Annotation of src/usr.bin/ssh/PROTOCOL, Revision 1.17
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:
54: 2. Connection protocol changes
55:
56: 2.1. connection: Channel write close extension "eow@openssh.com"
1.1 djm 57:
58: The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF
59: message to allow an endpoint to signal its peer that it will send no
60: more data over a channel. Unfortunately, there is no symmetric way for
61: an endpoint to request that its peer should cease sending data to it
62: while still keeping the channel open for the endpoint to send data to
63: the peer.
64:
1.2 djm 65: This is desirable, since it saves the transmission of data that would
1.1 djm 66: otherwise need to be discarded and it allows an endpoint to signal local
67: processes of the condition, e.g. by closing the corresponding file
68: descriptor.
69:
70: OpenSSH implements a channel extension message to perform this
1.10 djm 71: signalling: "eow@openssh.com" (End Of Write). This message is sent by
72: an endpoint when the local output of a session channel is closed or
73: experiences a write error. The message is formatted as follows:
1.1 djm 74:
75: byte SSH_MSG_CHANNEL_REQUEST
76: uint32 recipient channel
77: string "eow@openssh.com"
78: boolean FALSE
79:
80: On receiving this message, the peer SHOULD cease sending data of
81: the channel and MAY signal the process from which the channel data
82: originates (e.g. by closing its read file descriptor).
83:
84: As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does
85: remain open after a "eow@openssh.com" has been sent and more data may
86: still be sent in the other direction. This message does not consume
87: window space and may be sent even if no window space is available.
88:
1.12 djm 89: NB. due to certain broken SSH implementations aborting upon receipt
90: of this message (in contravention of RFC4254 section 5.4), this
91: message is only sent to OpenSSH peers (identified by banner).
92: Other SSH implementations may be whitelisted to receive this message
93: upon request.
94:
1.16 djm 95: 2.2. connection: disallow additional sessions extension
96: "no-more-sessions@openssh.com"
1.6 djm 97:
98: Most SSH connections will only ever request a single session, but a
99: attacker may abuse a running ssh client to surreptitiously open
100: additional sessions under their control. OpenSSH provides a global
101: request "no-more-sessions@openssh.com" to mitigate this attack.
102:
103: When an OpenSSH client expects that it will never open another session
104: (i.e. it has been started with connection multiplexing disabled), it
105: will send the following global request:
106:
107: byte SSH_MSG_GLOBAL_REQUEST
108: string "no-more-sessions@openssh.com"
109: char want-reply
110:
111: On receipt of such a message, an OpenSSH server will refuse to open
112: future channels of type "session" and instead immediately abort the
113: connection.
114:
115: Note that this is not a general defence against compromised clients
116: (that is impossible), but it thwarts a simple attack.
117:
1.12 djm 118: NB. due to certain broken SSH implementations aborting upon receipt
119: of this message, the no-more-sessions request is only sent to OpenSSH
120: servers (identified by banner). Other SSH implementations may be
121: whitelisted to receive this message upon request.
122:
1.16 djm 123: 2.3. connection: Tunnel forward extension "tun@openssh.com"
1.7 djm 124:
1.8 djm 125: OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com"
1.7 djm 126: channel type. This channel type supports forwarding of network packets
1.8 djm 127: with datagram boundaries intact between endpoints equipped with
1.7 djm 128: interfaces like the BSD tun(4) device. Tunnel forwarding channels are
129: requested by the client with the following packet:
130:
131: byte SSH_MSG_CHANNEL_OPEN
132: string "tun@openssh.com"
133: uint32 sender channel
134: uint32 initial window size
135: uint32 maximum packet size
136: uint32 tunnel mode
137: uint32 remote unit number
138:
139: The "tunnel mode" parameter specifies whether the tunnel should forward
140: layer 2 frames or layer 3 packets. It may take one of the following values:
141:
142: SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */
143: SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */
144:
145: The "tunnel unit number" specifies the remote interface number, or may
1.13 djm 146: be 0x7fffffff to allow the server to automatically chose an interface. A
147: server that is not willing to open a client-specified unit should refuse
148: the request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful
149: open, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS.
1.7 djm 150:
151: Once established the client and server may exchange packet or frames
152: over the tunnel channel by encapsulating them in SSH protocol strings
153: and sending them as channel data. This ensures that packet boundaries
154: are kept intact. Specifically, packets are transmitted using normal
155: SSH_MSG_CHANNEL_DATA packets:
156:
157: byte SSH_MSG_CHANNEL_DATA
158: uint32 recipient channel
159: string data
160:
161: The contents of the "data" field for layer 3 packets is:
162:
163: uint32 packet length
164: uint32 address family
165: byte[packet length - 4] packet data
166:
167: The "address family" field identifies the type of packet in the message.
168: It may be one of:
169:
170: SSH_TUN_AF_INET 2 /* IPv4 */
171: SSH_TUN_AF_INET6 24 /* IPv6 */
172:
173: The "packet data" field consists of the IPv4/IPv6 datagram itself
174: without any link layer header.
175:
1.13 djm 176: The contents of the "data" field for layer 2 packets is:
1.7 djm 177:
178: uint32 packet length
179: byte[packet length] frame
180:
1.8 djm 181: The "frame" field contains an IEEE 802.3 Ethernet frame, including
1.7 djm 182: header.
183:
1.16 djm 184: 3. SFTP protocol changes
185:
186: 3.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK
1.1 djm 187:
188: When OpenSSH's sftp-server was implemented, the order of the arguments
1.8 djm 189: to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
1.1 djm 190: the reversal was not noticed until the server was widely deployed. Since
191: fixing this to follow the specification would cause incompatibility, the
192: current order was retained. For correct operation, clients should send
193: SSH_FXP_SYMLINK as follows:
194:
195: uint32 id
196: string targetpath
197: string linkpath
198:
1.16 djm 199: 3.2. sftp: Server extension announcement in SSH_FXP_VERSION
1.1 djm 200:
201: OpenSSH's sftp-server lists the extensions it supports using the
202: standard extension announcement mechanism in the SSH_FXP_VERSION server
203: hello packet:
204:
205: uint32 3 /* protocol version */
206: string ext1-name
207: string ext1-version
208: string ext2-name
209: string ext2-version
210: ...
211: string extN-name
212: string extN-version
213:
214: Each extension reports its integer version number as an ASCII encoded
215: string, e.g. "1". The version will be incremented if the extension is
216: ever changed in an incompatible way. The server MAY advertise the same
217: extension with multiple versions (though this is unlikely). Clients MUST
1.8 djm 218: check the version number before attempting to use the extension.
1.1 djm 219:
1.16 djm 220: 3.3. sftp: Extension request "posix-rename@openssh.com"
1.1 djm 221:
222: This operation provides a rename operation with POSIX semantics, which
223: are different to those provided by the standard SSH_FXP_RENAME in
224: draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
225: SSH_FXP_EXTENDED request with the following format:
226:
227: uint32 id
228: string "posix-rename@openssh.com"
229: string oldpath
230: string newpath
231:
232: On receiving this request the server will perform the POSIX operation
233: rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
234: This extension is advertised in the SSH_FXP_VERSION hello with version
235: "1".
236:
1.16 djm 237: 3.4. sftp: Extension requests "statvfs@openssh.com" and
1.2 djm 238: "fstatvfs@openssh.com"
1.1 djm 239:
240: These requests correspond to the statvfs and fstatvfs POSIX system
241: interfaces. The "statvfs@openssh.com" request operates on an explicit
242: pathname, and is formatted as follows:
243:
244: uint32 id
245: string "statvfs@openssh.com"
246: string path
247:
1.8 djm 248: The "fstatvfs@openssh.com" operates on an open file handle:
1.1 djm 249:
250: uint32 id
1.2 djm 251: string "fstatvfs@openssh.com"
1.1 djm 252: string handle
253:
254: These requests return a SSH_FXP_STATUS reply on failure. On success they
255: return the following SSH_FXP_EXTENDED_REPLY reply:
256:
257: uint32 id
1.4 dtucker 258: uint64 f_bsize /* file system block size */
259: uint64 f_frsize /* fundamental fs block size */
1.1 djm 260: uint64 f_blocks /* number of blocks (unit f_frsize) */
261: uint64 f_bfree /* free blocks in file system */
262: uint64 f_bavail /* free blocks for non-root */
263: uint64 f_files /* total file inodes */
264: uint64 f_ffree /* free file inodes */
265: uint64 f_favail /* free file inodes for to non-root */
1.3 djm 266: uint64 f_fsid /* file system id */
1.4 dtucker 267: uint64 f_flag /* bit mask of f_flag values */
268: uint64 f_namemax /* maximum filename length */
1.1 djm 269:
270: The values of the f_flag bitmask are as follows:
271:
272: #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
273: #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
274:
1.11 djm 275: Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are
276: advertised in the SSH_FXP_VERSION hello with version "2".
1.3 djm 277:
1.17 ! djm 278: 10. sftp: Extension request "hardlink@openssh.com"
! 279:
! 280: This request is for creating a hard link to a regular file. This
! 281: request is implemented as a SSH_FXP_EXTENDED request with the
! 282: following format:
! 283:
! 284: uint32 id
! 285: string "hardlink@openssh.com"
! 286: string oldpath
! 287: string newpath
! 288:
! 289: On receiving this request the server will perform the operation
! 290: link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
! 291: This extension is advertised in the SSH_FXP_VERSION hello with version
! 292: "1".
! 293:
! 294: $OpenBSD: PROTOCOL,v 1.16 2010/08/31 11:54:45 djm Exp $
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