Annotation of src/usr.bin/ssh/PROTOCOL, Revision 1.49.2.1
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:
1.49 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:
1.49.2.1! bluhm 140: 1.9 transport: strict key exchange extension
! 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
! 145: initiial SSH2_MSG_KEXINIT kex_algorithms field. The client may append
! 146: "kex-strict-c-v00@openssh.com" to its kex_algorithms and the server
! 147: may append "kex-strict-s-v00@openssh.com". These pseudo-algorithms
! 148: are only valid in the initial SSH2_MSG_KEXINIT and MUST be ignored
! 149: if they are present in subsequent SSH2_MSG_KEXINIT packets.
! 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
! 153: the the protocol:
! 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:
1.16 djm 166: 2. Connection protocol changes
167:
168: 2.1. connection: Channel write close extension "eow@openssh.com"
1.1 djm 169:
170: The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF
171: message to allow an endpoint to signal its peer that it will send no
172: more data over a channel. Unfortunately, there is no symmetric way for
173: an endpoint to request that its peer should cease sending data to it
174: while still keeping the channel open for the endpoint to send data to
175: the peer.
176:
1.2 djm 177: This is desirable, since it saves the transmission of data that would
1.1 djm 178: otherwise need to be discarded and it allows an endpoint to signal local
179: processes of the condition, e.g. by closing the corresponding file
180: descriptor.
181:
182: OpenSSH implements a channel extension message to perform this
1.10 djm 183: signalling: "eow@openssh.com" (End Of Write). This message is sent by
184: an endpoint when the local output of a session channel is closed or
185: experiences a write error. The message is formatted as follows:
1.1 djm 186:
187: byte SSH_MSG_CHANNEL_REQUEST
188: uint32 recipient channel
189: string "eow@openssh.com"
190: boolean FALSE
191:
192: On receiving this message, the peer SHOULD cease sending data of
193: the channel and MAY signal the process from which the channel data
194: originates (e.g. by closing its read file descriptor).
195:
196: As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does
197: remain open after a "eow@openssh.com" has been sent and more data may
198: still be sent in the other direction. This message does not consume
199: window space and may be sent even if no window space is available.
200:
1.12 djm 201: NB. due to certain broken SSH implementations aborting upon receipt
202: of this message (in contravention of RFC4254 section 5.4), this
203: message is only sent to OpenSSH peers (identified by banner).
1.38 djm 204: Other SSH implementations may be listed to receive this message
1.12 djm 205: upon request.
206:
1.16 djm 207: 2.2. connection: disallow additional sessions extension
208: "no-more-sessions@openssh.com"
1.6 djm 209:
210: Most SSH connections will only ever request a single session, but a
211: attacker may abuse a running ssh client to surreptitiously open
212: additional sessions under their control. OpenSSH provides a global
213: request "no-more-sessions@openssh.com" to mitigate this attack.
214:
215: When an OpenSSH client expects that it will never open another session
216: (i.e. it has been started with connection multiplexing disabled), it
217: will send the following global request:
218:
219: byte SSH_MSG_GLOBAL_REQUEST
220: string "no-more-sessions@openssh.com"
221: char want-reply
222:
223: On receipt of such a message, an OpenSSH server will refuse to open
224: future channels of type "session" and instead immediately abort the
225: connection.
226:
227: Note that this is not a general defence against compromised clients
228: (that is impossible), but it thwarts a simple attack.
229:
1.12 djm 230: NB. due to certain broken SSH implementations aborting upon receipt
231: of this message, the no-more-sessions request is only sent to OpenSSH
232: servers (identified by banner). Other SSH implementations may be
1.38 djm 233: listed to receive this message upon request.
1.12 djm 234:
1.16 djm 235: 2.3. connection: Tunnel forward extension "tun@openssh.com"
1.7 djm 236:
1.8 djm 237: OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com"
1.7 djm 238: channel type. This channel type supports forwarding of network packets
1.28 djm 239: with datagram boundaries intact between endpoints equipped with
1.7 djm 240: interfaces like the BSD tun(4) device. Tunnel forwarding channels are
241: requested by the client with the following packet:
242:
243: byte SSH_MSG_CHANNEL_OPEN
244: string "tun@openssh.com"
245: uint32 sender channel
246: uint32 initial window size
247: uint32 maximum packet size
248: uint32 tunnel mode
249: uint32 remote unit number
250:
251: The "tunnel mode" parameter specifies whether the tunnel should forward
252: layer 2 frames or layer 3 packets. It may take one of the following values:
253:
254: SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */
255: SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */
256:
257: The "tunnel unit number" specifies the remote interface number, or may
1.37 dtucker 258: be 0x7fffffff to allow the server to automatically choose an interface. A
1.13 djm 259: server that is not willing to open a client-specified unit should refuse
260: the request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful
261: open, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS.
1.7 djm 262:
263: Once established the client and server may exchange packet or frames
264: over the tunnel channel by encapsulating them in SSH protocol strings
265: and sending them as channel data. This ensures that packet boundaries
266: are kept intact. Specifically, packets are transmitted using normal
267: SSH_MSG_CHANNEL_DATA packets:
268:
269: byte SSH_MSG_CHANNEL_DATA
270: uint32 recipient channel
271: string data
272:
273: The contents of the "data" field for layer 3 packets is:
274:
275: uint32 packet length
276: uint32 address family
277: byte[packet length - 4] packet data
278:
279: The "address family" field identifies the type of packet in the message.
280: It may be one of:
281:
282: SSH_TUN_AF_INET 2 /* IPv4 */
283: SSH_TUN_AF_INET6 24 /* IPv6 */
284:
285: The "packet data" field consists of the IPv4/IPv6 datagram itself
286: without any link layer header.
287:
1.13 djm 288: The contents of the "data" field for layer 2 packets is:
1.7 djm 289:
290: uint32 packet length
291: byte[packet length] frame
292:
1.8 djm 293: The "frame" field contains an IEEE 802.3 Ethernet frame, including
1.7 djm 294: header.
295:
1.24 millert 296: 2.4. connection: Unix domain socket forwarding
297:
298: OpenSSH supports local and remote Unix domain socket forwarding
299: using the "streamlocal" extension. Forwarding is initiated as per
300: TCP sockets but with a single path instead of a host and port.
301:
302: Similar to direct-tcpip, direct-streamlocal is sent by the client
303: to request that the server make a connection to a Unix domain socket.
304:
305: byte SSH_MSG_CHANNEL_OPEN
306: string "direct-streamlocal@openssh.com"
307: uint32 sender channel
308: uint32 initial window size
309: uint32 maximum packet size
310: string socket path
1.30 djm 311: string reserved
312: uint32 reserved
1.24 millert 313:
314: Similar to forwarded-tcpip, forwarded-streamlocal is sent by the
315: server when the client has previously send the server a streamlocal-forward
316: GLOBAL_REQUEST.
317:
318: byte SSH_MSG_CHANNEL_OPEN
319: string "forwarded-streamlocal@openssh.com"
320: uint32 sender channel
321: uint32 initial window size
322: uint32 maximum packet size
323: string socket path
324: string reserved for future use
325:
326: The reserved field is not currently defined and is ignored on the
327: remote end. It is intended to be used in the future to pass
328: information about the socket file, such as ownership and mode.
329: The client currently sends the empty string for this field.
330:
331: Similar to tcpip-forward, streamlocal-forward is sent by the client
332: to request remote forwarding of a Unix domain socket.
333:
334: byte SSH2_MSG_GLOBAL_REQUEST
335: string "streamlocal-forward@openssh.com"
336: boolean TRUE
337: string socket path
338:
339: Similar to cancel-tcpip-forward, cancel-streamlocal-forward is sent
340: by the client cancel the forwarding of a Unix domain socket.
341:
342: byte SSH2_MSG_GLOBAL_REQUEST
343: string "cancel-streamlocal-forward@openssh.com"
344: boolean FALSE
345: string socket path
346:
1.27 djm 347: 2.5. connection: hostkey update and rotation "hostkeys-00@openssh.com"
348: and "hostkeys-prove-00@openssh.com"
1.25 djm 349:
350: OpenSSH supports a protocol extension allowing a server to inform
1.26 djm 351: a client of all its protocol v.2 host keys after user-authentication
1.25 djm 352: has completed.
353:
354: byte SSH_MSG_GLOBAL_REQUEST
1.27 djm 355: string "hostkeys-00@openssh.com"
1.41 djm 356: char 0 /* want-reply */
1.25 djm 357: string[] hostkeys
358:
1.26 djm 359: Upon receiving this message, a client should check which of the
1.32 djm 360: supplied host keys are present in known_hosts.
361:
362: Note that the server may send key types that the client does not
1.37 dtucker 363: support. The client should disregard such keys if they are received.
1.32 djm 364:
365: If the client identifies any keys that are not present for the host,
366: it should send a "hostkeys-prove@openssh.com" message to request the
367: server prove ownership of the private half of the key.
1.26 djm 368:
369: byte SSH_MSG_GLOBAL_REQUEST
1.27 djm 370: string "hostkeys-prove-00@openssh.com"
1.26 djm 371: char 1 /* want-reply */
372: string[] hostkeys
373:
374: When a server receives this message, it should generate a signature
375: using each requested key over the following:
376:
1.27 djm 377: string "hostkeys-prove-00@openssh.com"
1.26 djm 378: string session identifier
379: string hostkey
380:
381: These signatures should be included in the reply, in the order matching
382: the hostkeys in the request:
383:
384: byte SSH_MSG_REQUEST_SUCCESS
385: string[] signatures
386:
387: When the client receives this reply (and not a failure), it should
388: validate the signatures and may update its known_hosts file, adding keys
389: that it has not seen before and deleting keys for the server host that
390: are no longer offered.
391:
392: These extensions let a client learn key types that it had not previously
393: encountered, thereby allowing it to potentially upgrade from weaker
394: key algorithms to better ones. It also supports graceful key rotation:
395: a server may offer multiple keys of the same type for a period (to
396: give clients an opportunity to learn them using this extension) before
397: removing the deprecated key from those offered.
1.25 djm 398:
1.36 djm 399: 2.6. connection: SIGINFO support for "signal" channel request
400:
401: The SSH channels protocol (RFC4254 section 6.9) supports sending a
402: signal to a session attached to a channel. OpenSSH supports one
403: extension signal "INFO@openssh.com" that allows sending SIGINFO on
404: BSD-derived systems.
405:
1.43 djm 406: 3. Authentication protocol changes
1.16 djm 407:
1.43 djm 408: 3.1. Host-bound public key authentication
409:
410: This is trivial change to the traditional "publickey" authentication
411: method. The authentication request is identical to the original method
412: but for the name and one additional field:
413:
414: byte SSH2_MSG_USERAUTH_REQUEST
415: string username
416: string "ssh-connection"
417: string "publickey-hostbound-v00@openssh.com"
418: bool has_signature
419: string pkalg
420: string public key
421: string server host key
422:
423: Because the entire SSH2_MSG_USERAUTH_REQUEST message is included in
424: the signed data, this ensures that a binding between the destination
425: user, the server identity and the session identifier is visible to the
426: signer. OpenSSH uses this binding via signed data to implement per-key
427: restrictions in ssh-agent.
428:
429: A server may advertise this method using the SSH2_MSG_EXT_INFO
430: mechanism (RFC8308), with the following message:
431:
432: string "publickey-hostbound@openssh.com"
433: string "0" (version)
434:
435: Clients should prefer host-bound authentication when advertised by
436: server.
437:
438: 4. SFTP protocol changes
439:
440: 4.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK
1.1 djm 441:
442: When OpenSSH's sftp-server was implemented, the order of the arguments
1.8 djm 443: to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
1.1 djm 444: the reversal was not noticed until the server was widely deployed. Since
445: fixing this to follow the specification would cause incompatibility, the
446: current order was retained. For correct operation, clients should send
447: SSH_FXP_SYMLINK as follows:
448:
449: uint32 id
450: string targetpath
451: string linkpath
452:
1.43 djm 453: 4.2. sftp: Server extension announcement in SSH_FXP_VERSION
1.1 djm 454:
455: OpenSSH's sftp-server lists the extensions it supports using the
456: standard extension announcement mechanism in the SSH_FXP_VERSION server
457: hello packet:
458:
459: uint32 3 /* protocol version */
460: string ext1-name
461: string ext1-version
462: string ext2-name
463: string ext2-version
464: ...
465: string extN-name
466: string extN-version
467:
468: Each extension reports its integer version number as an ASCII encoded
469: string, e.g. "1". The version will be incremented if the extension is
470: ever changed in an incompatible way. The server MAY advertise the same
471: extension with multiple versions (though this is unlikely). Clients MUST
1.8 djm 472: check the version number before attempting to use the extension.
1.1 djm 473:
1.43 djm 474: 4.3. sftp: Extension request "posix-rename@openssh.com"
1.1 djm 475:
476: This operation provides a rename operation with POSIX semantics, which
477: are different to those provided by the standard SSH_FXP_RENAME in
478: draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
479: SSH_FXP_EXTENDED request with the following format:
480:
481: uint32 id
482: string "posix-rename@openssh.com"
483: string oldpath
484: string newpath
485:
486: On receiving this request the server will perform the POSIX operation
487: rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
488: This extension is advertised in the SSH_FXP_VERSION hello with version
489: "1".
490:
1.43 djm 491: 4.4. sftp: Extension requests "statvfs@openssh.com" and
1.2 djm 492: "fstatvfs@openssh.com"
1.1 djm 493:
494: These requests correspond to the statvfs and fstatvfs POSIX system
495: interfaces. The "statvfs@openssh.com" request operates on an explicit
496: pathname, and is formatted as follows:
497:
498: uint32 id
499: string "statvfs@openssh.com"
500: string path
501:
1.8 djm 502: The "fstatvfs@openssh.com" operates on an open file handle:
1.1 djm 503:
504: uint32 id
1.2 djm 505: string "fstatvfs@openssh.com"
1.1 djm 506: string handle
507:
508: These requests return a SSH_FXP_STATUS reply on failure. On success they
509: return the following SSH_FXP_EXTENDED_REPLY reply:
510:
511: uint32 id
1.4 dtucker 512: uint64 f_bsize /* file system block size */
513: uint64 f_frsize /* fundamental fs block size */
1.1 djm 514: uint64 f_blocks /* number of blocks (unit f_frsize) */
515: uint64 f_bfree /* free blocks in file system */
516: uint64 f_bavail /* free blocks for non-root */
517: uint64 f_files /* total file inodes */
518: uint64 f_ffree /* free file inodes */
519: uint64 f_favail /* free file inodes for to non-root */
1.3 djm 520: uint64 f_fsid /* file system id */
1.4 dtucker 521: uint64 f_flag /* bit mask of f_flag values */
522: uint64 f_namemax /* maximum filename length */
1.1 djm 523:
524: The values of the f_flag bitmask are as follows:
525:
526: #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
527: #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
528:
1.11 djm 529: Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are
530: advertised in the SSH_FXP_VERSION hello with version "2".
1.3 djm 531:
1.43 djm 532: 4.5. sftp: Extension request "hardlink@openssh.com"
1.17 djm 533:
534: This request is for creating a hard link to a regular file. This
535: request is implemented as a SSH_FXP_EXTENDED request with the
536: following format:
537:
538: uint32 id
539: string "hardlink@openssh.com"
540: string oldpath
541: string newpath
542:
543: On receiving this request the server will perform the operation
544: link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
545: This extension is advertised in the SSH_FXP_VERSION hello with version
546: "1".
547:
1.43 djm 548: 4.6. sftp: Extension request "fsync@openssh.com"
1.21 djm 549:
550: This request asks the server to call fsync(2) on an open file handle.
551:
552: uint32 id
553: string "fsync@openssh.com"
554: string handle
555:
1.44 djm 556: On receiving this request, a server will call fsync(handle_fd) and will
1.21 djm 557: respond with a SSH_FXP_STATUS message.
558:
559: This extension is advertised in the SSH_FXP_VERSION hello with version
560: "1".
561:
1.43 djm 562: 4.7. sftp: Extension request "lsetstat@openssh.com"
1.39 djm 563:
564: This request is like the "setstat" command, but sets file attributes on
565: symlinks. It is implemented as a SSH_FXP_EXTENDED request with the
566: following format:
567:
568: uint32 id
569: string "lsetstat@openssh.com"
570: string path
571: ATTRS attrs
572:
573: See the "setstat" command for more details.
574:
575: This extension is advertised in the SSH_FXP_VERSION hello with version
576: "1".
577:
1.43 djm 578: 4.8. sftp: Extension request "limits@openssh.com"
1.40 djm 579:
580: This request is used to determine various limits the server might impose.
581: Clients should not attempt to exceed these limits as the server might sever
582: the connection immediately.
583:
584: uint32 id
585: string "limits@openssh.com"
586:
587: The server will respond with a SSH_FXP_EXTENDED_REPLY reply:
588:
589: uint32 id
590: uint64 max-packet-length
591: uint64 max-read-length
592: uint64 max-write-length
593: uint64 max-open-handles
594:
595: The 'max-packet-length' applies to the total number of bytes in a
596: single SFTP packet. Servers SHOULD set this at least to 34000.
597:
598: The 'max-read-length' is the largest length in a SSH_FXP_READ packet.
599: Even if the client requests a larger size, servers will usually respond
600: with a shorter SSH_FXP_DATA packet. Servers SHOULD set this at least to
601: 32768.
602:
603: The 'max-write-length' is the largest length in a SSH_FXP_WRITE packet
604: the server will accept. Servers SHOULD set this at least to 32768.
605:
606: The 'max-open-handles' is the maximum number of active handles that the
607: server allows (e.g. handles created by SSH_FXP_OPEN and SSH_FXP_OPENDIR
608: packets). Servers MAY count internal file handles against this limit
609: (e.g. system logging or stdout/stderr), so clients SHOULD NOT expect to
610: open this many handles in practice.
611:
612: If the server doesn't enforce a specific limit, then the field may be
613: set to 0. This implies the server relies on the OS to enforce limits
614: (e.g. available memory or file handles), and such limits might be
615: dynamic. The client SHOULD take care to not try to exceed reasonable
616: limits.
617:
618: This extension is advertised in the SSH_FXP_VERSION hello with version
619: "1".
620:
1.43 djm 621: 4.9. sftp: Extension request "expand-path@openssh.com"
1.42 djm 622:
623: This request supports canonicalisation of relative paths and
624: those that need tilde-expansion, i.e. "~", "~/..." and "~user/..."
625: These paths are expanded using shell-like rules and the resultant
626: path is canonicalised similarly to SSH2_FXP_REALPATH.
627:
628: It is implemented as a SSH_FXP_EXTENDED request with the following
629: format:
630:
631: uint32 id
632: string "expand-path@openssh.com"
633: string path
634:
635: Its reply is the same format as that of SSH2_FXP_REALPATH.
636:
637: This extension is advertised in the SSH_FXP_VERSION hello with version
638: "1".
639:
1.44 djm 640: 4.10. sftp: Extension request "copy-data"
641:
642: This request asks the server to copy data from one open file handle and
643: write it to a different open file handle. This avoids needing to transfer
644: the data across the network twice (a download followed by an upload).
645:
646: byte SSH_FXP_EXTENDED
647: uint32 id
648: string "copy-data"
649: string read-from-handle
650: uint64 read-from-offset
651: uint64 read-data-length
652: string write-to-handle
653: uint64 write-to-offset
654:
655: The server will copy read-data-length bytes starting from
656: read-from-offset from the read-from-handle and write them to
657: write-to-handle starting from write-to-offset, and then respond with a
658: SSH_FXP_STATUS message.
659:
660: It's equivalent to issuing a series of SSH_FXP_READ requests on
661: read-from-handle and a series of requests of SSH_FXP_WRITE on
662: write-to-handle.
663:
664: If read-from-handle and write-to-handle are the same, the server will
665: fail the request and respond with a SSH_FX_INVALID_PARAMETER message.
666:
667: If read-data-length is 0, then the server will read data from the
668: read-from-handle until EOF is reached.
669:
670: This extension is advertised in the SSH_FXP_VERSION hello with version
671: "1".
672:
673: This request is identical to the "copy-data" request documented in:
674:
675: https://tools.ietf.org/html/draft-ietf-secsh-filexfer-extensions-00#section-7
676:
1.46 djm 677: 4.11. sftp: Extension request "home-directory"
678:
679: This request asks the server to expand the specified user's home directory.
680: An empty username implies the current user. This can be used by the client
681: to expand ~/ type paths locally.
682:
683: byte SSH_FXP_EXTENDED
684: uint32 id
685: string "home-directory"
686: string username
687:
688: This extension is advertised in the SSH_FXP_VERSION hello with version
689: "1".
690:
691: This provides similar information as the "expand-path@openssh.com" extension.
692:
693: This request is identical to the "home-directory" request documented in:
694:
695: https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-extensions-00#section-5
696:
1.47 djm 697: 4.12. sftp: Extension request "users-groups-by-id@openssh.com"
698:
1.48 dtucker 699: This request asks the server to return user and/or group names that
1.47 djm 700: correspond to one or more IDs (e.g. as returned from a SSH_FXP_STAT
701: request). This may be used by the client to provide usernames in
702: directory listings.
703:
704: byte SSH_FXP_EXTENDED
705: uint32 id
706: string "users-groups-by-id@openssh.com"
707: string uids
708: string gids
709:
710: Where "uids" and "gids" consists of one or more integer user or group
711: identifiers:
712:
713: uint32 id-0
714: ...
715:
716: The server will reply with a SSH_FXP_EXTENDED_REPLY:
717:
718: byte SSH_FXP_EXTENDED_REPLY
719: string usernames
720: string groupnames
721:
722: Where "username" and "groupnames" consists of names in identical request
723: order to "uids" and "gids" respectively:
724:
725: string name-0
726: ...
727:
728: If a name cannot be identified for a given user or group ID, an empty
729: string will be returned in its place.
730:
731: It is acceptable for either "uids" or "gids" to be an empty set, in
732: which case the respective "usernames" or "groupnames" list will also
733: be empty.
734:
735: This extension is advertised in the SSH_FXP_VERSION hello with version
736: "1".
737:
1.43 djm 738: 5. Miscellaneous changes
1.34 djm 739:
1.43 djm 740: 5.1 Public key format
1.34 djm 741:
742: OpenSSH public keys, as generated by ssh-keygen(1) and appearing in
743: authorized_keys files, are formatted as a single line of text consisting
744: of the public key algorithm name followed by a base64-encoded key blob.
1.35 djm 745: The public key blob (before base64 encoding) is the same format used for
746: the encoding of public keys sent on the wire: as described in RFC4253
747: section 6.6 for RSA and DSA keys, RFC5656 section 3.1 for ECDSA keys
748: and the "New public key formats" section of PROTOCOL.certkeys for the
749: OpenSSH certificate formats.
1.34 djm 750:
1.43 djm 751: 5.2 Private key format
1.34 djm 752:
753: OpenSSH private keys, as generated by ssh-keygen(1) use the format
754: described in PROTOCOL.key by default. As a legacy option, PEM format
755: (RFC7468) private keys are also supported for RSA, DSA and ECDSA keys
756: and were the default format before OpenSSH 7.8.
757:
1.43 djm 758: 5.3 KRL format
1.34 djm 759:
760: OpenSSH supports a compact format for Key Revocation Lists (KRLs). This
761: format is described in the PROTOCOL.krl file.
762:
1.43 djm 763: 5.4 Connection multiplexing
1.34 djm 764:
765: OpenSSH's connection multiplexing uses messages as described in
766: PROTOCOL.mux over a Unix domain socket for communications between a
767: master instance and later clients.
768:
1.43 djm 769: 5.5. Agent protocol extensions
770:
771: OpenSSH extends the usual agent protocol. These changes are documented
772: in the PROTOCOL.agent file.
773:
1.49.2.1! bluhm 774: $OpenBSD: PROTOCOL,v 1.49 2023/08/28 03:28:43 djm Exp $
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