Annotation of src/usr.bin/ssh/PROTOCOL, Revision 1.18
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:
! 73: mac = MAC(key, sequence_number || encrypted_packet)
! 74:
! 75: where "encrypted_packet" contains:
! 76:
! 77: byte padding_length
! 78: byte[n1] payload; n1 = packet_length - padding_length - 1
! 79: byte[n2] random padding; n2 = padding_length
! 80:
1.16 djm 81: 2. Connection protocol changes
82:
83: 2.1. connection: Channel write close extension "eow@openssh.com"
1.1 djm 84:
85: The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF
86: message to allow an endpoint to signal its peer that it will send no
87: more data over a channel. Unfortunately, there is no symmetric way for
88: an endpoint to request that its peer should cease sending data to it
89: while still keeping the channel open for the endpoint to send data to
90: the peer.
91:
1.2 djm 92: This is desirable, since it saves the transmission of data that would
1.1 djm 93: otherwise need to be discarded and it allows an endpoint to signal local
94: processes of the condition, e.g. by closing the corresponding file
95: descriptor.
96:
97: OpenSSH implements a channel extension message to perform this
1.10 djm 98: signalling: "eow@openssh.com" (End Of Write). This message is sent by
99: an endpoint when the local output of a session channel is closed or
100: experiences a write error. The message is formatted as follows:
1.1 djm 101:
102: byte SSH_MSG_CHANNEL_REQUEST
103: uint32 recipient channel
104: string "eow@openssh.com"
105: boolean FALSE
106:
107: On receiving this message, the peer SHOULD cease sending data of
108: the channel and MAY signal the process from which the channel data
109: originates (e.g. by closing its read file descriptor).
110:
111: As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does
112: remain open after a "eow@openssh.com" has been sent and more data may
113: still be sent in the other direction. This message does not consume
114: window space and may be sent even if no window space is available.
115:
1.12 djm 116: NB. due to certain broken SSH implementations aborting upon receipt
117: of this message (in contravention of RFC4254 section 5.4), this
118: message is only sent to OpenSSH peers (identified by banner).
119: Other SSH implementations may be whitelisted to receive this message
120: upon request.
121:
1.16 djm 122: 2.2. connection: disallow additional sessions extension
123: "no-more-sessions@openssh.com"
1.6 djm 124:
125: Most SSH connections will only ever request a single session, but a
126: attacker may abuse a running ssh client to surreptitiously open
127: additional sessions under their control. OpenSSH provides a global
128: request "no-more-sessions@openssh.com" to mitigate this attack.
129:
130: When an OpenSSH client expects that it will never open another session
131: (i.e. it has been started with connection multiplexing disabled), it
132: will send the following global request:
133:
134: byte SSH_MSG_GLOBAL_REQUEST
135: string "no-more-sessions@openssh.com"
136: char want-reply
137:
138: On receipt of such a message, an OpenSSH server will refuse to open
139: future channels of type "session" and instead immediately abort the
140: connection.
141:
142: Note that this is not a general defence against compromised clients
143: (that is impossible), but it thwarts a simple attack.
144:
1.12 djm 145: NB. due to certain broken SSH implementations aborting upon receipt
146: of this message, the no-more-sessions request is only sent to OpenSSH
147: servers (identified by banner). Other SSH implementations may be
148: whitelisted to receive this message upon request.
149:
1.16 djm 150: 2.3. connection: Tunnel forward extension "tun@openssh.com"
1.7 djm 151:
1.8 djm 152: OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com"
1.7 djm 153: channel type. This channel type supports forwarding of network packets
1.8 djm 154: with datagram boundaries intact between endpoints equipped with
1.7 djm 155: interfaces like the BSD tun(4) device. Tunnel forwarding channels are
156: requested by the client with the following packet:
157:
158: byte SSH_MSG_CHANNEL_OPEN
159: string "tun@openssh.com"
160: uint32 sender channel
161: uint32 initial window size
162: uint32 maximum packet size
163: uint32 tunnel mode
164: uint32 remote unit number
165:
166: The "tunnel mode" parameter specifies whether the tunnel should forward
167: layer 2 frames or layer 3 packets. It may take one of the following values:
168:
169: SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */
170: SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */
171:
172: The "tunnel unit number" specifies the remote interface number, or may
1.13 djm 173: be 0x7fffffff to allow the server to automatically chose an interface. A
174: server that is not willing to open a client-specified unit should refuse
175: the request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful
176: open, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS.
1.7 djm 177:
178: Once established the client and server may exchange packet or frames
179: over the tunnel channel by encapsulating them in SSH protocol strings
180: and sending them as channel data. This ensures that packet boundaries
181: are kept intact. Specifically, packets are transmitted using normal
182: SSH_MSG_CHANNEL_DATA packets:
183:
184: byte SSH_MSG_CHANNEL_DATA
185: uint32 recipient channel
186: string data
187:
188: The contents of the "data" field for layer 3 packets is:
189:
190: uint32 packet length
191: uint32 address family
192: byte[packet length - 4] packet data
193:
194: The "address family" field identifies the type of packet in the message.
195: It may be one of:
196:
197: SSH_TUN_AF_INET 2 /* IPv4 */
198: SSH_TUN_AF_INET6 24 /* IPv6 */
199:
200: The "packet data" field consists of the IPv4/IPv6 datagram itself
201: without any link layer header.
202:
1.13 djm 203: The contents of the "data" field for layer 2 packets is:
1.7 djm 204:
205: uint32 packet length
206: byte[packet length] frame
207:
1.8 djm 208: The "frame" field contains an IEEE 802.3 Ethernet frame, including
1.7 djm 209: header.
210:
1.16 djm 211: 3. SFTP protocol changes
212:
213: 3.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK
1.1 djm 214:
215: When OpenSSH's sftp-server was implemented, the order of the arguments
1.8 djm 216: to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
1.1 djm 217: the reversal was not noticed until the server was widely deployed. Since
218: fixing this to follow the specification would cause incompatibility, the
219: current order was retained. For correct operation, clients should send
220: SSH_FXP_SYMLINK as follows:
221:
222: uint32 id
223: string targetpath
224: string linkpath
225:
1.16 djm 226: 3.2. sftp: Server extension announcement in SSH_FXP_VERSION
1.1 djm 227:
228: OpenSSH's sftp-server lists the extensions it supports using the
229: standard extension announcement mechanism in the SSH_FXP_VERSION server
230: hello packet:
231:
232: uint32 3 /* protocol version */
233: string ext1-name
234: string ext1-version
235: string ext2-name
236: string ext2-version
237: ...
238: string extN-name
239: string extN-version
240:
241: Each extension reports its integer version number as an ASCII encoded
242: string, e.g. "1". The version will be incremented if the extension is
243: ever changed in an incompatible way. The server MAY advertise the same
244: extension with multiple versions (though this is unlikely). Clients MUST
1.8 djm 245: check the version number before attempting to use the extension.
1.1 djm 246:
1.16 djm 247: 3.3. sftp: Extension request "posix-rename@openssh.com"
1.1 djm 248:
249: This operation provides a rename operation with POSIX semantics, which
250: are different to those provided by the standard SSH_FXP_RENAME in
251: draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
252: SSH_FXP_EXTENDED request with the following format:
253:
254: uint32 id
255: string "posix-rename@openssh.com"
256: string oldpath
257: string newpath
258:
259: On receiving this request the server will perform the POSIX operation
260: rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
261: This extension is advertised in the SSH_FXP_VERSION hello with version
262: "1".
263:
1.16 djm 264: 3.4. sftp: Extension requests "statvfs@openssh.com" and
1.2 djm 265: "fstatvfs@openssh.com"
1.1 djm 266:
267: These requests correspond to the statvfs and fstatvfs POSIX system
268: interfaces. The "statvfs@openssh.com" request operates on an explicit
269: pathname, and is formatted as follows:
270:
271: uint32 id
272: string "statvfs@openssh.com"
273: string path
274:
1.8 djm 275: The "fstatvfs@openssh.com" operates on an open file handle:
1.1 djm 276:
277: uint32 id
1.2 djm 278: string "fstatvfs@openssh.com"
1.1 djm 279: string handle
280:
281: These requests return a SSH_FXP_STATUS reply on failure. On success they
282: return the following SSH_FXP_EXTENDED_REPLY reply:
283:
284: uint32 id
1.4 dtucker 285: uint64 f_bsize /* file system block size */
286: uint64 f_frsize /* fundamental fs block size */
1.1 djm 287: uint64 f_blocks /* number of blocks (unit f_frsize) */
288: uint64 f_bfree /* free blocks in file system */
289: uint64 f_bavail /* free blocks for non-root */
290: uint64 f_files /* total file inodes */
291: uint64 f_ffree /* free file inodes */
292: uint64 f_favail /* free file inodes for to non-root */
1.3 djm 293: uint64 f_fsid /* file system id */
1.4 dtucker 294: uint64 f_flag /* bit mask of f_flag values */
295: uint64 f_namemax /* maximum filename length */
1.1 djm 296:
297: The values of the f_flag bitmask are as follows:
298:
299: #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
300: #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
301:
1.11 djm 302: Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are
303: advertised in the SSH_FXP_VERSION hello with version "2".
1.3 djm 304:
1.17 djm 305: 10. sftp: Extension request "hardlink@openssh.com"
306:
307: This request is for creating a hard link to a regular file. This
308: request is implemented as a SSH_FXP_EXTENDED request with the
309: following format:
310:
311: uint32 id
312: string "hardlink@openssh.com"
313: string oldpath
314: string newpath
315:
316: On receiving this request the server will perform the operation
317: link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
318: This extension is advertised in the SSH_FXP_VERSION hello with version
319: "1".
320:
1.18 ! markus 321: $OpenBSD: PROTOCOL,v 1.17 2010/12/04 00:18:01 djm Exp $
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