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 This is identical to curve25519-sha256 as later published in RFC8731.
107 1.9 transport: ping facility
109 OpenSSH implements a transport level ping message SSH2_MSG_PING
110 and a corresponding SSH2_MSG_PONG reply.
112 #define SSH2_MSG_PING 192
113 #define SSH2_MSG_PONG 193
115 The ping message is simply:
120 The reply copies the data (which may be the empty string) from the
126 Replies are sent in order. They are sent immediately except when rekeying
127 is in progress, in which case they are queued until rekeying completes.
129 The server advertises support for these messages using the
130 SSH2_MSG_EXT_INFO mechanism (RFC8308), with the following message:
132 string "ping@openssh.com"
135 The ping/reply message is implemented at the transport layer rather
136 than as a named global or channel request to allow pings with very
137 short packet lengths, which would not be possible with other
140 1.10 transport: strict key exchange extension
142 OpenSSH supports a number of transport-layer hardening measures under
143 a "strict KEX" feature. This feature is signalled similarly to the
144 RFC8308 ext-info feature: by including a additional algorithm in the
145 initial SSH2_MSG_KEXINIT kex_algorithms field. The client may append
146 "kex-strict-c-v00@openssh.com" to its kex_algorithms and the server
147 may append "kex-strict-s-v00@openssh.com". These pseudo-algorithms
148 are only valid in the initial SSH2_MSG_KEXINIT and MUST be ignored
149 if they are present in subsequent SSH2_MSG_KEXINIT packets.
151 When an endpoint that supports this extension observes this algorithm
152 name in a peer's KEXINIT packet, it MUST make the following changes to
155 a) During initial KEX, terminate the connection if out-of-sequence
156 packet or any message that is not strictly required by KEX is
157 received. This includes terminating the connection if the first
158 packet received is not SSH2_MSG_KEXINIT. Unexpected packets for
159 the purpose of strict KEX include messages that are otherwise
160 valid at any time during the connection such as SSH2_MSG_DEBUG,
161 SSH2_MSG_IGNORE or SSH2_MSG_UNIMPLEMENTED.
162 b) After sending or receiving a SSH2_MSG_NEWKEYS message, reset the
163 packet sequence number to zero. This behaviour persists for the
164 duration of the connection (i.e. not just the first
167 1.11 transport: SSH2_MSG_EXT_INFO during user authentication
169 This protocol extension allows the SSH2_MSG_EXT_INFO to be sent
170 during user authentication. RFC8308 does allow a second
171 SSH2_MSG_EXT_INFO notification, but it may only be sent at the end
172 of user authentication and this is too late to signal per-user
173 server signature algorithms.
175 Support for receiving the SSH2_MSG_EXT_INFO message during user
176 authentication is signalled by the client including a
177 "ext-info-in-auth@openssh.com" key via its initial SSH2_MSG_EXT_INFO
178 set after the SSH2_MSG_NEWKEYS message.
180 A server that supports this extension MAY send a second
181 SSH2_MSG_EXT_INFO message any time after the client's first
182 SSH2_MSG_USERAUTH_REQUEST, regardless of whether it succeed or fails.
183 The client SHOULD be prepared to update the server-sig-algs that
184 it received during an earlier SSH2_MSG_EXT_INFO with the later one.
186 2. Connection protocol changes
188 2.1. connection: Channel write close extension "eow@openssh.com"
190 The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF
191 message to allow an endpoint to signal its peer that it will send no
192 more data over a channel. Unfortunately, there is no symmetric way for
193 an endpoint to request that its peer should cease sending data to it
194 while still keeping the channel open for the endpoint to send data to
197 This is desirable, since it saves the transmission of data that would
198 otherwise need to be discarded and it allows an endpoint to signal local
199 processes of the condition, e.g. by closing the corresponding file
202 OpenSSH implements a channel extension message to perform this
203 signalling: "eow@openssh.com" (End Of Write). This message is sent by
204 an endpoint when the local output of a session channel is closed or
205 experiences a write error. The message is formatted as follows:
207 byte SSH_MSG_CHANNEL_REQUEST
208 uint32 recipient channel
209 string "eow@openssh.com"
212 On receiving this message, the peer SHOULD cease sending data of
213 the channel and MAY signal the process from which the channel data
214 originates (e.g. by closing its read file descriptor).
216 As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does
217 remain open after a "eow@openssh.com" has been sent and more data may
218 still be sent in the other direction. This message does not consume
219 window space and may be sent even if no window space is available.
221 NB. due to certain broken SSH implementations aborting upon receipt
222 of this message (in contravention of RFC4254 section 5.4), this
223 message is only sent to OpenSSH peers (identified by banner).
224 Other SSH implementations may be listed to receive this message
227 2.2. connection: disallow additional sessions extension
228 "no-more-sessions@openssh.com"
230 Most SSH connections will only ever request a single session, but a
231 attacker may abuse a running ssh client to surreptitiously open
232 additional sessions under their control. OpenSSH provides a global
233 request "no-more-sessions@openssh.com" to mitigate this attack.
235 When an OpenSSH client expects that it will never open another session
236 (i.e. it has been started with connection multiplexing disabled), it
237 will send the following global request:
239 byte SSH_MSG_GLOBAL_REQUEST
240 string "no-more-sessions@openssh.com"
243 On receipt of such a message, an OpenSSH server will refuse to open
244 future channels of type "session" and instead immediately abort the
247 Note that this is not a general defence against compromised clients
248 (that is impossible), but it thwarts a simple attack.
250 NB. due to certain broken SSH implementations aborting upon receipt
251 of this message, the no-more-sessions request is only sent to OpenSSH
252 servers (identified by banner). Other SSH implementations may be
253 listed to receive this message upon request.
255 2.3. connection: Tunnel forward extension "tun@openssh.com"
257 OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com"
258 channel type. This channel type supports forwarding of network packets
259 with datagram boundaries intact between endpoints equipped with
260 interfaces like the BSD tun(4) device. Tunnel forwarding channels are
261 requested by the client with the following packet:
263 byte SSH_MSG_CHANNEL_OPEN
264 string "tun@openssh.com"
265 uint32 sender channel
266 uint32 initial window size
267 uint32 maximum packet size
269 uint32 remote unit number
271 The "tunnel mode" parameter specifies whether the tunnel should forward
272 layer 2 frames or layer 3 packets. It may take one of the following values:
274 SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */
275 SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */
277 The "tunnel unit number" specifies the remote interface number, or may
278 be 0x7fffffff to allow the server to automatically choose an interface. A
279 server that is not willing to open a client-specified unit should refuse
280 the request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful
281 open, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS.
283 Once established the client and server may exchange packet or frames
284 over the tunnel channel by encapsulating them in SSH protocol strings
285 and sending them as channel data. This ensures that packet boundaries
286 are kept intact. Specifically, packets are transmitted using normal
287 SSH_MSG_CHANNEL_DATA packets:
289 byte SSH_MSG_CHANNEL_DATA
290 uint32 recipient channel
293 The contents of the "data" field for layer 3 packets is:
296 uint32 address family
297 byte[packet length - 4] packet data
299 The "address family" field identifies the type of packet in the message.
302 SSH_TUN_AF_INET 2 /* IPv4 */
303 SSH_TUN_AF_INET6 24 /* IPv6 */
305 The "packet data" field consists of the IPv4/IPv6 datagram itself
306 without any link layer header.
308 The contents of the "data" field for layer 2 packets is:
311 byte[packet length] frame
313 The "frame" field contains an IEEE 802.3 Ethernet frame, including
316 2.4. connection: Unix domain socket forwarding
318 OpenSSH supports local and remote Unix domain socket forwarding
319 using the "streamlocal" extension. Forwarding is initiated as per
320 TCP sockets but with a single path instead of a host and port.
322 Similar to direct-tcpip, direct-streamlocal is sent by the client
323 to request that the server make a connection to a Unix domain socket.
325 byte SSH_MSG_CHANNEL_OPEN
326 string "direct-streamlocal@openssh.com"
327 uint32 sender channel
328 uint32 initial window size
329 uint32 maximum packet size
334 Similar to forwarded-tcpip, forwarded-streamlocal is sent by the
335 server when the client has previously send the server a streamlocal-forward
338 byte SSH_MSG_CHANNEL_OPEN
339 string "forwarded-streamlocal@openssh.com"
340 uint32 sender channel
341 uint32 initial window size
342 uint32 maximum packet size
344 string reserved for future use
346 The reserved field is not currently defined and is ignored on the
347 remote end. It is intended to be used in the future to pass
348 information about the socket file, such as ownership and mode.
349 The client currently sends the empty string for this field.
351 Similar to tcpip-forward, streamlocal-forward is sent by the client
352 to request remote forwarding of a Unix domain socket.
354 byte SSH2_MSG_GLOBAL_REQUEST
355 string "streamlocal-forward@openssh.com"
359 Similar to cancel-tcpip-forward, cancel-streamlocal-forward is sent
360 by the client cancel the forwarding of a Unix domain socket.
362 byte SSH2_MSG_GLOBAL_REQUEST
363 string "cancel-streamlocal-forward@openssh.com"
367 2.5. connection: hostkey update and rotation "hostkeys-00@openssh.com"
368 and "hostkeys-prove-00@openssh.com"
370 OpenSSH supports a protocol extension allowing a server to inform
371 a client of all its protocol v.2 host keys after user-authentication
374 byte SSH_MSG_GLOBAL_REQUEST
375 string "hostkeys-00@openssh.com"
376 char 0 /* want-reply */
379 Upon receiving this message, a client should check which of the
380 supplied host keys are present in known_hosts.
382 Note that the server may send key types that the client does not
383 support. The client should disregard such keys if they are received.
385 If the client identifies any keys that are not present for the host,
386 it should send a "hostkeys-prove@openssh.com" message to request the
387 server prove ownership of the private half of the key.
389 byte SSH_MSG_GLOBAL_REQUEST
390 string "hostkeys-prove-00@openssh.com"
391 char 1 /* want-reply */
394 When a server receives this message, it should generate a signature
395 using each requested key over the following:
397 string "hostkeys-prove-00@openssh.com"
398 string session identifier
401 These signatures should be included in the reply, in the order matching
402 the hostkeys in the request:
404 byte SSH_MSG_REQUEST_SUCCESS
407 When the client receives this reply (and not a failure), it should
408 validate the signatures and may update its known_hosts file, adding keys
409 that it has not seen before and deleting keys for the server host that
410 are no longer offered.
412 These extensions let a client learn key types that it had not previously
413 encountered, thereby allowing it to potentially upgrade from weaker
414 key algorithms to better ones. It also supports graceful key rotation:
415 a server may offer multiple keys of the same type for a period (to
416 give clients an opportunity to learn them using this extension) before
417 removing the deprecated key from those offered.
419 2.6. connection: SIGINFO support for "signal" channel request
421 The SSH channels protocol (RFC4254 section 6.9) supports sending a
422 signal to a session attached to a channel. OpenSSH supports one
423 extension signal "INFO@openssh.com" that allows sending SIGINFO on
426 3. Authentication protocol changes
428 3.1. Host-bound public key authentication
430 This is trivial change to the traditional "publickey" authentication
431 method. The authentication request is identical to the original method
432 but for the name and one additional field:
434 byte SSH2_MSG_USERAUTH_REQUEST
436 string "ssh-connection"
437 string "publickey-hostbound-v00@openssh.com"
441 string server host key
443 Because the entire SSH2_MSG_USERAUTH_REQUEST message is included in
444 the signed data, this ensures that a binding between the destination
445 user, the server identity and the session identifier is visible to the
446 signer. OpenSSH uses this binding via signed data to implement per-key
447 restrictions in ssh-agent.
449 A server may advertise this method using the SSH2_MSG_EXT_INFO
450 mechanism (RFC8308), with the following message:
452 string "publickey-hostbound@openssh.com"
455 Clients should prefer host-bound authentication when advertised by
458 4. SFTP protocol changes
460 4.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK
462 When OpenSSH's sftp-server was implemented, the order of the arguments
463 to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
464 the reversal was not noticed until the server was widely deployed. Since
465 fixing this to follow the specification would cause incompatibility, the
466 current order was retained. For correct operation, clients should send
467 SSH_FXP_SYMLINK as follows:
473 4.2. sftp: Server extension announcement in SSH_FXP_VERSION
475 OpenSSH's sftp-server lists the extensions it supports using the
476 standard extension announcement mechanism in the SSH_FXP_VERSION server
479 uint32 3 /* protocol version */
488 Each extension reports its integer version number as an ASCII encoded
489 string, e.g. "1". The version will be incremented if the extension is
490 ever changed in an incompatible way. The server MAY advertise the same
491 extension with multiple versions (though this is unlikely). Clients MUST
492 check the version number before attempting to use the extension.
494 4.3. sftp: Extension request "posix-rename@openssh.com"
496 This operation provides a rename operation with POSIX semantics, which
497 are different to those provided by the standard SSH_FXP_RENAME in
498 draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
499 SSH_FXP_EXTENDED request with the following format:
502 string "posix-rename@openssh.com"
506 On receiving this request the server will perform the POSIX operation
507 rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
508 This extension is advertised in the SSH_FXP_VERSION hello with version
511 4.4. sftp: Extension requests "statvfs@openssh.com" and
512 "fstatvfs@openssh.com"
514 These requests correspond to the statvfs and fstatvfs POSIX system
515 interfaces. The "statvfs@openssh.com" request operates on an explicit
516 pathname, and is formatted as follows:
519 string "statvfs@openssh.com"
522 The "fstatvfs@openssh.com" operates on an open file handle:
525 string "fstatvfs@openssh.com"
528 These requests return a SSH_FXP_STATUS reply on failure. On success they
529 return the following SSH_FXP_EXTENDED_REPLY reply:
532 uint64 f_bsize /* file system block size */
533 uint64 f_frsize /* fundamental fs block size */
534 uint64 f_blocks /* number of blocks (unit f_frsize) */
535 uint64 f_bfree /* free blocks in file system */
536 uint64 f_bavail /* free blocks for non-root */
537 uint64 f_files /* total file inodes */
538 uint64 f_ffree /* free file inodes */
539 uint64 f_favail /* free file inodes for to non-root */
540 uint64 f_fsid /* file system id */
541 uint64 f_flag /* bit mask of f_flag values */
542 uint64 f_namemax /* maximum filename length */
544 The values of the f_flag bitmask are as follows:
546 #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
547 #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
549 Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are
550 advertised in the SSH_FXP_VERSION hello with version "2".
552 4.5. sftp: Extension request "hardlink@openssh.com"
554 This request is for creating a hard link to a regular file. This
555 request is implemented as a SSH_FXP_EXTENDED request with the
559 string "hardlink@openssh.com"
563 On receiving this request the server will perform the operation
564 link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
565 This extension is advertised in the SSH_FXP_VERSION hello with version
568 4.6. sftp: Extension request "fsync@openssh.com"
570 This request asks the server to call fsync(2) on an open file handle.
573 string "fsync@openssh.com"
576 On receiving this request, a server will call fsync(handle_fd) and will
577 respond with a SSH_FXP_STATUS message.
579 This extension is advertised in the SSH_FXP_VERSION hello with version
582 4.7. sftp: Extension request "lsetstat@openssh.com"
584 This request is like the "setstat" command, but sets file attributes on
585 symlinks. It is implemented as a SSH_FXP_EXTENDED request with the
589 string "lsetstat@openssh.com"
593 See the "setstat" command for more details.
595 This extension is advertised in the SSH_FXP_VERSION hello with version
598 4.8. sftp: Extension request "limits@openssh.com"
600 This request is used to determine various limits the server might impose.
601 Clients should not attempt to exceed these limits as the server might sever
602 the connection immediately.
605 string "limits@openssh.com"
607 The server will respond with a SSH_FXP_EXTENDED_REPLY reply:
610 uint64 max-packet-length
611 uint64 max-read-length
612 uint64 max-write-length
613 uint64 max-open-handles
615 The 'max-packet-length' applies to the total number of bytes in a
616 single SFTP packet. Servers SHOULD set this at least to 34000.
618 The 'max-read-length' is the largest length in a SSH_FXP_READ packet.
619 Even if the client requests a larger size, servers will usually respond
620 with a shorter SSH_FXP_DATA packet. Servers SHOULD set this at least to
623 The 'max-write-length' is the largest length in a SSH_FXP_WRITE packet
624 the server will accept. Servers SHOULD set this at least to 32768.
626 The 'max-open-handles' is the maximum number of active handles that the
627 server allows (e.g. handles created by SSH_FXP_OPEN and SSH_FXP_OPENDIR
628 packets). Servers MAY count internal file handles against this limit
629 (e.g. system logging or stdout/stderr), so clients SHOULD NOT expect to
630 open this many handles in practice.
632 If the server doesn't enforce a specific limit, then the field may be
633 set to 0. This implies the server relies on the OS to enforce limits
634 (e.g. available memory or file handles), and such limits might be
635 dynamic. The client SHOULD take care to not try to exceed reasonable
638 This extension is advertised in the SSH_FXP_VERSION hello with version
641 4.9. sftp: Extension request "expand-path@openssh.com"
643 This request supports canonicalisation of relative paths and
644 those that need tilde-expansion, i.e. "~", "~/..." and "~user/..."
645 These paths are expanded using shell-like rules and the resultant
646 path is canonicalised similarly to SSH2_FXP_REALPATH.
648 It is implemented as a SSH_FXP_EXTENDED request with the following
652 string "expand-path@openssh.com"
655 Its reply is the same format as that of SSH2_FXP_REALPATH.
657 This extension is advertised in the SSH_FXP_VERSION hello with version
660 4.10. sftp: Extension request "copy-data"
662 This request asks the server to copy data from one open file handle and
663 write it to a different open file handle. This avoids needing to transfer
664 the data across the network twice (a download followed by an upload).
666 byte SSH_FXP_EXTENDED
669 string read-from-handle
670 uint64 read-from-offset
671 uint64 read-data-length
672 string write-to-handle
673 uint64 write-to-offset
675 The server will copy read-data-length bytes starting from
676 read-from-offset from the read-from-handle and write them to
677 write-to-handle starting from write-to-offset, and then respond with a
678 SSH_FXP_STATUS message.
680 It's equivalent to issuing a series of SSH_FXP_READ requests on
681 read-from-handle and a series of requests of SSH_FXP_WRITE on
684 If read-from-handle and write-to-handle are the same, the server will
685 fail the request and respond with a SSH_FX_INVALID_PARAMETER message.
687 If read-data-length is 0, then the server will read data from the
688 read-from-handle until EOF is reached.
690 This extension is advertised in the SSH_FXP_VERSION hello with version
693 This request is identical to the "copy-data" request documented in:
695 https://tools.ietf.org/html/draft-ietf-secsh-filexfer-extensions-00#section-7
697 4.11. sftp: Extension request "home-directory"
699 This request asks the server to expand the specified user's home directory.
700 An empty username implies the current user. This can be used by the client
701 to expand ~/ type paths locally.
703 byte SSH_FXP_EXTENDED
705 string "home-directory"
708 This extension is advertised in the SSH_FXP_VERSION hello with version
711 This provides similar information as the "expand-path@openssh.com" extension.
713 This request is identical to the "home-directory" request documented in:
715 https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-extensions-00#section-5
717 4.12. sftp: Extension request "users-groups-by-id@openssh.com"
719 This request asks the server to return user and/or group names that
720 correspond to one or more IDs (e.g. as returned from a SSH_FXP_STAT
721 request). This may be used by the client to provide usernames in
724 byte SSH_FXP_EXTENDED
726 string "users-groups-by-id@openssh.com"
730 Where "uids" and "gids" consists of one or more integer user or group
736 The server will reply with a SSH_FXP_EXTENDED_REPLY:
738 byte SSH_FXP_EXTENDED_REPLY
743 Where "username" and "groupnames" consists of names in identical request
744 order to "uids" and "gids" respectively:
749 If a name cannot be identified for a given user or group ID, an empty
750 string will be returned in its place.
752 It is acceptable for either "uids" or "gids" to be an empty set, in
753 which case the respective "usernames" or "groupnames" list will also
756 This extension is advertised in the SSH_FXP_VERSION hello with version
759 5. Miscellaneous changes
761 5.1 Public key format
763 OpenSSH public keys, as generated by ssh-keygen(1) and appearing in
764 authorized_keys files, are formatted as a single line of text consisting
765 of the public key algorithm name followed by a base64-encoded key blob.
766 The public key blob (before base64 encoding) is the same format used for
767 the encoding of public keys sent on the wire: as described in RFC4253
768 section 6.6 for RSA and DSA keys, RFC5656 section 3.1 for ECDSA keys
769 and the "New public key formats" section of PROTOCOL.certkeys for the
770 OpenSSH certificate formats.
772 5.2 Private key format
774 OpenSSH private keys, as generated by ssh-keygen(1) use the format
775 described in PROTOCOL.key by default. As a legacy option, PEM format
776 (RFC7468) private keys are also supported for RSA, DSA and ECDSA keys
777 and were the default format before OpenSSH 7.8.
781 OpenSSH supports a compact format for Key Revocation Lists (KRLs). This
782 format is described in the PROTOCOL.krl file.
784 5.4 Connection multiplexing
786 OpenSSH's connection multiplexing uses messages as described in
787 PROTOCOL.mux over a Unix domain socket for communications between a
788 master instance and later clients.
790 5.5. Agent protocol extensions
792 OpenSSH extends the usual agent protocol. These changes are documented
793 in the PROTOCOL.agent file.
795 $OpenBSD: PROTOCOL,v 1.55 2024/01/08 05:05:15 djm Exp $