Annotation of src/usr.bin/ssh/PROTOCOL, Revision 1.19
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:
1.18 markus 54: 1.5 transport: Protocol 2 Encrypt-then-MAC MAC algorithms
55:
56: OpenSSH supports MAC algorithms, whose names contain "-etm", that
57: perform the calculations in a different order to that defined in RFC
58: 4253. These variants use the so-called "encrypt then MAC" ordering,
59: calculating the MAC over the packet ciphertext rather than the
60: plaintext. This ordering closes a security flaw in the SSH transport
61: protocol, where decryption of unauthenticated ciphertext provided a
62: "decryption oracle" that could, in conjunction with cipher flaws, reveal
63: session plaintext.
64:
65: Specifically, the "-etm" MAC algorithms modify the transport protocol
66: to calculate the MAC over the packet ciphertext and to send the packet
67: length unencrypted. This is necessary for the transport to obtain the
68: length of the packet and location of the MAC tag so that it may be
69: verified without decrypting unauthenticated data.
70:
71: As such, the MAC covers:
72:
1.19 ! djm 73: mac = MAC(key, sequence_number || packet_length || encrypted_packet)
1.18 markus 74:
1.19 ! djm 75: where "packet_length" is encoded as a uint32 and "encrypted_packet"
! 76: contains:
1.18 markus 77:
78: byte padding_length
79: byte[n1] payload; n1 = packet_length - padding_length - 1
80: byte[n2] random padding; n2 = padding_length
81:
1.16 djm 82: 2. Connection protocol changes
83:
84: 2.1. connection: Channel write close extension "eow@openssh.com"
1.1 djm 85:
86: The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF
87: message to allow an endpoint to signal its peer that it will send no
88: more data over a channel. Unfortunately, there is no symmetric way for
89: an endpoint to request that its peer should cease sending data to it
90: while still keeping the channel open for the endpoint to send data to
91: the peer.
92:
1.2 djm 93: This is desirable, since it saves the transmission of data that would
1.1 djm 94: otherwise need to be discarded and it allows an endpoint to signal local
95: processes of the condition, e.g. by closing the corresponding file
96: descriptor.
97:
98: OpenSSH implements a channel extension message to perform this
1.10 djm 99: signalling: "eow@openssh.com" (End Of Write). This message is sent by
100: an endpoint when the local output of a session channel is closed or
101: experiences a write error. The message is formatted as follows:
1.1 djm 102:
103: byte SSH_MSG_CHANNEL_REQUEST
104: uint32 recipient channel
105: string "eow@openssh.com"
106: boolean FALSE
107:
108: On receiving this message, the peer SHOULD cease sending data of
109: the channel and MAY signal the process from which the channel data
110: originates (e.g. by closing its read file descriptor).
111:
112: As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does
113: remain open after a "eow@openssh.com" has been sent and more data may
114: still be sent in the other direction. This message does not consume
115: window space and may be sent even if no window space is available.
116:
1.12 djm 117: NB. due to certain broken SSH implementations aborting upon receipt
118: of this message (in contravention of RFC4254 section 5.4), this
119: message is only sent to OpenSSH peers (identified by banner).
120: Other SSH implementations may be whitelisted to receive this message
121: upon request.
122:
1.16 djm 123: 2.2. connection: disallow additional sessions extension
124: "no-more-sessions@openssh.com"
1.6 djm 125:
126: Most SSH connections will only ever request a single session, but a
127: attacker may abuse a running ssh client to surreptitiously open
128: additional sessions under their control. OpenSSH provides a global
129: request "no-more-sessions@openssh.com" to mitigate this attack.
130:
131: When an OpenSSH client expects that it will never open another session
132: (i.e. it has been started with connection multiplexing disabled), it
133: will send the following global request:
134:
135: byte SSH_MSG_GLOBAL_REQUEST
136: string "no-more-sessions@openssh.com"
137: char want-reply
138:
139: On receipt of such a message, an OpenSSH server will refuse to open
140: future channels of type "session" and instead immediately abort the
141: connection.
142:
143: Note that this is not a general defence against compromised clients
144: (that is impossible), but it thwarts a simple attack.
145:
1.12 djm 146: NB. due to certain broken SSH implementations aborting upon receipt
147: of this message, the no-more-sessions request is only sent to OpenSSH
148: servers (identified by banner). Other SSH implementations may be
149: whitelisted to receive this message upon request.
150:
1.16 djm 151: 2.3. connection: Tunnel forward extension "tun@openssh.com"
1.7 djm 152:
1.8 djm 153: OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com"
1.7 djm 154: channel type. This channel type supports forwarding of network packets
1.8 djm 155: with datagram boundaries intact between endpoints equipped with
1.7 djm 156: interfaces like the BSD tun(4) device. Tunnel forwarding channels are
157: requested by the client with the following packet:
158:
159: byte SSH_MSG_CHANNEL_OPEN
160: string "tun@openssh.com"
161: uint32 sender channel
162: uint32 initial window size
163: uint32 maximum packet size
164: uint32 tunnel mode
165: uint32 remote unit number
166:
167: The "tunnel mode" parameter specifies whether the tunnel should forward
168: layer 2 frames or layer 3 packets. It may take one of the following values:
169:
170: SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */
171: SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */
172:
173: The "tunnel unit number" specifies the remote interface number, or may
1.13 djm 174: be 0x7fffffff to allow the server to automatically chose an interface. A
175: server that is not willing to open a client-specified unit should refuse
176: the request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful
177: open, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS.
1.7 djm 178:
179: Once established the client and server may exchange packet or frames
180: over the tunnel channel by encapsulating them in SSH protocol strings
181: and sending them as channel data. This ensures that packet boundaries
182: are kept intact. Specifically, packets are transmitted using normal
183: SSH_MSG_CHANNEL_DATA packets:
184:
185: byte SSH_MSG_CHANNEL_DATA
186: uint32 recipient channel
187: string data
188:
189: The contents of the "data" field for layer 3 packets is:
190:
191: uint32 packet length
192: uint32 address family
193: byte[packet length - 4] packet data
194:
195: The "address family" field identifies the type of packet in the message.
196: It may be one of:
197:
198: SSH_TUN_AF_INET 2 /* IPv4 */
199: SSH_TUN_AF_INET6 24 /* IPv6 */
200:
201: The "packet data" field consists of the IPv4/IPv6 datagram itself
202: without any link layer header.
203:
1.13 djm 204: The contents of the "data" field for layer 2 packets is:
1.7 djm 205:
206: uint32 packet length
207: byte[packet length] frame
208:
1.8 djm 209: The "frame" field contains an IEEE 802.3 Ethernet frame, including
1.7 djm 210: header.
211:
1.16 djm 212: 3. SFTP protocol changes
213:
214: 3.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK
1.1 djm 215:
216: When OpenSSH's sftp-server was implemented, the order of the arguments
1.8 djm 217: to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
1.1 djm 218: the reversal was not noticed until the server was widely deployed. Since
219: fixing this to follow the specification would cause incompatibility, the
220: current order was retained. For correct operation, clients should send
221: SSH_FXP_SYMLINK as follows:
222:
223: uint32 id
224: string targetpath
225: string linkpath
226:
1.16 djm 227: 3.2. sftp: Server extension announcement in SSH_FXP_VERSION
1.1 djm 228:
229: OpenSSH's sftp-server lists the extensions it supports using the
230: standard extension announcement mechanism in the SSH_FXP_VERSION server
231: hello packet:
232:
233: uint32 3 /* protocol version */
234: string ext1-name
235: string ext1-version
236: string ext2-name
237: string ext2-version
238: ...
239: string extN-name
240: string extN-version
241:
242: Each extension reports its integer version number as an ASCII encoded
243: string, e.g. "1". The version will be incremented if the extension is
244: ever changed in an incompatible way. The server MAY advertise the same
245: extension with multiple versions (though this is unlikely). Clients MUST
1.8 djm 246: check the version number before attempting to use the extension.
1.1 djm 247:
1.16 djm 248: 3.3. sftp: Extension request "posix-rename@openssh.com"
1.1 djm 249:
250: This operation provides a rename operation with POSIX semantics, which
251: are different to those provided by the standard SSH_FXP_RENAME in
252: draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
253: SSH_FXP_EXTENDED request with the following format:
254:
255: uint32 id
256: string "posix-rename@openssh.com"
257: string oldpath
258: string newpath
259:
260: On receiving this request the server will perform the POSIX operation
261: rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
262: This extension is advertised in the SSH_FXP_VERSION hello with version
263: "1".
264:
1.16 djm 265: 3.4. sftp: Extension requests "statvfs@openssh.com" and
1.2 djm 266: "fstatvfs@openssh.com"
1.1 djm 267:
268: These requests correspond to the statvfs and fstatvfs POSIX system
269: interfaces. The "statvfs@openssh.com" request operates on an explicit
270: pathname, and is formatted as follows:
271:
272: uint32 id
273: string "statvfs@openssh.com"
274: string path
275:
1.8 djm 276: The "fstatvfs@openssh.com" operates on an open file handle:
1.1 djm 277:
278: uint32 id
1.2 djm 279: string "fstatvfs@openssh.com"
1.1 djm 280: string handle
281:
282: These requests return a SSH_FXP_STATUS reply on failure. On success they
283: return the following SSH_FXP_EXTENDED_REPLY reply:
284:
285: uint32 id
1.4 dtucker 286: uint64 f_bsize /* file system block size */
287: uint64 f_frsize /* fundamental fs block size */
1.1 djm 288: uint64 f_blocks /* number of blocks (unit f_frsize) */
289: uint64 f_bfree /* free blocks in file system */
290: uint64 f_bavail /* free blocks for non-root */
291: uint64 f_files /* total file inodes */
292: uint64 f_ffree /* free file inodes */
293: uint64 f_favail /* free file inodes for to non-root */
1.3 djm 294: uint64 f_fsid /* file system id */
1.4 dtucker 295: uint64 f_flag /* bit mask of f_flag values */
296: uint64 f_namemax /* maximum filename length */
1.1 djm 297:
298: The values of the f_flag bitmask are as follows:
299:
300: #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
301: #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
302:
1.11 djm 303: Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are
304: advertised in the SSH_FXP_VERSION hello with version "2".
1.3 djm 305:
1.17 djm 306: 10. sftp: Extension request "hardlink@openssh.com"
307:
308: This request is for creating a hard link to a regular file. This
309: request is implemented as a SSH_FXP_EXTENDED request with the
310: following format:
311:
312: uint32 id
313: string "hardlink@openssh.com"
314: string oldpath
315: string newpath
316:
317: On receiving this request the server will perform the operation
318: link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
319: This extension is advertised in the SSH_FXP_VERSION hello with version
320: "1".
321:
1.19 ! djm 322: $OpenBSD: PROTOCOL,v 1.18 2012/12/11 22:31:18 markus Exp $
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