Annotation of src/usr.bin/ssh/PROTOCOL, Revision 1.53
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:
155: a) During initial KEX, terminate the connection if any unexpected or
156: out-of-sequence packet is received. This includes terminating the
157: connection if the first packet received is not SSH2_MSG_KEXINIT.
158: Unexpected packets for the purpose of strict KEX include messages
159: that are otherwise valid at any time during the connection such as
160: SSH2_MSG_DEBUG and SSH2_MSG_IGNORE.
161: b) After sending or receiving a SSH2_MSG_NEWKEYS message, reset the
162: packet sequence number to zero. This behaviour persists for the
163: duration of the connection (i.e. not just the first
164: SSH2_MSG_NEWKEYS).
165:
1.52 djm 166: 1.11 transport: SSH2_MSG_EXT_INFO during user authentication
1.51 djm 167:
168: This protocol extension allows the SSH2_MSG_EXT_INFO to be sent
169: during user authentication. RFC8308 does allow a second
170: SSH2_MSG_EXT_INFO notification, but it may only be sent at the end
171: of user authentication and this is too late to signal per-user
172: server signature algorithms.
173:
174: Support for receiving the SSH2_MSG_EXT_INFO message during user
175: authentication is signalled by the client including a
176: "ext-info-in-auth@openssh.com" key via its initial SSH2_MSG_EXT_INFO
177: set after the SSH2_MSG_NEWKEYS message.
178:
179: A server that supports this extension MAY send a second
180: SSH2_MSG_EXT_INFO message any time after the client's first
181: SSH2_MSG_USERAUTH_REQUEST, regardless of whether it succeed or fails.
182: The client SHOULD be prepared to update the server-sig-algs that
183: it received during an earlier SSH2_MSG_EXT_INFO with the later one.
184:
1.16 djm 185: 2. Connection protocol changes
186:
187: 2.1. connection: Channel write close extension "eow@openssh.com"
1.1 djm 188:
189: The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF
190: message to allow an endpoint to signal its peer that it will send no
191: more data over a channel. Unfortunately, there is no symmetric way for
192: an endpoint to request that its peer should cease sending data to it
193: while still keeping the channel open for the endpoint to send data to
194: the peer.
195:
1.2 djm 196: This is desirable, since it saves the transmission of data that would
1.1 djm 197: otherwise need to be discarded and it allows an endpoint to signal local
198: processes of the condition, e.g. by closing the corresponding file
199: descriptor.
200:
201: OpenSSH implements a channel extension message to perform this
1.10 djm 202: signalling: "eow@openssh.com" (End Of Write). This message is sent by
203: an endpoint when the local output of a session channel is closed or
204: experiences a write error. The message is formatted as follows:
1.1 djm 205:
206: byte SSH_MSG_CHANNEL_REQUEST
207: uint32 recipient channel
208: string "eow@openssh.com"
209: boolean FALSE
210:
211: On receiving this message, the peer SHOULD cease sending data of
212: the channel and MAY signal the process from which the channel data
213: originates (e.g. by closing its read file descriptor).
214:
215: As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does
216: remain open after a "eow@openssh.com" has been sent and more data may
217: still be sent in the other direction. This message does not consume
218: window space and may be sent even if no window space is available.
219:
1.12 djm 220: NB. due to certain broken SSH implementations aborting upon receipt
221: of this message (in contravention of RFC4254 section 5.4), this
222: message is only sent to OpenSSH peers (identified by banner).
1.38 djm 223: Other SSH implementations may be listed to receive this message
1.12 djm 224: upon request.
225:
1.16 djm 226: 2.2. connection: disallow additional sessions extension
227: "no-more-sessions@openssh.com"
1.6 djm 228:
229: Most SSH connections will only ever request a single session, but a
230: attacker may abuse a running ssh client to surreptitiously open
231: additional sessions under their control. OpenSSH provides a global
232: request "no-more-sessions@openssh.com" to mitigate this attack.
233:
234: When an OpenSSH client expects that it will never open another session
235: (i.e. it has been started with connection multiplexing disabled), it
236: will send the following global request:
237:
238: byte SSH_MSG_GLOBAL_REQUEST
239: string "no-more-sessions@openssh.com"
240: char want-reply
241:
242: On receipt of such a message, an OpenSSH server will refuse to open
243: future channels of type "session" and instead immediately abort the
244: connection.
245:
246: Note that this is not a general defence against compromised clients
247: (that is impossible), but it thwarts a simple attack.
248:
1.12 djm 249: NB. due to certain broken SSH implementations aborting upon receipt
250: of this message, the no-more-sessions request is only sent to OpenSSH
251: servers (identified by banner). Other SSH implementations may be
1.38 djm 252: listed to receive this message upon request.
1.12 djm 253:
1.16 djm 254: 2.3. connection: Tunnel forward extension "tun@openssh.com"
1.7 djm 255:
1.8 djm 256: OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com"
1.7 djm 257: channel type. This channel type supports forwarding of network packets
1.28 djm 258: with datagram boundaries intact between endpoints equipped with
1.7 djm 259: interfaces like the BSD tun(4) device. Tunnel forwarding channels are
260: requested by the client with the following packet:
261:
262: byte SSH_MSG_CHANNEL_OPEN
263: string "tun@openssh.com"
264: uint32 sender channel
265: uint32 initial window size
266: uint32 maximum packet size
267: uint32 tunnel mode
268: uint32 remote unit number
269:
270: The "tunnel mode" parameter specifies whether the tunnel should forward
271: layer 2 frames or layer 3 packets. It may take one of the following values:
272:
273: SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */
274: SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */
275:
276: The "tunnel unit number" specifies the remote interface number, or may
1.37 dtucker 277: be 0x7fffffff to allow the server to automatically choose an interface. A
1.13 djm 278: server that is not willing to open a client-specified unit should refuse
279: the request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful
280: open, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS.
1.7 djm 281:
282: Once established the client and server may exchange packet or frames
283: over the tunnel channel by encapsulating them in SSH protocol strings
284: and sending them as channel data. This ensures that packet boundaries
285: are kept intact. Specifically, packets are transmitted using normal
286: SSH_MSG_CHANNEL_DATA packets:
287:
288: byte SSH_MSG_CHANNEL_DATA
289: uint32 recipient channel
290: string data
291:
292: The contents of the "data" field for layer 3 packets is:
293:
294: uint32 packet length
295: uint32 address family
296: byte[packet length - 4] packet data
297:
298: The "address family" field identifies the type of packet in the message.
299: It may be one of:
300:
301: SSH_TUN_AF_INET 2 /* IPv4 */
302: SSH_TUN_AF_INET6 24 /* IPv6 */
303:
304: The "packet data" field consists of the IPv4/IPv6 datagram itself
305: without any link layer header.
306:
1.13 djm 307: The contents of the "data" field for layer 2 packets is:
1.7 djm 308:
309: uint32 packet length
310: byte[packet length] frame
311:
1.8 djm 312: The "frame" field contains an IEEE 802.3 Ethernet frame, including
1.7 djm 313: header.
314:
1.24 millert 315: 2.4. connection: Unix domain socket forwarding
316:
317: OpenSSH supports local and remote Unix domain socket forwarding
318: using the "streamlocal" extension. Forwarding is initiated as per
319: TCP sockets but with a single path instead of a host and port.
320:
321: Similar to direct-tcpip, direct-streamlocal is sent by the client
322: to request that the server make a connection to a Unix domain socket.
323:
324: byte SSH_MSG_CHANNEL_OPEN
325: string "direct-streamlocal@openssh.com"
326: uint32 sender channel
327: uint32 initial window size
328: uint32 maximum packet size
329: string socket path
1.30 djm 330: string reserved
331: uint32 reserved
1.24 millert 332:
333: Similar to forwarded-tcpip, forwarded-streamlocal is sent by the
334: server when the client has previously send the server a streamlocal-forward
335: GLOBAL_REQUEST.
336:
337: byte SSH_MSG_CHANNEL_OPEN
338: string "forwarded-streamlocal@openssh.com"
339: uint32 sender channel
340: uint32 initial window size
341: uint32 maximum packet size
342: string socket path
343: string reserved for future use
344:
345: The reserved field is not currently defined and is ignored on the
346: remote end. It is intended to be used in the future to pass
347: information about the socket file, such as ownership and mode.
348: The client currently sends the empty string for this field.
349:
350: Similar to tcpip-forward, streamlocal-forward is sent by the client
351: to request remote forwarding of a Unix domain socket.
352:
353: byte SSH2_MSG_GLOBAL_REQUEST
354: string "streamlocal-forward@openssh.com"
355: boolean TRUE
356: string socket path
357:
358: Similar to cancel-tcpip-forward, cancel-streamlocal-forward is sent
359: by the client cancel the forwarding of a Unix domain socket.
360:
361: byte SSH2_MSG_GLOBAL_REQUEST
362: string "cancel-streamlocal-forward@openssh.com"
363: boolean FALSE
364: string socket path
365:
1.27 djm 366: 2.5. connection: hostkey update and rotation "hostkeys-00@openssh.com"
367: and "hostkeys-prove-00@openssh.com"
1.25 djm 368:
369: OpenSSH supports a protocol extension allowing a server to inform
1.26 djm 370: a client of all its protocol v.2 host keys after user-authentication
1.25 djm 371: has completed.
372:
373: byte SSH_MSG_GLOBAL_REQUEST
1.27 djm 374: string "hostkeys-00@openssh.com"
1.41 djm 375: char 0 /* want-reply */
1.25 djm 376: string[] hostkeys
377:
1.26 djm 378: Upon receiving this message, a client should check which of the
1.32 djm 379: supplied host keys are present in known_hosts.
380:
381: Note that the server may send key types that the client does not
1.37 dtucker 382: support. The client should disregard such keys if they are received.
1.32 djm 383:
384: If the client identifies any keys that are not present for the host,
385: it should send a "hostkeys-prove@openssh.com" message to request the
386: server prove ownership of the private half of the key.
1.26 djm 387:
388: byte SSH_MSG_GLOBAL_REQUEST
1.27 djm 389: string "hostkeys-prove-00@openssh.com"
1.26 djm 390: char 1 /* want-reply */
391: string[] hostkeys
392:
393: When a server receives this message, it should generate a signature
394: using each requested key over the following:
395:
1.27 djm 396: string "hostkeys-prove-00@openssh.com"
1.26 djm 397: string session identifier
398: string hostkey
399:
400: These signatures should be included in the reply, in the order matching
401: the hostkeys in the request:
402:
403: byte SSH_MSG_REQUEST_SUCCESS
404: string[] signatures
405:
406: When the client receives this reply (and not a failure), it should
407: validate the signatures and may update its known_hosts file, adding keys
408: that it has not seen before and deleting keys for the server host that
409: are no longer offered.
410:
411: These extensions let a client learn key types that it had not previously
412: encountered, thereby allowing it to potentially upgrade from weaker
413: key algorithms to better ones. It also supports graceful key rotation:
414: a server may offer multiple keys of the same type for a period (to
415: give clients an opportunity to learn them using this extension) before
416: removing the deprecated key from those offered.
1.25 djm 417:
1.36 djm 418: 2.6. connection: SIGINFO support for "signal" channel request
419:
420: The SSH channels protocol (RFC4254 section 6.9) supports sending a
421: signal to a session attached to a channel. OpenSSH supports one
422: extension signal "INFO@openssh.com" that allows sending SIGINFO on
423: BSD-derived systems.
424:
1.43 djm 425: 3. Authentication protocol changes
1.16 djm 426:
1.43 djm 427: 3.1. Host-bound public key authentication
428:
429: This is trivial change to the traditional "publickey" authentication
430: method. The authentication request is identical to the original method
431: but for the name and one additional field:
432:
433: byte SSH2_MSG_USERAUTH_REQUEST
434: string username
435: string "ssh-connection"
436: string "publickey-hostbound-v00@openssh.com"
437: bool has_signature
438: string pkalg
439: string public key
440: string server host key
441:
442: Because the entire SSH2_MSG_USERAUTH_REQUEST message is included in
443: the signed data, this ensures that a binding between the destination
444: user, the server identity and the session identifier is visible to the
445: signer. OpenSSH uses this binding via signed data to implement per-key
446: restrictions in ssh-agent.
447:
448: A server may advertise this method using the SSH2_MSG_EXT_INFO
449: mechanism (RFC8308), with the following message:
450:
451: string "publickey-hostbound@openssh.com"
452: string "0" (version)
453:
454: Clients should prefer host-bound authentication when advertised by
455: server.
456:
457: 4. SFTP protocol changes
458:
459: 4.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK
1.1 djm 460:
461: When OpenSSH's sftp-server was implemented, the order of the arguments
1.8 djm 462: to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
1.1 djm 463: the reversal was not noticed until the server was widely deployed. Since
464: fixing this to follow the specification would cause incompatibility, the
465: current order was retained. For correct operation, clients should send
466: SSH_FXP_SYMLINK as follows:
467:
468: uint32 id
469: string targetpath
470: string linkpath
471:
1.43 djm 472: 4.2. sftp: Server extension announcement in SSH_FXP_VERSION
1.1 djm 473:
474: OpenSSH's sftp-server lists the extensions it supports using the
475: standard extension announcement mechanism in the SSH_FXP_VERSION server
476: hello packet:
477:
478: uint32 3 /* protocol version */
479: string ext1-name
480: string ext1-version
481: string ext2-name
482: string ext2-version
483: ...
484: string extN-name
485: string extN-version
486:
487: Each extension reports its integer version number as an ASCII encoded
488: string, e.g. "1". The version will be incremented if the extension is
489: ever changed in an incompatible way. The server MAY advertise the same
490: extension with multiple versions (though this is unlikely). Clients MUST
1.8 djm 491: check the version number before attempting to use the extension.
1.1 djm 492:
1.43 djm 493: 4.3. sftp: Extension request "posix-rename@openssh.com"
1.1 djm 494:
495: This operation provides a rename operation with POSIX semantics, which
496: are different to those provided by the standard SSH_FXP_RENAME in
497: draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
498: SSH_FXP_EXTENDED request with the following format:
499:
500: uint32 id
501: string "posix-rename@openssh.com"
502: string oldpath
503: string newpath
504:
505: On receiving this request the server will perform the POSIX operation
506: rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
507: This extension is advertised in the SSH_FXP_VERSION hello with version
508: "1".
509:
1.43 djm 510: 4.4. sftp: Extension requests "statvfs@openssh.com" and
1.2 djm 511: "fstatvfs@openssh.com"
1.1 djm 512:
513: These requests correspond to the statvfs and fstatvfs POSIX system
514: interfaces. The "statvfs@openssh.com" request operates on an explicit
515: pathname, and is formatted as follows:
516:
517: uint32 id
518: string "statvfs@openssh.com"
519: string path
520:
1.8 djm 521: The "fstatvfs@openssh.com" operates on an open file handle:
1.1 djm 522:
523: uint32 id
1.2 djm 524: string "fstatvfs@openssh.com"
1.1 djm 525: string handle
526:
527: These requests return a SSH_FXP_STATUS reply on failure. On success they
528: return the following SSH_FXP_EXTENDED_REPLY reply:
529:
530: uint32 id
1.4 dtucker 531: uint64 f_bsize /* file system block size */
532: uint64 f_frsize /* fundamental fs block size */
1.1 djm 533: uint64 f_blocks /* number of blocks (unit f_frsize) */
534: uint64 f_bfree /* free blocks in file system */
535: uint64 f_bavail /* free blocks for non-root */
536: uint64 f_files /* total file inodes */
537: uint64 f_ffree /* free file inodes */
538: uint64 f_favail /* free file inodes for to non-root */
1.3 djm 539: uint64 f_fsid /* file system id */
1.4 dtucker 540: uint64 f_flag /* bit mask of f_flag values */
541: uint64 f_namemax /* maximum filename length */
1.1 djm 542:
543: The values of the f_flag bitmask are as follows:
544:
545: #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
546: #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
547:
1.11 djm 548: Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are
549: advertised in the SSH_FXP_VERSION hello with version "2".
1.3 djm 550:
1.43 djm 551: 4.5. sftp: Extension request "hardlink@openssh.com"
1.17 djm 552:
553: This request is for creating a hard link to a regular file. This
554: request is implemented as a SSH_FXP_EXTENDED request with the
555: following format:
556:
557: uint32 id
558: string "hardlink@openssh.com"
559: string oldpath
560: string newpath
561:
562: On receiving this request the server will perform the operation
563: link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
564: This extension is advertised in the SSH_FXP_VERSION hello with version
565: "1".
566:
1.43 djm 567: 4.6. sftp: Extension request "fsync@openssh.com"
1.21 djm 568:
569: This request asks the server to call fsync(2) on an open file handle.
570:
571: uint32 id
572: string "fsync@openssh.com"
573: string handle
574:
1.44 djm 575: On receiving this request, a server will call fsync(handle_fd) and will
1.21 djm 576: respond with a SSH_FXP_STATUS message.
577:
578: This extension is advertised in the SSH_FXP_VERSION hello with version
579: "1".
580:
1.43 djm 581: 4.7. sftp: Extension request "lsetstat@openssh.com"
1.39 djm 582:
583: This request is like the "setstat" command, but sets file attributes on
584: symlinks. It is implemented as a SSH_FXP_EXTENDED request with the
585: following format:
586:
587: uint32 id
588: string "lsetstat@openssh.com"
589: string path
590: ATTRS attrs
591:
592: See the "setstat" command for more details.
593:
594: This extension is advertised in the SSH_FXP_VERSION hello with version
595: "1".
596:
1.43 djm 597: 4.8. sftp: Extension request "limits@openssh.com"
1.40 djm 598:
599: This request is used to determine various limits the server might impose.
600: Clients should not attempt to exceed these limits as the server might sever
601: the connection immediately.
602:
603: uint32 id
604: string "limits@openssh.com"
605:
606: The server will respond with a SSH_FXP_EXTENDED_REPLY reply:
607:
608: uint32 id
609: uint64 max-packet-length
610: uint64 max-read-length
611: uint64 max-write-length
612: uint64 max-open-handles
613:
614: The 'max-packet-length' applies to the total number of bytes in a
615: single SFTP packet. Servers SHOULD set this at least to 34000.
616:
617: The 'max-read-length' is the largest length in a SSH_FXP_READ packet.
618: Even if the client requests a larger size, servers will usually respond
619: with a shorter SSH_FXP_DATA packet. Servers SHOULD set this at least to
620: 32768.
621:
622: The 'max-write-length' is the largest length in a SSH_FXP_WRITE packet
623: the server will accept. Servers SHOULD set this at least to 32768.
624:
625: The 'max-open-handles' is the maximum number of active handles that the
626: server allows (e.g. handles created by SSH_FXP_OPEN and SSH_FXP_OPENDIR
627: packets). Servers MAY count internal file handles against this limit
628: (e.g. system logging or stdout/stderr), so clients SHOULD NOT expect to
629: open this many handles in practice.
630:
631: If the server doesn't enforce a specific limit, then the field may be
632: set to 0. This implies the server relies on the OS to enforce limits
633: (e.g. available memory or file handles), and such limits might be
634: dynamic. The client SHOULD take care to not try to exceed reasonable
635: limits.
636:
637: This extension is advertised in the SSH_FXP_VERSION hello with version
638: "1".
639:
1.43 djm 640: 4.9. sftp: Extension request "expand-path@openssh.com"
1.42 djm 641:
642: This request supports canonicalisation of relative paths and
643: those that need tilde-expansion, i.e. "~", "~/..." and "~user/..."
644: These paths are expanded using shell-like rules and the resultant
645: path is canonicalised similarly to SSH2_FXP_REALPATH.
646:
647: It is implemented as a SSH_FXP_EXTENDED request with the following
648: format:
649:
650: uint32 id
651: string "expand-path@openssh.com"
652: string path
653:
654: Its reply is the same format as that of SSH2_FXP_REALPATH.
655:
656: This extension is advertised in the SSH_FXP_VERSION hello with version
657: "1".
658:
1.44 djm 659: 4.10. sftp: Extension request "copy-data"
660:
661: This request asks the server to copy data from one open file handle and
662: write it to a different open file handle. This avoids needing to transfer
663: the data across the network twice (a download followed by an upload).
664:
665: byte SSH_FXP_EXTENDED
666: uint32 id
667: string "copy-data"
668: string read-from-handle
669: uint64 read-from-offset
670: uint64 read-data-length
671: string write-to-handle
672: uint64 write-to-offset
673:
674: The server will copy read-data-length bytes starting from
675: read-from-offset from the read-from-handle and write them to
676: write-to-handle starting from write-to-offset, and then respond with a
677: SSH_FXP_STATUS message.
678:
679: It's equivalent to issuing a series of SSH_FXP_READ requests on
680: read-from-handle and a series of requests of SSH_FXP_WRITE on
681: write-to-handle.
682:
683: If read-from-handle and write-to-handle are the same, the server will
684: fail the request and respond with a SSH_FX_INVALID_PARAMETER message.
685:
686: If read-data-length is 0, then the server will read data from the
687: read-from-handle until EOF is reached.
688:
689: This extension is advertised in the SSH_FXP_VERSION hello with version
690: "1".
691:
692: This request is identical to the "copy-data" request documented in:
693:
694: https://tools.ietf.org/html/draft-ietf-secsh-filexfer-extensions-00#section-7
695:
1.46 djm 696: 4.11. sftp: Extension request "home-directory"
697:
698: This request asks the server to expand the specified user's home directory.
699: An empty username implies the current user. This can be used by the client
700: to expand ~/ type paths locally.
701:
702: byte SSH_FXP_EXTENDED
703: uint32 id
704: string "home-directory"
705: string username
706:
707: This extension is advertised in the SSH_FXP_VERSION hello with version
708: "1".
709:
710: This provides similar information as the "expand-path@openssh.com" extension.
711:
712: This request is identical to the "home-directory" request documented in:
713:
714: https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-extensions-00#section-5
715:
1.47 djm 716: 4.12. sftp: Extension request "users-groups-by-id@openssh.com"
717:
1.48 dtucker 718: This request asks the server to return user and/or group names that
1.47 djm 719: correspond to one or more IDs (e.g. as returned from a SSH_FXP_STAT
720: request). This may be used by the client to provide usernames in
721: directory listings.
722:
723: byte SSH_FXP_EXTENDED
724: uint32 id
725: string "users-groups-by-id@openssh.com"
726: string uids
727: string gids
728:
729: Where "uids" and "gids" consists of one or more integer user or group
730: identifiers:
731:
732: uint32 id-0
733: ...
734:
735: The server will reply with a SSH_FXP_EXTENDED_REPLY:
736:
737: byte SSH_FXP_EXTENDED_REPLY
738: string usernames
739: string groupnames
740:
741: Where "username" and "groupnames" consists of names in identical request
742: order to "uids" and "gids" respectively:
743:
744: string name-0
745: ...
746:
747: If a name cannot be identified for a given user or group ID, an empty
748: string will be returned in its place.
749:
750: It is acceptable for either "uids" or "gids" to be an empty set, in
751: which case the respective "usernames" or "groupnames" list will also
752: be empty.
753:
754: This extension is advertised in the SSH_FXP_VERSION hello with version
755: "1".
756:
1.43 djm 757: 5. Miscellaneous changes
1.34 djm 758:
1.43 djm 759: 5.1 Public key format
1.34 djm 760:
761: OpenSSH public keys, as generated by ssh-keygen(1) and appearing in
762: authorized_keys files, are formatted as a single line of text consisting
763: of the public key algorithm name followed by a base64-encoded key blob.
1.35 djm 764: The public key blob (before base64 encoding) is the same format used for
765: the encoding of public keys sent on the wire: as described in RFC4253
766: section 6.6 for RSA and DSA keys, RFC5656 section 3.1 for ECDSA keys
767: and the "New public key formats" section of PROTOCOL.certkeys for the
768: OpenSSH certificate formats.
1.34 djm 769:
1.43 djm 770: 5.2 Private key format
1.34 djm 771:
772: OpenSSH private keys, as generated by ssh-keygen(1) use the format
773: described in PROTOCOL.key by default. As a legacy option, PEM format
774: (RFC7468) private keys are also supported for RSA, DSA and ECDSA keys
775: and were the default format before OpenSSH 7.8.
776:
1.43 djm 777: 5.3 KRL format
1.34 djm 778:
779: OpenSSH supports a compact format for Key Revocation Lists (KRLs). This
780: format is described in the PROTOCOL.krl file.
781:
1.43 djm 782: 5.4 Connection multiplexing
1.34 djm 783:
784: OpenSSH's connection multiplexing uses messages as described in
785: PROTOCOL.mux over a Unix domain socket for communications between a
786: master instance and later clients.
787:
1.43 djm 788: 5.5. Agent protocol extensions
789:
790: OpenSSH extends the usual agent protocol. These changes are documented
791: in the PROTOCOL.agent file.
792:
1.53 ! jsg 793: $OpenBSD: PROTOCOL,v 1.52 2023/12/19 06:41:14 djm Exp $
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