openssh.com" request operates on an explicit
435 pathname, and is formatted as follows:
438 string "statvfs@openssh.com"
441 The "fstatvfs@openssh.com" operates on an open file handle:
444 string "fstatvfs@openssh.com"
447 These requests return a SSH_FXP_STATUS reply on failure. On success they
448 return the following SSH_FXP_EXTENDED_REPLY reply:
451 uint64 f_bsize /* file system block size */
452 uint64 f_frsize /* fundamental fs block size */
453 uint64 f_blocks /* number of blocks (unit f_frsize) */
454 uint64 f_bfree /* free blocks in file system */
455 uint64 f_bavail /* free blocks for non-root */
456 uint64 f_files /* total file inodes */
457 uint64 f_ffree /* free file inodes */
458 uint64 f_favail /* free file inodes for to non-root */
459 uint64 f_fsid /* file system id */
460 uint64 f_flag /* bit mask of f_flag values */
461 uint64 f_namemax /* maximum filename length */
463 The values of the f_flag bitmask are as follows:
465 #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
466 #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
468 Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are
469 advertised in the SSH_FXP_VERSION hello with version "2".
471 4.5. sftp: Extension request "hardlink@openssh.com"
473 This request is for creating a hard link to a regular file. This
474 request is implemented as a SSH_FXP_EXTENDED request with the
478 string "hardlink@openssh.com"
482 On receiving this request the server will perform the operation
483 link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
484 This extension is advertised in the SSH_FXP_VERSION hello with version
487 4.6. sftp: Extension request "fsync@openssh.com"
489 This request asks the server to call fsync(2) on an open file handle.
492 string "fsync@openssh.com"
495 On receiving this request, a server will call fsync(handle_fd) and will
496 respond with a SSH_FXP_STATUS message.
498 This extension is advertised in the SSH_FXP_VERSION hello with version
501 4.7. sftp: Extension request "lsetstat@openssh.com"
503 This request is like the "setstat" command, but sets file attributes on
504 symlinks. It is implemented as a SSH_FXP_EXTENDED request with the
508 string "lsetstat@openssh.com"
512 See the "setstat" command for more details.
514 This extension is advertised in the SSH_FXP_VERSION hello with version
517 4.8. sftp: Extension request "limits@openssh.com"
519 This request is used to determine various limits the server might impose.
520 Clients should not attempt to exceed these limits as the server might sever
521 the connection immediately.
524 string "limits@openssh.com"
526 The server will respond with a SSH_FXP_EXTENDED_REPLY reply:
529 uint64 max-packet-length
530 uint64 max-read-length
531 uint64 max-write-length
532 uint64 max-open-handles
534 The 'max-packet-length' applies to the total number of bytes in a
535 single SFTP packet. Servers SHOULD set this at least to 34000.
537 The 'max-read-length' is the largest length in a SSH_FXP_READ packet.
538 Even if the client requests a larger size, servers will usually respond
539 with a shorter SSH_FXP_DATA packet. Servers SHOULD set this at least to
542 The 'max-write-length' is the largest length in a SSH_FXP_WRITE packet
543 the server will accept. Servers SHOULD set this at least to 32768.
545 The 'max-open-handles' is the maximum number of active handles that the
546 server allows (e.g. handles created by SSH_FXP_OPEN and SSH_FXP_OPENDIR
547 packets). Servers MAY count internal file handles against this limit
548 (e.g. system logging or stdout/stderr), so clients SHOULD NOT expect to
549 open this many handles in practice.
551 If the server doesn't enforce a specific limit, then the field may be
552 set to 0. This implies the server relies on the OS to enforce limits
553 (e.g. available memory or file handles), and such limits might be
554 dynamic. The client SHOULD take care to not try to exceed reasonable
557 This extension is advertised in the SSH_FXP_VERSION hello with version
560 4.9. sftp: Extension request "expand-path@openssh.com"
562 This request supports canonicalisation of relative paths and
563 those that need tilde-expansion, i.e. "~", "~/..." and "~user/..."
564 These paths are expanded using shell-like rules and the resultant
565 path is canonicalised similarly to SSH2_FXP_REALPATH.
567 It is implemented as a SSH_FXP_EXTENDED request with the following
571 string "expand-path@openssh.com"
574 Its reply is the same format as that of SSH2_FXP_REALPATH.
576 This extension is advertised in the SSH_FXP_VERSION hello with version
579 4.10. sftp: Extension request "copy-data"
581 This request asks the server to copy data from one open file handle and
582 write it to a different open file handle. This avoids needing to transfer
583 the data across the network twice (a download followed by an upload).
585 byte SSH_FXP_EXTENDED
588 string read-from-handle
589 uint64 read-from-offset
590 uint64 read-data-length
591 string write-to-handle
592 uint64 write-to-offset
594 The server will copy read-data-length bytes starting from
595 read-from-offset from the read-from-handle and write them to
596 write-to-handle starting from write-to-offset, and then respond with a
597 SSH_FXP_STATUS message.
599 It's equivalent to issuing a series of SSH_FXP_READ requests on
600 read-from-handle and a series of requests of SSH_FXP_WRITE on
603 If read-from-handle and write-to-handle are the same, the server will
604 fail the request and respond with a SSH_FX_INVALID_PARAMETER message.
606 If read-data-length is 0, then the server will read data from the
607 read-from-handle until EOF is reached.
609 This extension is advertised in the SSH_FXP_VERSION hello with version
612 This request is identical to the "copy-data" request documented in:
614 https://tools.ietf.org/html/draft-ietf-secsh-filexfer-extensions-00#section-7
616 5. Miscellaneous changes
618 5.1 Public key format
620 OpenSSH public keys, as generated by ssh-keygen(1) and appearing in
621 authorized_keys files, are formatted as a single line of text consisting
622 of the public key algorithm name followed by a base64-encoded key blob.
623 The public key blob (before base64 encoding) is the same format used for
624 the encoding of public keys sent on the wire: as described in RFC4253
625 section 6.6 for RSA and DSA keys, RFC5656 section 3.1 for ECDSA keys
626 and the "New public key formats" section of PROTOCOL.certkeys for the
627 OpenSSH certificate formats.
629 5.2 Private key format
631 OpenSSH private keys, as generated by ssh-keygen(1) use the format
632 described in PROTOCOL.key by default. As a legacy option, PEM format
633 (RFC7468) private keys are also supported for RSA, DSA and ECDSA keys
634 and were the default format before OpenSSH 7.8.
638 OpenSSH supports a compact format for Key Revocation Lists (KRLs). This
639 format is described in the PROTOCOL.krl file.
641 5.4 Connection multiplexing
643 OpenSSH's connection multiplexing uses messages as described in
644 PROTOCOL.mux over a Unix domain socket for communications between a
645 master instance and later clients.
647 5.5. Agent protocol extensions
649 OpenSSH extends the usual agent protocol. These changes are documented
650 in the PROTOCOL.agent file.
652 $OpenBSD: PROTOCOL,v 1.44 2022/03/31 03:05:49 djm Exp $