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1This documents OpenSSH's deviations and extensions to the published SSH
2protocol.
3
4Note that OpenSSH's sftp and sftp-server implement revision 3 of the SSH
5filexfer protocol described in:
6
7http://www.openssh.com/txt/draft-ietf-secsh-filexfer-02.txt
8
9Features from newer versions of the draft are not supported, unless
10explicitly implemented as extensions described below.
11
12The protocol used by OpenSSH's ssh-agent is described in the file
13PROTOCOL.agent
14
151. transport: Protocol 2 MAC algorithm "umac-64@openssh.com"
16
17This is a new transport-layer MAC method using the UMAC algorithm
18(rfc4418). This method is identical to the "umac-64" method documented
19in:
20
21http://www.openssh.com/txt/draft-miller-secsh-umac-01.txt
22
232. transport: Protocol 2 compression algorithm "zlib@openssh.com"
24
25This transport-layer compression method uses the zlib compression
26algorithm (identical to the "zlib" method in rfc4253), but delays the
27start of compression until after authentication has completed. This
28avoids exposing compression code to attacks from unauthenticated users.
29
30The method is documented in:
31
32http://www.openssh.com/txt/draft-miller-secsh-compression-delayed-00.txt
33
343. connection: Channel write close extension "eow@openssh.com"
35
36The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF
37message to allow an endpoint to signal its peer that it will send no
38more data over a channel. Unfortunately, there is no symmetric way for
39an endpoint to request that its peer should cease sending data to it
40while still keeping the channel open for the endpoint to send data to
41the peer.
42
43This is desirable, since it saves the transmission of data that would
44otherwise need to be discarded and it allows an endpoint to signal local
45processes of the condition, e.g. by closing the corresponding file
46descriptor.
47
48OpenSSH implements a channel extension message to perform this
49signalling: "eow@openssh.com" (End Of Write). This message is sent by
50an endpoint when the local output of a session channel is closed or
51experiences a write error. The message is formatted as follows:
52
53 byte SSH_MSG_CHANNEL_REQUEST
54 uint32 recipient channel
55 string "eow@openssh.com"
56 boolean FALSE
57
58On receiving this message, the peer SHOULD cease sending data of
59the channel and MAY signal the process from which the channel data
60originates (e.g. by closing its read file descriptor).
61
62As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does
63remain open after a "eow@openssh.com" has been sent and more data may
64still be sent in the other direction. This message does not consume
65window space and may be sent even if no window space is available.
66
674. connection: disallow additional sessions extension
68 "no-more-sessions@openssh.com"
69
70Most SSH connections will only ever request a single session, but a
71attacker may abuse a running ssh client to surreptitiously open
72additional sessions under their control. OpenSSH provides a global
73request "no-more-sessions@openssh.com" to mitigate this attack.
74
75When an OpenSSH client expects that it will never open another session
76(i.e. it has been started with connection multiplexing disabled), it
77will send the following global request:
78
79 byte SSH_MSG_GLOBAL_REQUEST
80 string "no-more-sessions@openssh.com"
81 char want-reply
82
83On receipt of such a message, an OpenSSH server will refuse to open
84future channels of type "session" and instead immediately abort the
85connection.
86
87Note that this is not a general defence against compromised clients
88(that is impossible), but it thwarts a simple attack.
89
905. connection: Tunnel forward extension "tun@openssh.com"
91
92OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com"
93channel type. This channel type supports forwarding of network packets
94with datagram boundaries intact between endpoints equipped with
95interfaces like the BSD tun(4) device. Tunnel forwarding channels are
96requested by the client with the following packet:
97
98 byte SSH_MSG_CHANNEL_OPEN
99 string "tun@openssh.com"
100 uint32 sender channel
101 uint32 initial window size
102 uint32 maximum packet size
103 uint32 tunnel mode
104 uint32 remote unit number
105
106The "tunnel mode" parameter specifies whether the tunnel should forward
107layer 2 frames or layer 3 packets. It may take one of the following values:
108
109 SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */
110 SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */
111
112The "tunnel unit number" specifies the remote interface number, or may
113be zero to allow the server to automatically chose an interface. A server
114that is not willing to open a client-specified unit should refuse the
115request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful open,
116the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS.
117
118Once established the client and server may exchange packet or frames
119over the tunnel channel by encapsulating them in SSH protocol strings
120and sending them as channel data. This ensures that packet boundaries
121are kept intact. Specifically, packets are transmitted using normal
122SSH_MSG_CHANNEL_DATA packets:
123
124 byte SSH_MSG_CHANNEL_DATA
125 uint32 recipient channel
126 string data
127
128The contents of the "data" field for layer 3 packets is:
129
130 uint32 packet length
131 uint32 address family
132 byte[packet length - 4] packet data
133
134The "address family" field identifies the type of packet in the message.
135It may be one of:
136
137 SSH_TUN_AF_INET 2 /* IPv4 */
138 SSH_TUN_AF_INET6 24 /* IPv6 */
139
140The "packet data" field consists of the IPv4/IPv6 datagram itself
141without any link layer header.
142
143The contents of the "data" field for layer 3 packets is:
144
145 uint32 packet length
146 byte[packet length] frame
147
148The "frame" field contains an IEEE 802.3 Ethernet frame, including
149header.
150
1516. sftp: Reversal of arguments to SSH_FXP_SYMLINK
152
153When OpenSSH's sftp-server was implemented, the order of the arguments
154to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
155the reversal was not noticed until the server was widely deployed. Since
156fixing this to follow the specification would cause incompatibility, the
157current order was retained. For correct operation, clients should send
158SSH_FXP_SYMLINK as follows:
159
160 uint32 id
161 string targetpath
162 string linkpath
163
1647. sftp: Server extension announcement in SSH_FXP_VERSION
165
166OpenSSH's sftp-server lists the extensions it supports using the
167standard extension announcement mechanism in the SSH_FXP_VERSION server
168hello packet:
169
170 uint32 3 /* protocol version */
171 string ext1-name
172 string ext1-version
173 string ext2-name
174 string ext2-version
175 ...
176 string extN-name
177 string extN-version
178
179Each extension reports its integer version number as an ASCII encoded
180string, e.g. "1". The version will be incremented if the extension is
181ever changed in an incompatible way. The server MAY advertise the same
182extension with multiple versions (though this is unlikely). Clients MUST
183check the version number before attempting to use the extension.
184
1858. sftp: Extension request "posix-rename@openssh.com"
186
187This operation provides a rename operation with POSIX semantics, which
188are different to those provided by the standard SSH_FXP_RENAME in
189draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
190SSH_FXP_EXTENDED request with the following format:
191
192 uint32 id
193 string "posix-rename@openssh.com"
194 string oldpath
195 string newpath
196
197On receiving this request the server will perform the POSIX operation
198rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
199This extension is advertised in the SSH_FXP_VERSION hello with version
200"1".
201
2029. sftp: Extension requests "statvfs@openssh.com" and
203 "fstatvfs@openssh.com"
204
205These requests correspond to the statvfs and fstatvfs POSIX system
206interfaces. The "statvfs@openssh.com" request operates on an explicit
207pathname, and is formatted as follows:
208
209 uint32 id
210 string "statvfs@openssh.com"
211 string path
212
213The "fstatvfs@openssh.com" operates on an open file handle:
214
215 uint32 id
216 string "fstatvfs@openssh.com"
217 string handle
218
219These requests return a SSH_FXP_STATUS reply on failure. On success they
220return the following SSH_FXP_EXTENDED_REPLY reply:
221
222 uint32 id
223 uint64 f_bsize /* file system block size */
224 uint64 f_frsize /* fundamental fs block size */
225 uint64 f_blocks /* number of blocks (unit f_frsize) */
226 uint64 f_bfree /* free blocks in file system */
227 uint64 f_bavail /* free blocks for non-root */
228 uint64 f_files /* total file inodes */
229 uint64 f_ffree /* free file inodes */
230 uint64 f_favail /* free file inodes for to non-root */
231 uint64 f_fsid /* file system id */
232 uint64 f_flag /* bit mask of f_flag values */
233 uint64 f_namemax /* maximum filename length */
234
235The values of the f_flag bitmask are as follows:
236
237 #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
238 #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
239
240Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are
241advertised in the SSH_FXP_VERSION hello with version "2".
242
243$OpenBSD: PROTOCOL,v 1.11 2008/07/05 05:16:01 djm Exp $