Annotation of src/usr.bin/ssh/PROTOCOL, Revision 1.48.4.1
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:
1.48.4.1! bluhm 107: 1.9 transport: strict key exchange extension
! 108:
! 109: OpenSSH supports a number of transport-layer hardening measures under
! 110: a "strict KEX" feature. This feature is signalled similarly to the
! 111: RFC8308 ext-info feature: by including a additional algorithm in the
! 112: initiial SSH2_MSG_KEXINIT kex_algorithms field. The client may append
! 113: "kex-strict-c-v00@openssh.com" to its kex_algorithms and the server
! 114: may append "kex-strict-s-v00@openssh.com". These pseudo-algorithms
! 115: are only valid in the initial SSH2_MSG_KEXINIT and MUST be ignored
! 116: if they are present in subsequent SSH2_MSG_KEXINIT packets.
! 117:
! 118: When an endpoint that supports this extension observes this algorithm
! 119: name in a peer's KEXINIT packet, it MUST make the following changes to
! 120: the the protocol:
! 121:
! 122: a) During initial KEX, terminate the connection if any unexpected or
! 123: out-of-sequence packet is received. This includes terminating the
! 124: connection if the first packet received is not SSH2_MSG_KEXINIT.
! 125: Unexpected packets for the purpose of strict KEX include messages
! 126: that are otherwise valid at any time during the connection such as
! 127: SSH2_MSG_DEBUG and SSH2_MSG_IGNORE.
! 128: b) After sending or receiving a SSH2_MSG_NEWKEYS message, reset the
! 129: packet sequence number to zero. This behaviour persists for the
! 130: duration of the connection (i.e. not just the first
! 131: SSH2_MSG_NEWKEYS).
! 132:
1.16 djm 133: 2. Connection protocol changes
134:
135: 2.1. connection: Channel write close extension "eow@openssh.com"
1.1 djm 136:
137: The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF
138: message to allow an endpoint to signal its peer that it will send no
139: more data over a channel. Unfortunately, there is no symmetric way for
140: an endpoint to request that its peer should cease sending data to it
141: while still keeping the channel open for the endpoint to send data to
142: the peer.
143:
1.2 djm 144: This is desirable, since it saves the transmission of data that would
1.1 djm 145: otherwise need to be discarded and it allows an endpoint to signal local
146: processes of the condition, e.g. by closing the corresponding file
147: descriptor.
148:
149: OpenSSH implements a channel extension message to perform this
1.10 djm 150: signalling: "eow@openssh.com" (End Of Write). This message is sent by
151: an endpoint when the local output of a session channel is closed or
152: experiences a write error. The message is formatted as follows:
1.1 djm 153:
154: byte SSH_MSG_CHANNEL_REQUEST
155: uint32 recipient channel
156: string "eow@openssh.com"
157: boolean FALSE
158:
159: On receiving this message, the peer SHOULD cease sending data of
160: the channel and MAY signal the process from which the channel data
161: originates (e.g. by closing its read file descriptor).
162:
163: As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does
164: remain open after a "eow@openssh.com" has been sent and more data may
165: still be sent in the other direction. This message does not consume
166: window space and may be sent even if no window space is available.
167:
1.12 djm 168: NB. due to certain broken SSH implementations aborting upon receipt
169: of this message (in contravention of RFC4254 section 5.4), this
170: message is only sent to OpenSSH peers (identified by banner).
1.38 djm 171: Other SSH implementations may be listed to receive this message
1.12 djm 172: upon request.
173:
1.16 djm 174: 2.2. connection: disallow additional sessions extension
175: "no-more-sessions@openssh.com"
1.6 djm 176:
177: Most SSH connections will only ever request a single session, but a
178: attacker may abuse a running ssh client to surreptitiously open
179: additional sessions under their control. OpenSSH provides a global
180: request "no-more-sessions@openssh.com" to mitigate this attack.
181:
182: When an OpenSSH client expects that it will never open another session
183: (i.e. it has been started with connection multiplexing disabled), it
184: will send the following global request:
185:
186: byte SSH_MSG_GLOBAL_REQUEST
187: string "no-more-sessions@openssh.com"
188: char want-reply
189:
190: On receipt of such a message, an OpenSSH server will refuse to open
191: future channels of type "session" and instead immediately abort the
192: connection.
193:
194: Note that this is not a general defence against compromised clients
195: (that is impossible), but it thwarts a simple attack.
196:
1.12 djm 197: NB. due to certain broken SSH implementations aborting upon receipt
198: of this message, the no-more-sessions request is only sent to OpenSSH
199: servers (identified by banner). Other SSH implementations may be
1.38 djm 200: listed to receive this message upon request.
1.12 djm 201:
1.16 djm 202: 2.3. connection: Tunnel forward extension "tun@openssh.com"
1.7 djm 203:
1.8 djm 204: OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com"
1.7 djm 205: channel type. This channel type supports forwarding of network packets
1.28 djm 206: with datagram boundaries intact between endpoints equipped with
1.7 djm 207: interfaces like the BSD tun(4) device. Tunnel forwarding channels are
208: requested by the client with the following packet:
209:
210: byte SSH_MSG_CHANNEL_OPEN
211: string "tun@openssh.com"
212: uint32 sender channel
213: uint32 initial window size
214: uint32 maximum packet size
215: uint32 tunnel mode
216: uint32 remote unit number
217:
218: The "tunnel mode" parameter specifies whether the tunnel should forward
219: layer 2 frames or layer 3 packets. It may take one of the following values:
220:
221: SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */
222: SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */
223:
224: The "tunnel unit number" specifies the remote interface number, or may
1.37 dtucker 225: be 0x7fffffff to allow the server to automatically choose an interface. A
1.13 djm 226: server that is not willing to open a client-specified unit should refuse
227: the request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful
228: open, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS.
1.7 djm 229:
230: Once established the client and server may exchange packet or frames
231: over the tunnel channel by encapsulating them in SSH protocol strings
232: and sending them as channel data. This ensures that packet boundaries
233: are kept intact. Specifically, packets are transmitted using normal
234: SSH_MSG_CHANNEL_DATA packets:
235:
236: byte SSH_MSG_CHANNEL_DATA
237: uint32 recipient channel
238: string data
239:
240: The contents of the "data" field for layer 3 packets is:
241:
242: uint32 packet length
243: uint32 address family
244: byte[packet length - 4] packet data
245:
246: The "address family" field identifies the type of packet in the message.
247: It may be one of:
248:
249: SSH_TUN_AF_INET 2 /* IPv4 */
250: SSH_TUN_AF_INET6 24 /* IPv6 */
251:
252: The "packet data" field consists of the IPv4/IPv6 datagram itself
253: without any link layer header.
254:
1.13 djm 255: The contents of the "data" field for layer 2 packets is:
1.7 djm 256:
257: uint32 packet length
258: byte[packet length] frame
259:
1.8 djm 260: The "frame" field contains an IEEE 802.3 Ethernet frame, including
1.7 djm 261: header.
262:
1.24 millert 263: 2.4. connection: Unix domain socket forwarding
264:
265: OpenSSH supports local and remote Unix domain socket forwarding
266: using the "streamlocal" extension. Forwarding is initiated as per
267: TCP sockets but with a single path instead of a host and port.
268:
269: Similar to direct-tcpip, direct-streamlocal is sent by the client
270: to request that the server make a connection to a Unix domain socket.
271:
272: byte SSH_MSG_CHANNEL_OPEN
273: string "direct-streamlocal@openssh.com"
274: uint32 sender channel
275: uint32 initial window size
276: uint32 maximum packet size
277: string socket path
1.30 djm 278: string reserved
279: uint32 reserved
1.24 millert 280:
281: Similar to forwarded-tcpip, forwarded-streamlocal is sent by the
282: server when the client has previously send the server a streamlocal-forward
283: GLOBAL_REQUEST.
284:
285: byte SSH_MSG_CHANNEL_OPEN
286: string "forwarded-streamlocal@openssh.com"
287: uint32 sender channel
288: uint32 initial window size
289: uint32 maximum packet size
290: string socket path
291: string reserved for future use
292:
293: The reserved field is not currently defined and is ignored on the
294: remote end. It is intended to be used in the future to pass
295: information about the socket file, such as ownership and mode.
296: The client currently sends the empty string for this field.
297:
298: Similar to tcpip-forward, streamlocal-forward is sent by the client
299: to request remote forwarding of a Unix domain socket.
300:
301: byte SSH2_MSG_GLOBAL_REQUEST
302: string "streamlocal-forward@openssh.com"
303: boolean TRUE
304: string socket path
305:
306: Similar to cancel-tcpip-forward, cancel-streamlocal-forward is sent
307: by the client cancel the forwarding of a Unix domain socket.
308:
309: byte SSH2_MSG_GLOBAL_REQUEST
310: string "cancel-streamlocal-forward@openssh.com"
311: boolean FALSE
312: string socket path
313:
1.27 djm 314: 2.5. connection: hostkey update and rotation "hostkeys-00@openssh.com"
315: and "hostkeys-prove-00@openssh.com"
1.25 djm 316:
317: OpenSSH supports a protocol extension allowing a server to inform
1.26 djm 318: a client of all its protocol v.2 host keys after user-authentication
1.25 djm 319: has completed.
320:
321: byte SSH_MSG_GLOBAL_REQUEST
1.27 djm 322: string "hostkeys-00@openssh.com"
1.41 djm 323: char 0 /* want-reply */
1.25 djm 324: string[] hostkeys
325:
1.26 djm 326: Upon receiving this message, a client should check which of the
1.32 djm 327: supplied host keys are present in known_hosts.
328:
329: Note that the server may send key types that the client does not
1.37 dtucker 330: support. The client should disregard such keys if they are received.
1.32 djm 331:
332: If the client identifies any keys that are not present for the host,
333: it should send a "hostkeys-prove@openssh.com" message to request the
334: server prove ownership of the private half of the key.
1.26 djm 335:
336: byte SSH_MSG_GLOBAL_REQUEST
1.27 djm 337: string "hostkeys-prove-00@openssh.com"
1.26 djm 338: char 1 /* want-reply */
339: string[] hostkeys
340:
341: When a server receives this message, it should generate a signature
342: using each requested key over the following:
343:
1.27 djm 344: string "hostkeys-prove-00@openssh.com"
1.26 djm 345: string session identifier
346: string hostkey
347:
348: These signatures should be included in the reply, in the order matching
349: the hostkeys in the request:
350:
351: byte SSH_MSG_REQUEST_SUCCESS
352: string[] signatures
353:
354: When the client receives this reply (and not a failure), it should
355: validate the signatures and may update its known_hosts file, adding keys
356: that it has not seen before and deleting keys for the server host that
357: are no longer offered.
358:
359: These extensions let a client learn key types that it had not previously
360: encountered, thereby allowing it to potentially upgrade from weaker
361: key algorithms to better ones. It also supports graceful key rotation:
362: a server may offer multiple keys of the same type for a period (to
363: give clients an opportunity to learn them using this extension) before
364: removing the deprecated key from those offered.
1.25 djm 365:
1.36 djm 366: 2.6. connection: SIGINFO support for "signal" channel request
367:
368: The SSH channels protocol (RFC4254 section 6.9) supports sending a
369: signal to a session attached to a channel. OpenSSH supports one
370: extension signal "INFO@openssh.com" that allows sending SIGINFO on
371: BSD-derived systems.
372:
1.43 djm 373: 3. Authentication protocol changes
1.16 djm 374:
1.43 djm 375: 3.1. Host-bound public key authentication
376:
377: This is trivial change to the traditional "publickey" authentication
378: method. The authentication request is identical to the original method
379: but for the name and one additional field:
380:
381: byte SSH2_MSG_USERAUTH_REQUEST
382: string username
383: string "ssh-connection"
384: string "publickey-hostbound-v00@openssh.com"
385: bool has_signature
386: string pkalg
387: string public key
388: string server host key
389:
390: Because the entire SSH2_MSG_USERAUTH_REQUEST message is included in
391: the signed data, this ensures that a binding between the destination
392: user, the server identity and the session identifier is visible to the
393: signer. OpenSSH uses this binding via signed data to implement per-key
394: restrictions in ssh-agent.
395:
396: A server may advertise this method using the SSH2_MSG_EXT_INFO
397: mechanism (RFC8308), with the following message:
398:
399: string "publickey-hostbound@openssh.com"
400: string "0" (version)
401:
402: Clients should prefer host-bound authentication when advertised by
403: server.
404:
405: 4. SFTP protocol changes
406:
407: 4.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK
1.1 djm 408:
409: When OpenSSH's sftp-server was implemented, the order of the arguments
1.8 djm 410: to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
1.1 djm 411: the reversal was not noticed until the server was widely deployed. Since
412: fixing this to follow the specification would cause incompatibility, the
413: current order was retained. For correct operation, clients should send
414: SSH_FXP_SYMLINK as follows:
415:
416: uint32 id
417: string targetpath
418: string linkpath
419:
1.43 djm 420: 4.2. sftp: Server extension announcement in SSH_FXP_VERSION
1.1 djm 421:
422: OpenSSH's sftp-server lists the extensions it supports using the
423: standard extension announcement mechanism in the SSH_FXP_VERSION server
424: hello packet:
425:
426: uint32 3 /* protocol version */
427: string ext1-name
428: string ext1-version
429: string ext2-name
430: string ext2-version
431: ...
432: string extN-name
433: string extN-version
434:
435: Each extension reports its integer version number as an ASCII encoded
436: string, e.g. "1". The version will be incremented if the extension is
437: ever changed in an incompatible way. The server MAY advertise the same
438: extension with multiple versions (though this is unlikely). Clients MUST
1.8 djm 439: check the version number before attempting to use the extension.
1.1 djm 440:
1.43 djm 441: 4.3. sftp: Extension request "posix-rename@openssh.com"
1.1 djm 442:
443: This operation provides a rename operation with POSIX semantics, which
444: are different to those provided by the standard SSH_FXP_RENAME in
445: draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
446: SSH_FXP_EXTENDED request with the following format:
447:
448: uint32 id
449: string "posix-rename@openssh.com"
450: string oldpath
451: string newpath
452:
453: On receiving this request the server will perform the POSIX operation
454: rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
455: This extension is advertised in the SSH_FXP_VERSION hello with version
456: "1".
457:
1.43 djm 458: 4.4. sftp: Extension requests "statvfs@openssh.com" and
1.2 djm 459: "fstatvfs@openssh.com"
1.1 djm 460:
461: These requests correspond to the statvfs and fstatvfs POSIX system
462: interfaces. The "statvfs@openssh.com" request operates on an explicit
463: pathname, and is formatted as follows:
464:
465: uint32 id
466: string "statvfs@openssh.com"
467: string path
468:
1.8 djm 469: The "fstatvfs@openssh.com" operates on an open file handle:
1.1 djm 470:
471: uint32 id
1.2 djm 472: string "fstatvfs@openssh.com"
1.1 djm 473: string handle
474:
475: These requests return a SSH_FXP_STATUS reply on failure. On success they
476: return the following SSH_FXP_EXTENDED_REPLY reply:
477:
478: uint32 id
1.4 dtucker 479: uint64 f_bsize /* file system block size */
480: uint64 f_frsize /* fundamental fs block size */
1.1 djm 481: uint64 f_blocks /* number of blocks (unit f_frsize) */
482: uint64 f_bfree /* free blocks in file system */
483: uint64 f_bavail /* free blocks for non-root */
484: uint64 f_files /* total file inodes */
485: uint64 f_ffree /* free file inodes */
486: uint64 f_favail /* free file inodes for to non-root */
1.3 djm 487: uint64 f_fsid /* file system id */
1.4 dtucker 488: uint64 f_flag /* bit mask of f_flag values */
489: uint64 f_namemax /* maximum filename length */
1.1 djm 490:
491: The values of the f_flag bitmask are as follows:
492:
493: #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
494: #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
495:
1.11 djm 496: Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are
497: advertised in the SSH_FXP_VERSION hello with version "2".
1.3 djm 498:
1.43 djm 499: 4.5. sftp: Extension request "hardlink@openssh.com"
1.17 djm 500:
501: This request is for creating a hard link to a regular file. This
502: request is implemented as a SSH_FXP_EXTENDED request with the
503: following format:
504:
505: uint32 id
506: string "hardlink@openssh.com"
507: string oldpath
508: string newpath
509:
510: On receiving this request the server will perform the operation
511: link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
512: This extension is advertised in the SSH_FXP_VERSION hello with version
513: "1".
514:
1.43 djm 515: 4.6. sftp: Extension request "fsync@openssh.com"
1.21 djm 516:
517: This request asks the server to call fsync(2) on an open file handle.
518:
519: uint32 id
520: string "fsync@openssh.com"
521: string handle
522:
1.44 djm 523: On receiving this request, a server will call fsync(handle_fd) and will
1.21 djm 524: respond with a SSH_FXP_STATUS message.
525:
526: This extension is advertised in the SSH_FXP_VERSION hello with version
527: "1".
528:
1.43 djm 529: 4.7. sftp: Extension request "lsetstat@openssh.com"
1.39 djm 530:
531: This request is like the "setstat" command, but sets file attributes on
532: symlinks. It is implemented as a SSH_FXP_EXTENDED request with the
533: following format:
534:
535: uint32 id
536: string "lsetstat@openssh.com"
537: string path
538: ATTRS attrs
539:
540: See the "setstat" command for more details.
541:
542: This extension is advertised in the SSH_FXP_VERSION hello with version
543: "1".
544:
1.43 djm 545: 4.8. sftp: Extension request "limits@openssh.com"
1.40 djm 546:
547: This request is used to determine various limits the server might impose.
548: Clients should not attempt to exceed these limits as the server might sever
549: the connection immediately.
550:
551: uint32 id
552: string "limits@openssh.com"
553:
554: The server will respond with a SSH_FXP_EXTENDED_REPLY reply:
555:
556: uint32 id
557: uint64 max-packet-length
558: uint64 max-read-length
559: uint64 max-write-length
560: uint64 max-open-handles
561:
562: The 'max-packet-length' applies to the total number of bytes in a
563: single SFTP packet. Servers SHOULD set this at least to 34000.
564:
565: The 'max-read-length' is the largest length in a SSH_FXP_READ packet.
566: Even if the client requests a larger size, servers will usually respond
567: with a shorter SSH_FXP_DATA packet. Servers SHOULD set this at least to
568: 32768.
569:
570: The 'max-write-length' is the largest length in a SSH_FXP_WRITE packet
571: the server will accept. Servers SHOULD set this at least to 32768.
572:
573: The 'max-open-handles' is the maximum number of active handles that the
574: server allows (e.g. handles created by SSH_FXP_OPEN and SSH_FXP_OPENDIR
575: packets). Servers MAY count internal file handles against this limit
576: (e.g. system logging or stdout/stderr), so clients SHOULD NOT expect to
577: open this many handles in practice.
578:
579: If the server doesn't enforce a specific limit, then the field may be
580: set to 0. This implies the server relies on the OS to enforce limits
581: (e.g. available memory or file handles), and such limits might be
582: dynamic. The client SHOULD take care to not try to exceed reasonable
583: limits.
584:
585: This extension is advertised in the SSH_FXP_VERSION hello with version
586: "1".
587:
1.43 djm 588: 4.9. sftp: Extension request "expand-path@openssh.com"
1.42 djm 589:
590: This request supports canonicalisation of relative paths and
591: those that need tilde-expansion, i.e. "~", "~/..." and "~user/..."
592: These paths are expanded using shell-like rules and the resultant
593: path is canonicalised similarly to SSH2_FXP_REALPATH.
594:
595: It is implemented as a SSH_FXP_EXTENDED request with the following
596: format:
597:
598: uint32 id
599: string "expand-path@openssh.com"
600: string path
601:
602: Its reply is the same format as that of SSH2_FXP_REALPATH.
603:
604: This extension is advertised in the SSH_FXP_VERSION hello with version
605: "1".
606:
1.44 djm 607: 4.10. sftp: Extension request "copy-data"
608:
609: This request asks the server to copy data from one open file handle and
610: write it to a different open file handle. This avoids needing to transfer
611: the data across the network twice (a download followed by an upload).
612:
613: byte SSH_FXP_EXTENDED
614: uint32 id
615: string "copy-data"
616: string read-from-handle
617: uint64 read-from-offset
618: uint64 read-data-length
619: string write-to-handle
620: uint64 write-to-offset
621:
622: The server will copy read-data-length bytes starting from
623: read-from-offset from the read-from-handle and write them to
624: write-to-handle starting from write-to-offset, and then respond with a
625: SSH_FXP_STATUS message.
626:
627: It's equivalent to issuing a series of SSH_FXP_READ requests on
628: read-from-handle and a series of requests of SSH_FXP_WRITE on
629: write-to-handle.
630:
631: If read-from-handle and write-to-handle are the same, the server will
632: fail the request and respond with a SSH_FX_INVALID_PARAMETER message.
633:
634: If read-data-length is 0, then the server will read data from the
635: read-from-handle until EOF is reached.
636:
637: This extension is advertised in the SSH_FXP_VERSION hello with version
638: "1".
639:
640: This request is identical to the "copy-data" request documented in:
641:
642: https://tools.ietf.org/html/draft-ietf-secsh-filexfer-extensions-00#section-7
643:
1.46 djm 644: 4.11. sftp: Extension request "home-directory"
645:
646: This request asks the server to expand the specified user's home directory.
647: An empty username implies the current user. This can be used by the client
648: to expand ~/ type paths locally.
649:
650: byte SSH_FXP_EXTENDED
651: uint32 id
652: string "home-directory"
653: string username
654:
655: This extension is advertised in the SSH_FXP_VERSION hello with version
656: "1".
657:
658: This provides similar information as the "expand-path@openssh.com" extension.
659:
660: This request is identical to the "home-directory" request documented in:
661:
662: https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-extensions-00#section-5
663:
1.47 djm 664: 4.12. sftp: Extension request "users-groups-by-id@openssh.com"
665:
1.48 dtucker 666: This request asks the server to return user and/or group names that
1.47 djm 667: correspond to one or more IDs (e.g. as returned from a SSH_FXP_STAT
668: request). This may be used by the client to provide usernames in
669: directory listings.
670:
671: byte SSH_FXP_EXTENDED
672: uint32 id
673: string "users-groups-by-id@openssh.com"
674: string uids
675: string gids
676:
677: Where "uids" and "gids" consists of one or more integer user or group
678: identifiers:
679:
680: uint32 id-0
681: ...
682:
683: The server will reply with a SSH_FXP_EXTENDED_REPLY:
684:
685: byte SSH_FXP_EXTENDED_REPLY
686: string usernames
687: string groupnames
688:
689: Where "username" and "groupnames" consists of names in identical request
690: order to "uids" and "gids" respectively:
691:
692: string name-0
693: ...
694:
695: If a name cannot be identified for a given user or group ID, an empty
696: string will be returned in its place.
697:
698: It is acceptable for either "uids" or "gids" to be an empty set, in
699: which case the respective "usernames" or "groupnames" list will also
700: be empty.
701:
702: This extension is advertised in the SSH_FXP_VERSION hello with version
703: "1".
704:
1.43 djm 705: 5. Miscellaneous changes
1.34 djm 706:
1.43 djm 707: 5.1 Public key format
1.34 djm 708:
709: OpenSSH public keys, as generated by ssh-keygen(1) and appearing in
710: authorized_keys files, are formatted as a single line of text consisting
711: of the public key algorithm name followed by a base64-encoded key blob.
1.35 djm 712: The public key blob (before base64 encoding) is the same format used for
713: the encoding of public keys sent on the wire: as described in RFC4253
714: section 6.6 for RSA and DSA keys, RFC5656 section 3.1 for ECDSA keys
715: and the "New public key formats" section of PROTOCOL.certkeys for the
716: OpenSSH certificate formats.
1.34 djm 717:
1.43 djm 718: 5.2 Private key format
1.34 djm 719:
720: OpenSSH private keys, as generated by ssh-keygen(1) use the format
721: described in PROTOCOL.key by default. As a legacy option, PEM format
722: (RFC7468) private keys are also supported for RSA, DSA and ECDSA keys
723: and were the default format before OpenSSH 7.8.
724:
1.43 djm 725: 5.3 KRL format
1.34 djm 726:
727: OpenSSH supports a compact format for Key Revocation Lists (KRLs). This
728: format is described in the PROTOCOL.krl file.
729:
1.43 djm 730: 5.4 Connection multiplexing
1.34 djm 731:
732: OpenSSH's connection multiplexing uses messages as described in
733: PROTOCOL.mux over a Unix domain socket for communications between a
734: master instance and later clients.
735:
1.43 djm 736: 5.5. Agent protocol extensions
737:
738: OpenSSH extends the usual agent protocol. These changes are documented
739: in the PROTOCOL.agent file.
740:
1.48.4.1! bluhm 741: $OpenBSD: PROTOCOL,v 1.48 2022/11/07 01:53:01 dtucker Exp $
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