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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
121. transport: Protocol 2 MAC algorithm "umac-64@openssh.com"
13
14This is a new transport-layer MAC method using the UMAC algorithm
15(rfc4418). This method is identical to the "umac-64" method documented
16in:
17
18http://www.openssh.com/txt/draft-miller-secsh-umac-01.txt
19
202. transport: Protocol 2 compression algorithm "zlib@openssh.com"
21
22This transport-layer compression method uses the zlib compression
23algorithm (identical to the "zlib" method in rfc4253), but delays the
24start of compression until after authentication has completed. This
25avoids exposing compression code to attacks from unauthenticated users.
26
27The method is documented in:
28
29http://www.openssh.com/txt/draft-miller-secsh-compression-delayed-00.txt
30
313. connection: Channel write close extension "eow@openssh.com"
32
33The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF
34message to allow an endpoint to signal its peer that it will send no
35more data over a channel. Unfortunately, there is no symmetric way for
36an endpoint to request that its peer should cease sending data to it
37while still keeping the channel open for the endpoint to send data to
38the peer.
39
40This is desirable, since it saves the transmission of data that would
41otherwise need to be discarded and it allows an endpoint to signal local
42processes of the condition, e.g. by closing the corresponding file
43descriptor.
44
45OpenSSH implements a channel extension message to perform this
46signalling: "eow@openssh.com" (End Of Write). This message is sent by an
47endpoint when the local output of a channel is closed or experiences a
48write error. The message is formatted as follows:
49
50 byte SSH_MSG_CHANNEL_REQUEST
51 uint32 recipient channel
52 string "eow@openssh.com"
53 boolean FALSE
54
55On receiving this message, the peer SHOULD cease sending data of
56the channel and MAY signal the process from which the channel data
57originates (e.g. by closing its read file descriptor).
58
59As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does
60remain open after a "eow@openssh.com" has been sent and more data may
61still be sent in the other direction. This message does not consume
62window space and may be sent even if no window space is available.
63
644. sftp: Reversal of arguments to SSH_FXP_SYMLINK
65
66When OpenSSH's sftp-server was implemented, the order of the arguments
67to the SSH_FXP_SYMLINK method was inadvertendly reversed. Unfortunately,
68the reversal was not noticed until the server was widely deployed. Since
69fixing this to follow the specification would cause incompatibility, the
70current order was retained. For correct operation, clients should send
71SSH_FXP_SYMLINK as follows:
72
73 uint32 id
74 string targetpath
75 string linkpath
76
775. sftp: Server extension announcement in SSH_FXP_VERSION
78
79OpenSSH's sftp-server lists the extensions it supports using the
80standard extension announcement mechanism in the SSH_FXP_VERSION server
81hello packet:
82
83 uint32 3 /* protocol version */
84 string ext1-name
85 string ext1-version
86 string ext2-name
87 string ext2-version
88 ...
89 string extN-name
90 string extN-version
91
92Each extension reports its integer version number as an ASCII encoded
93string, e.g. "1". The version will be incremented if the extension is
94ever changed in an incompatible way. The server MAY advertise the same
95extension with multiple versions (though this is unlikely). Clients MUST
96check the version number before attemping to use the extension.
97
986. sftp: Extension request "posix-rename@openssh.com"
99
100This operation provides a rename operation with POSIX semantics, which
101are different to those provided by the standard SSH_FXP_RENAME in
102draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
103SSH_FXP_EXTENDED request with the following format:
104
105 uint32 id
106 string "posix-rename@openssh.com"
107 string oldpath
108 string newpath
109
110On receiving this request the server will perform the POSIX operation
111rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
112This extension is advertised in the SSH_FXP_VERSION hello with version
113"1".
114
1157. sftp: Extension requests "statvfs@openssh.com" and
116 "fstatvfs@openssh.com"
117
118These requests correspond to the statvfs and fstatvfs POSIX system
119interfaces. The "statvfs@openssh.com" request operates on an explicit
120pathname, and is formatted as follows:
121
122 uint32 id
123 string "statvfs@openssh.com"
124 string path
125
126The "fstatvfs@openssh.com" operates on an open filehandle:
127
128 uint32 id
129 string "fstatvfs@openssh.com"
130 string handle
131
132These requests return a SSH_FXP_STATUS reply on failure. On success they
133return the following SSH_FXP_EXTENDED_REPLY reply:
134
135 uint32 id
136 uint32 f_bsize /* file system block size */
137 uint32 f_frsize /* fundamental fs block size */
138 uint64 f_blocks /* number of blocks (unit f_frsize) */
139 uint64 f_bfree /* free blocks in file system */
140 uint64 f_bavail /* free blocks for non-root */
141 uint64 f_files /* total file inodes */
142 uint64 f_ffree /* free file inodes */
143 uint64 f_favail /* free file inodes for to non-root */
144 uint32 f_fsid /* file system id */
145 uint32 f_flag /* bit mask of f_flag values */
146 uint32 f_namemax /* maximum filename length */
147
148The values of the f_flag bitmask are as follows:
149
150 #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
151 #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
152
153$Id: PROTOCOL,v 1.1 2008/05/19 06:11:56 djm Exp $
154