Annotation of src/usr.bin/ssh/PROTOCOL, Revision HEAD
1.1 djm 1: This documents OpenSSH's deviations and extensions to the published SSH
2: protocol.
3:
1.2 djm 4: Note that OpenSSH's sftp and sftp-server implement revision 3 of the SSH
5: filexfer protocol described in:
1.1 djm 6:
7: http://www.openssh.com/txt/draft-ietf-secsh-filexfer-02.txt
8:
1.14 djm 9: Newer versions of the draft will not be supported, though some features
10: are individually implemented as extensions described below.
1.1 djm 11:
1.9 djm 12: The protocol used by OpenSSH's ssh-agent is described in the file
13: PROTOCOL.agent
14:
1.16 djm 15: 1. Transport protocol changes
16:
17: 1.1. transport: Protocol 2 MAC algorithm "umac-64@openssh.com"
1.1 djm 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:
1.16 djm 25: 1.2. transport: Protocol 2 compression algorithm "zlib@openssh.com"
1.1 djm 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
1.2 djm 30: avoids exposing compression code to attacks from unauthenticated users.
1.1 djm 31:
32: The method is documented in:
33:
34: http://www.openssh.com/txt/draft-miller-secsh-compression-delayed-00.txt
35:
1.31 djm 36: 1.3. transport: New public key algorithms "ssh-rsa-cert-v01@openssh.com",
37: "ssh-dsa-cert-v01@openssh.com",
1.16 djm 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"
1.15 djm 41:
1.16 djm 42: OpenSSH introduces new public key algorithms to support certificate
1.26 djm 43: authentication for users and host keys. These methods are documented
44: in the file PROTOCOL.certkeys
1.15 djm 45:
1.16 djm 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:
1.18 markus 54: 1.5 transport: Protocol 2 Encrypt-then-MAC MAC algorithms
55:
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
63: session plaintext.
64:
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.
70:
71: As such, the MAC covers:
72:
1.19 djm 73: mac = MAC(key, sequence_number || packet_length || encrypted_packet)
1.18 markus 74:
1.19 djm 75: where "packet_length" is encoded as a uint32 and "encrypted_packet"
76: contains:
1.18 markus 77:
78: byte padding_length
79: byte[n1] payload; n1 = packet_length - padding_length - 1
80: byte[n2] random padding; n2 = padding_length
81:
1.20 markus 82: 1.6 transport: AES-GCM
83:
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:
87:
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
92: a matching MAC.
93:
1.22 djm 94: 1.7 transport: chacha20-poly1305@openssh.com authenticated encryption
95:
96: OpenSSH supports authenticated encryption using ChaCha20 and Poly1305
97: as described in PROTOCOL.chacha20poly1305.
98:
1.23 djm 99: 1.8 transport: curve25519-sha256@libssh.org key exchange algorithm
100:
101: OpenSSH supports the use of ECDH in Curve25519 for key exchange as
102: described at:
103: http://git.libssh.org/users/aris/libssh.git/plain/doc/curve25519-sha256@libssh.org.txt?h=curve25519
104:
1.45 dtucker 105: This is identical to curve25519-sha256 as later published in RFC8731.
106:
1.49 djm 107: 1.9 transport: ping facility
108:
109: OpenSSH implements a transport level ping message SSH2_MSG_PING
110: and a corresponding SSH2_MSG_PONG reply.
111:
112: #define SSH2_MSG_PING 192
113: #define SSH2_MSG_PONG 193
114:
115: The ping message is simply:
116:
117: byte SSH_MSG_PING
118: string data
119:
120: The reply copies the data (which may be the empty string) from the
121: ping:
122:
123: byte SSH_MSG_PONG
124: string data
125:
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.
128:
129: The server advertises support for these messages using the
130: SSH2_MSG_EXT_INFO mechanism (RFC8308), with the following message:
131:
132: string "ping@openssh.com"
133: string "0" (version)
134:
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
138: approaches.
139:
1.52 djm 140: 1.10 transport: strict key exchange extension
1.50 djm 141:
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
1.53 jsg 145: initial SSH2_MSG_KEXINIT kex_algorithms field. The client may append
1.50 djm 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.
150:
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
1.53 jsg 153: the protocol:
1.50 djm 154:
1.54 djm 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.
1.50 djm 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
165: SSH2_MSG_NEWKEYS).
166:
1.52 djm 167: 1.11 transport: SSH2_MSG_EXT_INFO during user authentication
1.51 djm 168:
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.
174:
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.
179:
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.
185:
1.16 djm 186: 2. Connection protocol changes
187:
188: 2.1. connection: Channel write close extension "eow@openssh.com"
1.1 djm 189:
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
195: the peer.
196:
1.2 djm 197: This is desirable, since it saves the transmission of data that would
1.1 djm 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
200: descriptor.
201:
202: OpenSSH implements a channel extension message to perform this
1.10 djm 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:
1.1 djm 206:
207: byte SSH_MSG_CHANNEL_REQUEST
208: uint32 recipient channel
209: string "eow@openssh.com"
210: boolean FALSE
211:
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).
215:
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.
220:
1.12 djm 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).
1.38 djm 224: Other SSH implementations may be listed to receive this message
1.12 djm 225: upon request.
226:
1.16 djm 227: 2.2. connection: disallow additional sessions extension
228: "no-more-sessions@openssh.com"
1.6 djm 229:
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.
234:
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:
238:
239: byte SSH_MSG_GLOBAL_REQUEST
240: string "no-more-sessions@openssh.com"
241: char want-reply
242:
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
245: connection.
246:
247: Note that this is not a general defence against compromised clients
248: (that is impossible), but it thwarts a simple attack.
249:
1.12 djm 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
1.38 djm 253: listed to receive this message upon request.
1.12 djm 254:
1.16 djm 255: 2.3. connection: Tunnel forward extension "tun@openssh.com"
1.7 djm 256:
1.8 djm 257: OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com"
1.7 djm 258: channel type. This channel type supports forwarding of network packets
1.28 djm 259: with datagram boundaries intact between endpoints equipped with
1.7 djm 260: interfaces like the BSD tun(4) device. Tunnel forwarding channels are
261: requested by the client with the following packet:
262:
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
268: uint32 tunnel mode
269: uint32 remote unit number
270:
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:
273:
274: SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */
275: SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */
276:
277: The "tunnel unit number" specifies the remote interface number, or may
1.37 dtucker 278: be 0x7fffffff to allow the server to automatically choose an interface. A
1.13 djm 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.
1.7 djm 282:
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:
288:
289: byte SSH_MSG_CHANNEL_DATA
290: uint32 recipient channel
291: string data
292:
293: The contents of the "data" field for layer 3 packets is:
294:
295: uint32 packet length
296: uint32 address family
297: byte[packet length - 4] packet data
298:
299: The "address family" field identifies the type of packet in the message.
300: It may be one of:
301:
302: SSH_TUN_AF_INET 2 /* IPv4 */
303: SSH_TUN_AF_INET6 24 /* IPv6 */
304:
305: The "packet data" field consists of the IPv4/IPv6 datagram itself
306: without any link layer header.
307:
1.13 djm 308: The contents of the "data" field for layer 2 packets is:
1.7 djm 309:
310: uint32 packet length
311: byte[packet length] frame
312:
1.8 djm 313: The "frame" field contains an IEEE 802.3 Ethernet frame, including
1.7 djm 314: header.
315:
1.24 millert 316: 2.4. connection: Unix domain socket forwarding
317:
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.
321:
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.
324:
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
330: string socket path
1.30 djm 331: string reserved
332: uint32 reserved
1.24 millert 333:
334: Similar to forwarded-tcpip, forwarded-streamlocal is sent by the
335: server when the client has previously send the server a streamlocal-forward
336: GLOBAL_REQUEST.
337:
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
343: string socket path
344: string reserved for future use
345:
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.
350:
351: Similar to tcpip-forward, streamlocal-forward is sent by the client
352: to request remote forwarding of a Unix domain socket.
353:
354: byte SSH2_MSG_GLOBAL_REQUEST
355: string "streamlocal-forward@openssh.com"
356: boolean TRUE
357: string socket path
358:
359: Similar to cancel-tcpip-forward, cancel-streamlocal-forward is sent
360: by the client cancel the forwarding of a Unix domain socket.
361:
362: byte SSH2_MSG_GLOBAL_REQUEST
363: string "cancel-streamlocal-forward@openssh.com"
364: boolean FALSE
365: string socket path
366:
1.27 djm 367: 2.5. connection: hostkey update and rotation "hostkeys-00@openssh.com"
368: and "hostkeys-prove-00@openssh.com"
1.25 djm 369:
370: OpenSSH supports a protocol extension allowing a server to inform
1.26 djm 371: a client of all its protocol v.2 host keys after user-authentication
1.25 djm 372: has completed.
373:
374: byte SSH_MSG_GLOBAL_REQUEST
1.27 djm 375: string "hostkeys-00@openssh.com"
1.41 djm 376: char 0 /* want-reply */
1.25 djm 377: string[] hostkeys
378:
1.26 djm 379: Upon receiving this message, a client should check which of the
1.32 djm 380: supplied host keys are present in known_hosts.
381:
382: Note that the server may send key types that the client does not
1.37 dtucker 383: support. The client should disregard such keys if they are received.
1.32 djm 384:
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.
1.26 djm 388:
389: byte SSH_MSG_GLOBAL_REQUEST
1.27 djm 390: string "hostkeys-prove-00@openssh.com"
1.26 djm 391: char 1 /* want-reply */
392: string[] hostkeys
393:
394: When a server receives this message, it should generate a signature
395: using each requested key over the following:
396:
1.27 djm 397: string "hostkeys-prove-00@openssh.com"
1.26 djm 398: string session identifier
399: string hostkey
400:
401: These signatures should be included in the reply, in the order matching
402: the hostkeys in the request:
403:
404: byte SSH_MSG_REQUEST_SUCCESS
405: string[] signatures
406:
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.
411:
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.
1.25 djm 418:
1.36 djm 419: 2.6. connection: SIGINFO support for "signal" channel request
420:
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
424: BSD-derived systems.
425:
1.43 djm 426: 3. Authentication protocol changes
1.16 djm 427:
1.43 djm 428: 3.1. Host-bound public key authentication
429:
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:
433:
434: byte SSH2_MSG_USERAUTH_REQUEST
435: string username
436: string "ssh-connection"
437: string "publickey-hostbound-v00@openssh.com"
438: bool has_signature
439: string pkalg
440: string public key
441: string server host key
442:
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.
448:
449: A server may advertise this method using the SSH2_MSG_EXT_INFO
450: mechanism (RFC8308), with the following message:
451:
452: string "publickey-hostbound@openssh.com"
453: string "0" (version)
454:
455: Clients should prefer host-bound authentication when advertised by
456: server.
457:
458: 4. SFTP protocol changes
459:
460: 4.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK
1.1 djm 461:
462: When OpenSSH's sftp-server was implemented, the order of the arguments
1.8 djm 463: to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
1.1 djm 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:
468:
469: uint32 id
470: string targetpath
471: string linkpath
472:
1.43 djm 473: 4.2. sftp: Server extension announcement in SSH_FXP_VERSION
1.1 djm 474:
475: OpenSSH's sftp-server lists the extensions it supports using the
476: standard extension announcement mechanism in the SSH_FXP_VERSION server
477: hello packet:
478:
479: uint32 3 /* protocol version */
480: string ext1-name
481: string ext1-version
482: string ext2-name
483: string ext2-version
484: ...
485: string extN-name
486: string extN-version
487:
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
1.8 djm 492: check the version number before attempting to use the extension.
1.1 djm 493:
1.43 djm 494: 4.3. sftp: Extension request "posix-rename@openssh.com"
1.1 djm 495:
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:
500:
501: uint32 id
502: string "posix-rename@openssh.com"
503: string oldpath
504: string newpath
505:
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
509: "1".
510:
1.43 djm 511: 4.4. sftp: Extension requests "statvfs@openssh.com" and
1.2 djm 512: "fstatvfs@openssh.com"
1.1 djm 513:
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:
517:
518: uint32 id
519: string "statvfs@openssh.com"
520: string path
521:
1.8 djm 522: The "fstatvfs@openssh.com" operates on an open file handle:
1.1 djm 523:
524: uint32 id
1.2 djm 525: string "fstatvfs@openssh.com"
1.1 djm 526: string handle
527:
528: These requests return a SSH_FXP_STATUS reply on failure. On success they
529: return the following SSH_FXP_EXTENDED_REPLY reply:
530:
531: uint32 id
1.4 dtucker 532: uint64 f_bsize /* file system block size */
533: uint64 f_frsize /* fundamental fs block size */
1.1 djm 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 */
1.3 djm 540: uint64 f_fsid /* file system id */
1.4 dtucker 541: uint64 f_flag /* bit mask of f_flag values */
542: uint64 f_namemax /* maximum filename length */
1.1 djm 543:
544: The values of the f_flag bitmask are as follows:
545:
546: #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
547: #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
548:
1.11 djm 549: Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are
550: advertised in the SSH_FXP_VERSION hello with version "2".
1.3 djm 551:
1.43 djm 552: 4.5. sftp: Extension request "hardlink@openssh.com"
1.17 djm 553:
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
556: following format:
557:
558: uint32 id
559: string "hardlink@openssh.com"
560: string oldpath
561: string newpath
562:
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
566: "1".
567:
1.43 djm 568: 4.6. sftp: Extension request "fsync@openssh.com"
1.21 djm 569:
570: This request asks the server to call fsync(2) on an open file handle.
571:
572: uint32 id
573: string "fsync@openssh.com"
574: string handle
575:
1.44 djm 576: On receiving this request, a server will call fsync(handle_fd) and will
1.21 djm 577: respond with a SSH_FXP_STATUS message.
578:
579: This extension is advertised in the SSH_FXP_VERSION hello with version
580: "1".
581:
1.43 djm 582: 4.7. sftp: Extension request "lsetstat@openssh.com"
1.39 djm 583:
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
586: following format:
587:
588: uint32 id
589: string "lsetstat@openssh.com"
590: string path
591: ATTRS attrs
592:
593: See the "setstat" command for more details.
594:
595: This extension is advertised in the SSH_FXP_VERSION hello with version
596: "1".
597:
1.43 djm 598: 4.8. sftp: Extension request "limits@openssh.com"
1.40 djm 599:
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.
603:
604: uint32 id
605: string "limits@openssh.com"
606:
607: The server will respond with a SSH_FXP_EXTENDED_REPLY reply:
608:
609: uint32 id
610: uint64 max-packet-length
611: uint64 max-read-length
612: uint64 max-write-length
613: uint64 max-open-handles
614:
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.
617:
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
621: 32768.
622:
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.
625:
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.
631:
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
636: limits.
637:
638: This extension is advertised in the SSH_FXP_VERSION hello with version
639: "1".
640:
1.43 djm 641: 4.9. sftp: Extension request "expand-path@openssh.com"
1.42 djm 642:
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.
647:
648: It is implemented as a SSH_FXP_EXTENDED request with the following
649: format:
650:
651: uint32 id
652: string "expand-path@openssh.com"
653: string path
654:
655: Its reply is the same format as that of SSH2_FXP_REALPATH.
656:
657: This extension is advertised in the SSH_FXP_VERSION hello with version
658: "1".
659:
1.44 djm 660: 4.10. sftp: Extension request "copy-data"
661:
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).
665:
666: byte SSH_FXP_EXTENDED
667: uint32 id
668: string "copy-data"
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
674:
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.
679:
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
682: write-to-handle.
683:
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.
686:
687: If read-data-length is 0, then the server will read data from the
688: read-from-handle until EOF is reached.
689:
690: This extension is advertised in the SSH_FXP_VERSION hello with version
691: "1".
692:
693: This request is identical to the "copy-data" request documented in:
694:
695: https://tools.ietf.org/html/draft-ietf-secsh-filexfer-extensions-00#section-7
696:
1.46 djm 697: 4.11. sftp: Extension request "home-directory"
698:
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.
702:
703: byte SSH_FXP_EXTENDED
704: uint32 id
705: string "home-directory"
706: string username
707:
708: This extension is advertised in the SSH_FXP_VERSION hello with version
709: "1".
710:
711: This provides similar information as the "expand-path@openssh.com" extension.
712:
713: This request is identical to the "home-directory" request documented in:
714:
715: https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-extensions-00#section-5
716:
1.47 djm 717: 4.12. sftp: Extension request "users-groups-by-id@openssh.com"
718:
1.48 dtucker 719: This request asks the server to return user and/or group names that
1.47 djm 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
722: directory listings.
723:
724: byte SSH_FXP_EXTENDED
725: uint32 id
726: string "users-groups-by-id@openssh.com"
727: string uids
728: string gids
729:
730: Where "uids" and "gids" consists of one or more integer user or group
731: identifiers:
732:
733: uint32 id-0
734: ...
735:
736: The server will reply with a SSH_FXP_EXTENDED_REPLY:
737:
738: byte SSH_FXP_EXTENDED_REPLY
1.55 djm 739: uint32 id
1.47 djm 740: string usernames
741: string groupnames
742:
743: Where "username" and "groupnames" consists of names in identical request
744: order to "uids" and "gids" respectively:
745:
746: string name-0
747: ...
748:
749: If a name cannot be identified for a given user or group ID, an empty
750: string will be returned in its place.
751:
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
754: be empty.
755:
756: This extension is advertised in the SSH_FXP_VERSION hello with version
757: "1".
758:
1.43 djm 759: 5. Miscellaneous changes
1.34 djm 760:
1.43 djm 761: 5.1 Public key format
1.34 djm 762:
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.
1.35 djm 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.
1.34 djm 771:
1.43 djm 772: 5.2 Private key format
1.34 djm 773:
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.
778:
1.43 djm 779: 5.3 KRL format
1.34 djm 780:
781: OpenSSH supports a compact format for Key Revocation Lists (KRLs). This
782: format is described in the PROTOCOL.krl file.
783:
1.43 djm 784: 5.4 Connection multiplexing
1.34 djm 785:
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.
789:
1.43 djm 790: 5.5. Agent protocol extensions
791:
792: OpenSSH extends the usual agent protocol. These changes are documented
793: in the PROTOCOL.agent file.
794:
1.55 djm 795: $OpenBSD: PROTOCOL,v 1.54 2024/01/08 04:10:03 djm Exp $
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