Annotation of src/usr.bin/ssh/PROTOCOL, Revision 1.15
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:
1.1 djm 15: 1. transport: Protocol 2 MAC algorithm "umac-64@openssh.com"
16:
17: This is a new transport-layer MAC method using the UMAC algorithm
18: (rfc4418). This method is identical to the "umac-64" method documented
19: in:
20:
21: http://www.openssh.com/txt/draft-miller-secsh-umac-01.txt
22:
23: 2. transport: Protocol 2 compression algorithm "zlib@openssh.com"
24:
25: This transport-layer compression method uses the zlib compression
26: algorithm (identical to the "zlib" method in rfc4253), but delays the
27: start of compression until after authentication has completed. This
1.2 djm 28: avoids exposing compression code to attacks from unauthenticated users.
1.1 djm 29:
30: The method is documented in:
31:
32: http://www.openssh.com/txt/draft-miller-secsh-compression-delayed-00.txt
33:
1.15 ! djm 34: 3. transport: New public key algorithms "ssh-rsa-cert-v00@openssh.com" and
! 35: "ssh-dsa-cert-v00@openssh.com"
! 36:
! 37: OpenSSH introduces two new public key algorithms to support certificate
! 38: authentication for users and hostkeys. These methods are documented in
! 39: the file PROTOCOL.certkeys
! 40:
! 41: 4. connection: Channel write close extension "eow@openssh.com"
1.1 djm 42:
43: The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF
44: message to allow an endpoint to signal its peer that it will send no
45: more data over a channel. Unfortunately, there is no symmetric way for
46: an endpoint to request that its peer should cease sending data to it
47: while still keeping the channel open for the endpoint to send data to
48: the peer.
49:
1.2 djm 50: This is desirable, since it saves the transmission of data that would
1.1 djm 51: otherwise need to be discarded and it allows an endpoint to signal local
52: processes of the condition, e.g. by closing the corresponding file
53: descriptor.
54:
55: OpenSSH implements a channel extension message to perform this
1.10 djm 56: signalling: "eow@openssh.com" (End Of Write). This message is sent by
57: an endpoint when the local output of a session channel is closed or
58: experiences a write error. The message is formatted as follows:
1.1 djm 59:
60: byte SSH_MSG_CHANNEL_REQUEST
61: uint32 recipient channel
62: string "eow@openssh.com"
63: boolean FALSE
64:
65: On receiving this message, the peer SHOULD cease sending data of
66: the channel and MAY signal the process from which the channel data
67: originates (e.g. by closing its read file descriptor).
68:
69: As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does
70: remain open after a "eow@openssh.com" has been sent and more data may
71: still be sent in the other direction. This message does not consume
72: window space and may be sent even if no window space is available.
73:
1.12 djm 74: NB. due to certain broken SSH implementations aborting upon receipt
75: of this message (in contravention of RFC4254 section 5.4), this
76: message is only sent to OpenSSH peers (identified by banner).
77: Other SSH implementations may be whitelisted to receive this message
78: upon request.
79:
1.15 ! djm 80: 5. connection: disallow additional sessions extension
1.6 djm 81: "no-more-sessions@openssh.com"
82:
83: Most SSH connections will only ever request a single session, but a
84: attacker may abuse a running ssh client to surreptitiously open
85: additional sessions under their control. OpenSSH provides a global
86: request "no-more-sessions@openssh.com" to mitigate this attack.
87:
88: When an OpenSSH client expects that it will never open another session
89: (i.e. it has been started with connection multiplexing disabled), it
90: will send the following global request:
91:
92: byte SSH_MSG_GLOBAL_REQUEST
93: string "no-more-sessions@openssh.com"
94: char want-reply
95:
96: On receipt of such a message, an OpenSSH server will refuse to open
97: future channels of type "session" and instead immediately abort the
98: connection.
99:
100: Note that this is not a general defence against compromised clients
101: (that is impossible), but it thwarts a simple attack.
102:
1.12 djm 103: NB. due to certain broken SSH implementations aborting upon receipt
104: of this message, the no-more-sessions request is only sent to OpenSSH
105: servers (identified by banner). Other SSH implementations may be
106: whitelisted to receive this message upon request.
107:
1.15 ! djm 108: 6. connection: Tunnel forward extension "tun@openssh.com"
1.7 djm 109:
1.8 djm 110: OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com"
1.7 djm 111: channel type. This channel type supports forwarding of network packets
1.8 djm 112: with datagram boundaries intact between endpoints equipped with
1.7 djm 113: interfaces like the BSD tun(4) device. Tunnel forwarding channels are
114: requested by the client with the following packet:
115:
116: byte SSH_MSG_CHANNEL_OPEN
117: string "tun@openssh.com"
118: uint32 sender channel
119: uint32 initial window size
120: uint32 maximum packet size
121: uint32 tunnel mode
122: uint32 remote unit number
123:
124: The "tunnel mode" parameter specifies whether the tunnel should forward
125: layer 2 frames or layer 3 packets. It may take one of the following values:
126:
127: SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */
128: SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */
129:
130: The "tunnel unit number" specifies the remote interface number, or may
1.13 djm 131: be 0x7fffffff to allow the server to automatically chose an interface. A
132: server that is not willing to open a client-specified unit should refuse
133: the request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful
134: open, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS.
1.7 djm 135:
136: Once established the client and server may exchange packet or frames
137: over the tunnel channel by encapsulating them in SSH protocol strings
138: and sending them as channel data. This ensures that packet boundaries
139: are kept intact. Specifically, packets are transmitted using normal
140: SSH_MSG_CHANNEL_DATA packets:
141:
142: byte SSH_MSG_CHANNEL_DATA
143: uint32 recipient channel
144: string data
145:
146: The contents of the "data" field for layer 3 packets is:
147:
148: uint32 packet length
149: uint32 address family
150: byte[packet length - 4] packet data
151:
152: The "address family" field identifies the type of packet in the message.
153: It may be one of:
154:
155: SSH_TUN_AF_INET 2 /* IPv4 */
156: SSH_TUN_AF_INET6 24 /* IPv6 */
157:
158: The "packet data" field consists of the IPv4/IPv6 datagram itself
159: without any link layer header.
160:
1.13 djm 161: The contents of the "data" field for layer 2 packets is:
1.7 djm 162:
163: uint32 packet length
164: byte[packet length] frame
165:
1.8 djm 166: The "frame" field contains an IEEE 802.3 Ethernet frame, including
1.7 djm 167: header.
168:
1.15 ! djm 169: 7. sftp: Reversal of arguments to SSH_FXP_SYMLINK
1.1 djm 170:
171: When OpenSSH's sftp-server was implemented, the order of the arguments
1.8 djm 172: to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
1.1 djm 173: the reversal was not noticed until the server was widely deployed. Since
174: fixing this to follow the specification would cause incompatibility, the
175: current order was retained. For correct operation, clients should send
176: SSH_FXP_SYMLINK as follows:
177:
178: uint32 id
179: string targetpath
180: string linkpath
181:
1.15 ! djm 182: 8. sftp: Server extension announcement in SSH_FXP_VERSION
1.1 djm 183:
184: OpenSSH's sftp-server lists the extensions it supports using the
185: standard extension announcement mechanism in the SSH_FXP_VERSION server
186: hello packet:
187:
188: uint32 3 /* protocol version */
189: string ext1-name
190: string ext1-version
191: string ext2-name
192: string ext2-version
193: ...
194: string extN-name
195: string extN-version
196:
197: Each extension reports its integer version number as an ASCII encoded
198: string, e.g. "1". The version will be incremented if the extension is
199: ever changed in an incompatible way. The server MAY advertise the same
200: extension with multiple versions (though this is unlikely). Clients MUST
1.8 djm 201: check the version number before attempting to use the extension.
1.1 djm 202:
1.15 ! djm 203: 9. sftp: Extension request "posix-rename@openssh.com"
1.1 djm 204:
205: This operation provides a rename operation with POSIX semantics, which
206: are different to those provided by the standard SSH_FXP_RENAME in
207: draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
208: SSH_FXP_EXTENDED request with the following format:
209:
210: uint32 id
211: string "posix-rename@openssh.com"
212: string oldpath
213: string newpath
214:
215: On receiving this request the server will perform the POSIX operation
216: rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
217: This extension is advertised in the SSH_FXP_VERSION hello with version
218: "1".
219:
1.15 ! djm 220: 10. sftp: Extension requests "statvfs@openssh.com" and
1.2 djm 221: "fstatvfs@openssh.com"
1.1 djm 222:
223: These requests correspond to the statvfs and fstatvfs POSIX system
224: interfaces. The "statvfs@openssh.com" request operates on an explicit
225: pathname, and is formatted as follows:
226:
227: uint32 id
228: string "statvfs@openssh.com"
229: string path
230:
1.8 djm 231: The "fstatvfs@openssh.com" operates on an open file handle:
1.1 djm 232:
233: uint32 id
1.2 djm 234: string "fstatvfs@openssh.com"
1.1 djm 235: string handle
236:
237: These requests return a SSH_FXP_STATUS reply on failure. On success they
238: return the following SSH_FXP_EXTENDED_REPLY reply:
239:
240: uint32 id
1.4 dtucker 241: uint64 f_bsize /* file system block size */
242: uint64 f_frsize /* fundamental fs block size */
1.1 djm 243: uint64 f_blocks /* number of blocks (unit f_frsize) */
244: uint64 f_bfree /* free blocks in file system */
245: uint64 f_bavail /* free blocks for non-root */
246: uint64 f_files /* total file inodes */
247: uint64 f_ffree /* free file inodes */
248: uint64 f_favail /* free file inodes for to non-root */
1.3 djm 249: uint64 f_fsid /* file system id */
1.4 dtucker 250: uint64 f_flag /* bit mask of f_flag values */
251: uint64 f_namemax /* maximum filename length */
1.1 djm 252:
253: The values of the f_flag bitmask are as follows:
254:
255: #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
256: #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
257:
1.11 djm 258: Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are
259: advertised in the SSH_FXP_VERSION hello with version "2".
1.3 djm 260:
1.15 ! djm 261: $OpenBSD: PROTOCOL,v 1.14 2010/01/09 00:57:10 djm Exp $
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