Annotation of src/usr.bin/ssh/PROTOCOL, Revision 1.8
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:
9: Features from newer versions of the draft are not supported, unless
10: explicitly implemented as extensions described below.
11:
12: 1. transport: Protocol 2 MAC algorithm "umac-64@openssh.com"
13:
14: This is a new transport-layer MAC method using the UMAC algorithm
15: (rfc4418). This method is identical to the "umac-64" method documented
16: in:
17:
18: http://www.openssh.com/txt/draft-miller-secsh-umac-01.txt
19:
20: 2. transport: Protocol 2 compression algorithm "zlib@openssh.com"
21:
22: This transport-layer compression method uses the zlib compression
23: algorithm (identical to the "zlib" method in rfc4253), but delays the
24: start of compression until after authentication has completed. This
1.2 djm 25: avoids exposing compression code to attacks from unauthenticated users.
1.1 djm 26:
27: The method is documented in:
28:
29: http://www.openssh.com/txt/draft-miller-secsh-compression-delayed-00.txt
30:
31: 3. connection: Channel write close extension "eow@openssh.com"
32:
33: The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF
34: message to allow an endpoint to signal its peer that it will send no
35: more data over a channel. Unfortunately, there is no symmetric way for
36: an endpoint to request that its peer should cease sending data to it
37: while still keeping the channel open for the endpoint to send data to
38: the peer.
39:
1.2 djm 40: This is desirable, since it saves the transmission of data that would
1.1 djm 41: otherwise need to be discarded and it allows an endpoint to signal local
42: processes of the condition, e.g. by closing the corresponding file
43: descriptor.
44:
45: OpenSSH implements a channel extension message to perform this
46: signalling: "eow@openssh.com" (End Of Write). This message is sent by an
47: endpoint when the local output of a channel is closed or experiences a
48: write 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:
55: On receiving this message, the peer SHOULD cease sending data of
56: the channel and MAY signal the process from which the channel data
57: originates (e.g. by closing its read file descriptor).
58:
59: As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does
60: remain open after a "eow@openssh.com" has been sent and more data may
61: still be sent in the other direction. This message does not consume
62: window space and may be sent even if no window space is available.
63:
1.6 djm 64: 4. connection: disallow additional sessions extension
65: "no-more-sessions@openssh.com"
66:
67: Most SSH connections will only ever request a single session, but a
68: attacker may abuse a running ssh client to surreptitiously open
69: additional sessions under their control. OpenSSH provides a global
70: request "no-more-sessions@openssh.com" to mitigate this attack.
71:
72: When an OpenSSH client expects that it will never open another session
73: (i.e. it has been started with connection multiplexing disabled), it
74: will send the following global request:
75:
76: byte SSH_MSG_GLOBAL_REQUEST
77: string "no-more-sessions@openssh.com"
78: char want-reply
79:
80: On receipt of such a message, an OpenSSH server will refuse to open
81: future channels of type "session" and instead immediately abort the
82: connection.
83:
84: Note that this is not a general defence against compromised clients
85: (that is impossible), but it thwarts a simple attack.
86:
1.7 djm 87: 5. connection: Tunnel forward extension "tun@openssh.com"
88:
1.8 ! djm 89: OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com"
1.7 djm 90: channel type. This channel type supports forwarding of network packets
1.8 ! djm 91: with datagram boundaries intact between endpoints equipped with
1.7 djm 92: interfaces like the BSD tun(4) device. Tunnel forwarding channels are
93: requested by the client with the following packet:
94:
95: byte SSH_MSG_CHANNEL_OPEN
96: string "tun@openssh.com"
97: uint32 sender channel
98: uint32 initial window size
99: uint32 maximum packet size
100: uint32 tunnel mode
101: uint32 remote unit number
102:
103: The "tunnel mode" parameter specifies whether the tunnel should forward
104: layer 2 frames or layer 3 packets. It may take one of the following values:
105:
106: SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */
107: SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */
108:
109: The "tunnel unit number" specifies the remote interface number, or may
110: be zero to allow the server to automatically chose an interface. A server
111: that is not willing to open a client-specified unit should refuse the
112: request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful open,
113: the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS.
114:
115: Once established the client and server may exchange packet or frames
116: over the tunnel channel by encapsulating them in SSH protocol strings
117: and sending them as channel data. This ensures that packet boundaries
118: are kept intact. Specifically, packets are transmitted using normal
119: SSH_MSG_CHANNEL_DATA packets:
120:
121: byte SSH_MSG_CHANNEL_DATA
122: uint32 recipient channel
123: string data
124:
125: The contents of the "data" field for layer 3 packets is:
126:
127: uint32 packet length
128: uint32 address family
129: byte[packet length - 4] packet data
130:
131: The "address family" field identifies the type of packet in the message.
132: It may be one of:
133:
134: SSH_TUN_AF_INET 2 /* IPv4 */
135: SSH_TUN_AF_INET6 24 /* IPv6 */
136:
137: The "packet data" field consists of the IPv4/IPv6 datagram itself
138: without any link layer header.
139:
140: The contents of the "data" field for layer 3 packets is:
141:
142: uint32 packet length
143: byte[packet length] frame
144:
1.8 ! djm 145: The "frame" field contains an IEEE 802.3 Ethernet frame, including
1.7 djm 146: header.
147:
148: 6. sftp: Reversal of arguments to SSH_FXP_SYMLINK
1.1 djm 149:
150: When OpenSSH's sftp-server was implemented, the order of the arguments
1.8 ! djm 151: to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
1.1 djm 152: the reversal was not noticed until the server was widely deployed. Since
153: fixing this to follow the specification would cause incompatibility, the
154: current order was retained. For correct operation, clients should send
155: SSH_FXP_SYMLINK as follows:
156:
157: uint32 id
158: string targetpath
159: string linkpath
160:
1.7 djm 161: 7. sftp: Server extension announcement in SSH_FXP_VERSION
1.1 djm 162:
163: OpenSSH's sftp-server lists the extensions it supports using the
164: standard extension announcement mechanism in the SSH_FXP_VERSION server
165: hello packet:
166:
167: uint32 3 /* protocol version */
168: string ext1-name
169: string ext1-version
170: string ext2-name
171: string ext2-version
172: ...
173: string extN-name
174: string extN-version
175:
176: Each extension reports its integer version number as an ASCII encoded
177: string, e.g. "1". The version will be incremented if the extension is
178: ever changed in an incompatible way. The server MAY advertise the same
179: extension with multiple versions (though this is unlikely). Clients MUST
1.8 ! djm 180: check the version number before attempting to use the extension.
1.1 djm 181:
1.7 djm 182: 8. sftp: Extension request "posix-rename@openssh.com"
1.1 djm 183:
184: This operation provides a rename operation with POSIX semantics, which
185: are different to those provided by the standard SSH_FXP_RENAME in
186: draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
187: SSH_FXP_EXTENDED request with the following format:
188:
189: uint32 id
190: string "posix-rename@openssh.com"
191: string oldpath
192: string newpath
193:
194: On receiving this request the server will perform the POSIX operation
195: rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
196: This extension is advertised in the SSH_FXP_VERSION hello with version
197: "1".
198:
1.7 djm 199: 9. sftp: Extension requests "statvfs@openssh.com" and
1.2 djm 200: "fstatvfs@openssh.com"
1.1 djm 201:
202: These requests correspond to the statvfs and fstatvfs POSIX system
203: interfaces. The "statvfs@openssh.com" request operates on an explicit
204: pathname, and is formatted as follows:
205:
206: uint32 id
207: string "statvfs@openssh.com"
208: string path
209:
1.8 ! djm 210: The "fstatvfs@openssh.com" operates on an open file handle:
1.1 djm 211:
212: uint32 id
1.2 djm 213: string "fstatvfs@openssh.com"
1.1 djm 214: string handle
215:
216: These requests return a SSH_FXP_STATUS reply on failure. On success they
217: return the following SSH_FXP_EXTENDED_REPLY reply:
218:
219: uint32 id
1.4 dtucker 220: uint64 f_bsize /* file system block size */
221: uint64 f_frsize /* fundamental fs block size */
1.1 djm 222: uint64 f_blocks /* number of blocks (unit f_frsize) */
223: uint64 f_bfree /* free blocks in file system */
224: uint64 f_bavail /* free blocks for non-root */
225: uint64 f_files /* total file inodes */
226: uint64 f_ffree /* free file inodes */
227: uint64 f_favail /* free file inodes for to non-root */
1.3 djm 228: uint64 f_fsid /* file system id */
1.4 dtucker 229: uint64 f_flag /* bit mask of f_flag values */
230: uint64 f_namemax /* maximum filename length */
1.1 djm 231:
232: The values of the f_flag bitmask are as follows:
233:
234: #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
235: #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
236:
1.3 djm 237: This extension is advertised in the SSH_FXP_VERSION hello with version
238: "2".
239:
1.8 ! djm 240: $OpenBSD: PROTOCOL,v 1.7 2008/06/12 05:15:41 djm Exp $
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