Annotation of src/usr.bin/ssh/PROTOCOL, Revision 1.22
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:
1.16 djm 36: 1.3. transport: New public key algorithms "ssh-rsa-cert-v00@openssh.com",
37: "ssh-dsa-cert-v00@openssh.com",
38: "ecdsa-sha2-nistp256-cert-v01@openssh.com",
39: "ecdsa-sha2-nistp384-cert-v01@openssh.com" and
40: "ecdsa-sha2-nistp521-cert-v01@openssh.com"
1.15 djm 41:
1.16 djm 42: OpenSSH introduces new public key algorithms to support certificate
1.15 djm 43: authentication for users and hostkeys. These methods are documented in
44: the file PROTOCOL.certkeys
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:
1.16 djm 99: 2. Connection protocol changes
100:
101: 2.1. connection: Channel write close extension "eow@openssh.com"
1.1 djm 102:
103: The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF
104: message to allow an endpoint to signal its peer that it will send no
105: more data over a channel. Unfortunately, there is no symmetric way for
106: an endpoint to request that its peer should cease sending data to it
107: while still keeping the channel open for the endpoint to send data to
108: the peer.
109:
1.2 djm 110: This is desirable, since it saves the transmission of data that would
1.1 djm 111: otherwise need to be discarded and it allows an endpoint to signal local
112: processes of the condition, e.g. by closing the corresponding file
113: descriptor.
114:
115: OpenSSH implements a channel extension message to perform this
1.10 djm 116: signalling: "eow@openssh.com" (End Of Write). This message is sent by
117: an endpoint when the local output of a session channel is closed or
118: experiences a write error. The message is formatted as follows:
1.1 djm 119:
120: byte SSH_MSG_CHANNEL_REQUEST
121: uint32 recipient channel
122: string "eow@openssh.com"
123: boolean FALSE
124:
125: On receiving this message, the peer SHOULD cease sending data of
126: the channel and MAY signal the process from which the channel data
127: originates (e.g. by closing its read file descriptor).
128:
129: As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does
130: remain open after a "eow@openssh.com" has been sent and more data may
131: still be sent in the other direction. This message does not consume
132: window space and may be sent even if no window space is available.
133:
1.12 djm 134: NB. due to certain broken SSH implementations aborting upon receipt
135: of this message (in contravention of RFC4254 section 5.4), this
136: message is only sent to OpenSSH peers (identified by banner).
137: Other SSH implementations may be whitelisted to receive this message
138: upon request.
139:
1.16 djm 140: 2.2. connection: disallow additional sessions extension
141: "no-more-sessions@openssh.com"
1.6 djm 142:
143: Most SSH connections will only ever request a single session, but a
144: attacker may abuse a running ssh client to surreptitiously open
145: additional sessions under their control. OpenSSH provides a global
146: request "no-more-sessions@openssh.com" to mitigate this attack.
147:
148: When an OpenSSH client expects that it will never open another session
149: (i.e. it has been started with connection multiplexing disabled), it
150: will send the following global request:
151:
152: byte SSH_MSG_GLOBAL_REQUEST
153: string "no-more-sessions@openssh.com"
154: char want-reply
155:
156: On receipt of such a message, an OpenSSH server will refuse to open
157: future channels of type "session" and instead immediately abort the
158: connection.
159:
160: Note that this is not a general defence against compromised clients
161: (that is impossible), but it thwarts a simple attack.
162:
1.12 djm 163: NB. due to certain broken SSH implementations aborting upon receipt
164: of this message, the no-more-sessions request is only sent to OpenSSH
165: servers (identified by banner). Other SSH implementations may be
166: whitelisted to receive this message upon request.
167:
1.16 djm 168: 2.3. connection: Tunnel forward extension "tun@openssh.com"
1.7 djm 169:
1.8 djm 170: OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com"
1.7 djm 171: channel type. This channel type supports forwarding of network packets
1.8 djm 172: with datagram boundaries intact between endpoints equipped with
1.7 djm 173: interfaces like the BSD tun(4) device. Tunnel forwarding channels are
174: requested by the client with the following packet:
175:
176: byte SSH_MSG_CHANNEL_OPEN
177: string "tun@openssh.com"
178: uint32 sender channel
179: uint32 initial window size
180: uint32 maximum packet size
181: uint32 tunnel mode
182: uint32 remote unit number
183:
184: The "tunnel mode" parameter specifies whether the tunnel should forward
185: layer 2 frames or layer 3 packets. It may take one of the following values:
186:
187: SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */
188: SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */
189:
190: The "tunnel unit number" specifies the remote interface number, or may
1.13 djm 191: be 0x7fffffff to allow the server to automatically chose an interface. A
192: server that is not willing to open a client-specified unit should refuse
193: the request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful
194: open, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS.
1.7 djm 195:
196: Once established the client and server may exchange packet or frames
197: over the tunnel channel by encapsulating them in SSH protocol strings
198: and sending them as channel data. This ensures that packet boundaries
199: are kept intact. Specifically, packets are transmitted using normal
200: SSH_MSG_CHANNEL_DATA packets:
201:
202: byte SSH_MSG_CHANNEL_DATA
203: uint32 recipient channel
204: string data
205:
206: The contents of the "data" field for layer 3 packets is:
207:
208: uint32 packet length
209: uint32 address family
210: byte[packet length - 4] packet data
211:
212: The "address family" field identifies the type of packet in the message.
213: It may be one of:
214:
215: SSH_TUN_AF_INET 2 /* IPv4 */
216: SSH_TUN_AF_INET6 24 /* IPv6 */
217:
218: The "packet data" field consists of the IPv4/IPv6 datagram itself
219: without any link layer header.
220:
1.13 djm 221: The contents of the "data" field for layer 2 packets is:
1.7 djm 222:
223: uint32 packet length
224: byte[packet length] frame
225:
1.8 djm 226: The "frame" field contains an IEEE 802.3 Ethernet frame, including
1.7 djm 227: header.
228:
1.16 djm 229: 3. SFTP protocol changes
230:
231: 3.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK
1.1 djm 232:
233: When OpenSSH's sftp-server was implemented, the order of the arguments
1.8 djm 234: to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
1.1 djm 235: the reversal was not noticed until the server was widely deployed. Since
236: fixing this to follow the specification would cause incompatibility, the
237: current order was retained. For correct operation, clients should send
238: SSH_FXP_SYMLINK as follows:
239:
240: uint32 id
241: string targetpath
242: string linkpath
243:
1.16 djm 244: 3.2. sftp: Server extension announcement in SSH_FXP_VERSION
1.1 djm 245:
246: OpenSSH's sftp-server lists the extensions it supports using the
247: standard extension announcement mechanism in the SSH_FXP_VERSION server
248: hello packet:
249:
250: uint32 3 /* protocol version */
251: string ext1-name
252: string ext1-version
253: string ext2-name
254: string ext2-version
255: ...
256: string extN-name
257: string extN-version
258:
259: Each extension reports its integer version number as an ASCII encoded
260: string, e.g. "1". The version will be incremented if the extension is
261: ever changed in an incompatible way. The server MAY advertise the same
262: extension with multiple versions (though this is unlikely). Clients MUST
1.8 djm 263: check the version number before attempting to use the extension.
1.1 djm 264:
1.16 djm 265: 3.3. sftp: Extension request "posix-rename@openssh.com"
1.1 djm 266:
267: This operation provides a rename operation with POSIX semantics, which
268: are different to those provided by the standard SSH_FXP_RENAME in
269: draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
270: SSH_FXP_EXTENDED request with the following format:
271:
272: uint32 id
273: string "posix-rename@openssh.com"
274: string oldpath
275: string newpath
276:
277: On receiving this request the server will perform the POSIX operation
278: rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
279: This extension is advertised in the SSH_FXP_VERSION hello with version
280: "1".
281:
1.16 djm 282: 3.4. sftp: Extension requests "statvfs@openssh.com" and
1.2 djm 283: "fstatvfs@openssh.com"
1.1 djm 284:
285: These requests correspond to the statvfs and fstatvfs POSIX system
286: interfaces. The "statvfs@openssh.com" request operates on an explicit
287: pathname, and is formatted as follows:
288:
289: uint32 id
290: string "statvfs@openssh.com"
291: string path
292:
1.8 djm 293: The "fstatvfs@openssh.com" operates on an open file handle:
1.1 djm 294:
295: uint32 id
1.2 djm 296: string "fstatvfs@openssh.com"
1.1 djm 297: string handle
298:
299: These requests return a SSH_FXP_STATUS reply on failure. On success they
300: return the following SSH_FXP_EXTENDED_REPLY reply:
301:
302: uint32 id
1.4 dtucker 303: uint64 f_bsize /* file system block size */
304: uint64 f_frsize /* fundamental fs block size */
1.1 djm 305: uint64 f_blocks /* number of blocks (unit f_frsize) */
306: uint64 f_bfree /* free blocks in file system */
307: uint64 f_bavail /* free blocks for non-root */
308: uint64 f_files /* total file inodes */
309: uint64 f_ffree /* free file inodes */
310: uint64 f_favail /* free file inodes for to non-root */
1.3 djm 311: uint64 f_fsid /* file system id */
1.4 dtucker 312: uint64 f_flag /* bit mask of f_flag values */
313: uint64 f_namemax /* maximum filename length */
1.1 djm 314:
315: The values of the f_flag bitmask are as follows:
316:
317: #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
318: #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
319:
1.11 djm 320: Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are
321: advertised in the SSH_FXP_VERSION hello with version "2".
1.3 djm 322:
1.17 djm 323: 10. sftp: Extension request "hardlink@openssh.com"
324:
325: This request is for creating a hard link to a regular file. This
326: request is implemented as a SSH_FXP_EXTENDED request with the
327: following format:
328:
329: uint32 id
330: string "hardlink@openssh.com"
331: string oldpath
332: string newpath
333:
334: On receiving this request the server will perform the operation
335: link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
336: This extension is advertised in the SSH_FXP_VERSION hello with version
337: "1".
338:
1.21 djm 339: 10. sftp: Extension request "fsync@openssh.com"
340:
341: This request asks the server to call fsync(2) on an open file handle.
342:
343: uint32 id
344: string "fsync@openssh.com"
345: string handle
346:
347: One receiving this request, a server will call fsync(handle_fd) and will
348: respond with a SSH_FXP_STATUS message.
349:
350: This extension is advertised in the SSH_FXP_VERSION hello with version
351: "1".
352:
1.22 ! djm 353: $OpenBSD: PROTOCOL,v 1.21 2013/10/17 00:30:13 djm Exp $
![[BACK]](/icons/back.gif){kind=icon}
Return to PROTOCOL CVS log ![[TXT]](/icons/text.gif){kind=icon}
![[DIR]](/icons/dir.gif){kind=icon}
Up to [local] / src / usr.bin / ssh