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Annotation of src/usr.bin/ssh/PROTOCOL, Revision 1.24

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:
1.20 markus 82: 1.6 transport: AES-GCM
83:
84: OpenSSH supports the AES-GCM algorithm as specified in RFC 5647.
85: Because of problems with the specification of the key exchange
86: the behaviour of OpenSSH differs from the RFC as follows:
87:
88: AES-GCM is only negotiated as the cipher algorithms
89: "aes128-gcm@openssh.com" or "aes256-gcm@openssh.com" and never as
90: an MAC algorithm. Additionally, if AES-GCM is selected as the cipher
91: the exchanged MAC algorithms are ignored and there doesn't have to be
92: a matching MAC.
93:
1.22 djm 94: 1.7 transport: chacha20-poly1305@openssh.com authenticated encryption
95:
96: OpenSSH supports authenticated encryption using ChaCha20 and Poly1305
97: as described in PROTOCOL.chacha20poly1305.
98:
1.23 djm 99: 1.8 transport: curve25519-sha256@libssh.org key exchange algorithm
100:
101: OpenSSH supports the use of ECDH in Curve25519 for key exchange as
102: described at:
103: http://git.libssh.org/users/aris/libssh.git/plain/doc/curve25519-sha256@libssh.org.txt?h=curve25519
104:
1.16 djm 105: 2. Connection protocol changes
106:
107: 2.1. connection: Channel write close extension "eow@openssh.com"
1.1 djm 108:
109: The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF
110: message to allow an endpoint to signal its peer that it will send no
111: more data over a channel. Unfortunately, there is no symmetric way for
112: an endpoint to request that its peer should cease sending data to it
113: while still keeping the channel open for the endpoint to send data to
114: the peer.
115:
1.2 djm 116: This is desirable, since it saves the transmission of data that would
1.1 djm 117: otherwise need to be discarded and it allows an endpoint to signal local
118: processes of the condition, e.g. by closing the corresponding file
119: descriptor.
120:
121: OpenSSH implements a channel extension message to perform this
1.10 djm 122: signalling: "eow@openssh.com" (End Of Write). This message is sent by
123: an endpoint when the local output of a session channel is closed or
124: experiences a write error. The message is formatted as follows:
1.1 djm 125:
126: byte SSH_MSG_CHANNEL_REQUEST
127: uint32 recipient channel
128: string "eow@openssh.com"
129: boolean FALSE
130:
131: On receiving this message, the peer SHOULD cease sending data of
132: the channel and MAY signal the process from which the channel data
133: originates (e.g. by closing its read file descriptor).
134:
135: As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does
136: remain open after a "eow@openssh.com" has been sent and more data may
137: still be sent in the other direction. This message does not consume
138: window space and may be sent even if no window space is available.
139:
1.12 djm 140: NB. due to certain broken SSH implementations aborting upon receipt
141: of this message (in contravention of RFC4254 section 5.4), this
142: message is only sent to OpenSSH peers (identified by banner).
143: Other SSH implementations may be whitelisted to receive this message
144: upon request.
145:
1.16 djm 146: 2.2. connection: disallow additional sessions extension
147: "no-more-sessions@openssh.com"
1.6 djm 148:
149: Most SSH connections will only ever request a single session, but a
150: attacker may abuse a running ssh client to surreptitiously open
151: additional sessions under their control. OpenSSH provides a global
152: request "no-more-sessions@openssh.com" to mitigate this attack.
153:
154: When an OpenSSH client expects that it will never open another session
155: (i.e. it has been started with connection multiplexing disabled), it
156: will send the following global request:
157:
158: byte SSH_MSG_GLOBAL_REQUEST
159: string "no-more-sessions@openssh.com"
160: char want-reply
161:
162: On receipt of such a message, an OpenSSH server will refuse to open
163: future channels of type "session" and instead immediately abort the
164: connection.
165:
166: Note that this is not a general defence against compromised clients
167: (that is impossible), but it thwarts a simple attack.
168:
1.12 djm 169: NB. due to certain broken SSH implementations aborting upon receipt
170: of this message, the no-more-sessions request is only sent to OpenSSH
171: servers (identified by banner). Other SSH implementations may be
172: whitelisted to receive this message upon request.
173:
1.16 djm 174: 2.3. connection: Tunnel forward extension "tun@openssh.com"
1.7 djm 175:
1.8 djm 176: OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com"
1.7 djm 177: channel type. This channel type supports forwarding of network packets
1.8 djm 178: with datagram boundaries intact between endpoints equipped with
1.7 djm 179: interfaces like the BSD tun(4) device. Tunnel forwarding channels are
180: requested by the client with the following packet:
181:
182: byte SSH_MSG_CHANNEL_OPEN
183: string "tun@openssh.com"
184: uint32 sender channel
185: uint32 initial window size
186: uint32 maximum packet size
187: uint32 tunnel mode
188: uint32 remote unit number
189:
190: The "tunnel mode" parameter specifies whether the tunnel should forward
191: layer 2 frames or layer 3 packets. It may take one of the following values:
192:
193: SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */
194: SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */
195:
196: The "tunnel unit number" specifies the remote interface number, or may
1.13 djm 197: be 0x7fffffff to allow the server to automatically chose an interface. A
198: server that is not willing to open a client-specified unit should refuse
199: the request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful
200: open, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS.
1.7 djm 201:
202: Once established the client and server may exchange packet or frames
203: over the tunnel channel by encapsulating them in SSH protocol strings
204: and sending them as channel data. This ensures that packet boundaries
205: are kept intact. Specifically, packets are transmitted using normal
206: SSH_MSG_CHANNEL_DATA packets:
207:
208: byte SSH_MSG_CHANNEL_DATA
209: uint32 recipient channel
210: string data
211:
212: The contents of the "data" field for layer 3 packets is:
213:
214: uint32 packet length
215: uint32 address family
216: byte[packet length - 4] packet data
217:
218: The "address family" field identifies the type of packet in the message.
219: It may be one of:
220:
221: SSH_TUN_AF_INET 2 /* IPv4 */
222: SSH_TUN_AF_INET6 24 /* IPv6 */
223:
224: The "packet data" field consists of the IPv4/IPv6 datagram itself
225: without any link layer header.
226:
1.13 djm 227: The contents of the "data" field for layer 2 packets is:
1.7 djm 228:
229: uint32 packet length
230: byte[packet length] frame
231:
1.8 djm 232: The "frame" field contains an IEEE 802.3 Ethernet frame, including
1.7 djm 233: header.
234:
1.24 ! millert 235: 2.4. connection: Unix domain socket forwarding
! 236:
! 237: OpenSSH supports local and remote Unix domain socket forwarding
! 238: using the "streamlocal" extension. Forwarding is initiated as per
! 239: TCP sockets but with a single path instead of a host and port.
! 240:
! 241: Similar to direct-tcpip, direct-streamlocal is sent by the client
! 242: to request that the server make a connection to a Unix domain socket.
! 243:
! 244: byte SSH_MSG_CHANNEL_OPEN
! 245: string "direct-streamlocal@openssh.com"
! 246: uint32 sender channel
! 247: uint32 initial window size
! 248: uint32 maximum packet size
! 249: string socket path
! 250: string reserved for future use
! 251:
! 252: Similar to forwarded-tcpip, forwarded-streamlocal is sent by the
! 253: server when the client has previously send the server a streamlocal-forward
! 254: GLOBAL_REQUEST.
! 255:
! 256: byte SSH_MSG_CHANNEL_OPEN
! 257: string "forwarded-streamlocal@openssh.com"
! 258: uint32 sender channel
! 259: uint32 initial window size
! 260: uint32 maximum packet size
! 261: string socket path
! 262: string reserved for future use
! 263:
! 264: The reserved field is not currently defined and is ignored on the
! 265: remote end. It is intended to be used in the future to pass
! 266: information about the socket file, such as ownership and mode.
! 267: The client currently sends the empty string for this field.
! 268:
! 269: Similar to tcpip-forward, streamlocal-forward is sent by the client
! 270: to request remote forwarding of a Unix domain socket.
! 271:
! 272: byte SSH2_MSG_GLOBAL_REQUEST
! 273: string "streamlocal-forward@openssh.com"
! 274: boolean TRUE
! 275: string socket path
! 276:
! 277: Similar to cancel-tcpip-forward, cancel-streamlocal-forward is sent
! 278: by the client cancel the forwarding of a Unix domain socket.
! 279:
! 280: byte SSH2_MSG_GLOBAL_REQUEST
! 281: string "cancel-streamlocal-forward@openssh.com"
! 282: boolean FALSE
! 283: string socket path
! 284:
1.16 djm 285: 3. SFTP protocol changes
286:
287: 3.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK
1.1 djm 288:
289: When OpenSSH's sftp-server was implemented, the order of the arguments
1.8 djm 290: to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
1.1 djm 291: the reversal was not noticed until the server was widely deployed. Since
292: fixing this to follow the specification would cause incompatibility, the
293: current order was retained. For correct operation, clients should send
294: SSH_FXP_SYMLINK as follows:
295:
296: uint32 id
297: string targetpath
298: string linkpath
299:
1.16 djm 300: 3.2. sftp: Server extension announcement in SSH_FXP_VERSION
1.1 djm 301:
302: OpenSSH's sftp-server lists the extensions it supports using the
303: standard extension announcement mechanism in the SSH_FXP_VERSION server
304: hello packet:
305:
306: uint32 3 /* protocol version */
307: string ext1-name
308: string ext1-version
309: string ext2-name
310: string ext2-version
311: ...
312: string extN-name
313: string extN-version
314:
315: Each extension reports its integer version number as an ASCII encoded
316: string, e.g. "1". The version will be incremented if the extension is
317: ever changed in an incompatible way. The server MAY advertise the same
318: extension with multiple versions (though this is unlikely). Clients MUST
1.8 djm 319: check the version number before attempting to use the extension.
1.1 djm 320:
1.16 djm 321: 3.3. sftp: Extension request "posix-rename@openssh.com"
1.1 djm 322:
323: This operation provides a rename operation with POSIX semantics, which
324: are different to those provided by the standard SSH_FXP_RENAME in
325: draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
326: SSH_FXP_EXTENDED request with the following format:
327:
328: uint32 id
329: string "posix-rename@openssh.com"
330: string oldpath
331: string newpath
332:
333: On receiving this request the server will perform the POSIX operation
334: rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
335: This extension is advertised in the SSH_FXP_VERSION hello with version
336: "1".
337:
1.16 djm 338: 3.4. sftp: Extension requests "statvfs@openssh.com" and
1.2 djm 339: "fstatvfs@openssh.com"
1.1 djm 340:
341: These requests correspond to the statvfs and fstatvfs POSIX system
342: interfaces. The "statvfs@openssh.com" request operates on an explicit
343: pathname, and is formatted as follows:
344:
345: uint32 id
346: string "statvfs@openssh.com"
347: string path
348:
1.8 djm 349: The "fstatvfs@openssh.com" operates on an open file handle:
1.1 djm 350:
351: uint32 id
1.2 djm 352: string "fstatvfs@openssh.com"
1.1 djm 353: string handle
354:
355: These requests return a SSH_FXP_STATUS reply on failure. On success they
356: return the following SSH_FXP_EXTENDED_REPLY reply:
357:
358: uint32 id
1.4 dtucker 359: uint64 f_bsize /* file system block size */
360: uint64 f_frsize /* fundamental fs block size */
1.1 djm 361: uint64 f_blocks /* number of blocks (unit f_frsize) */
362: uint64 f_bfree /* free blocks in file system */
363: uint64 f_bavail /* free blocks for non-root */
364: uint64 f_files /* total file inodes */
365: uint64 f_ffree /* free file inodes */
366: uint64 f_favail /* free file inodes for to non-root */
1.3 djm 367: uint64 f_fsid /* file system id */
1.4 dtucker 368: uint64 f_flag /* bit mask of f_flag values */
369: uint64 f_namemax /* maximum filename length */
1.1 djm 370:
371: The values of the f_flag bitmask are as follows:
372:
373: #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
374: #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
375:
1.11 djm 376: Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are
377: advertised in the SSH_FXP_VERSION hello with version "2".
1.3 djm 378:
1.17 djm 379: 10. sftp: Extension request "hardlink@openssh.com"
380:
381: This request is for creating a hard link to a regular file. This
382: request is implemented as a SSH_FXP_EXTENDED request with the
383: following format:
384:
385: uint32 id
386: string "hardlink@openssh.com"
387: string oldpath
388: string newpath
389:
390: On receiving this request the server will perform the operation
391: link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
392: This extension is advertised in the SSH_FXP_VERSION hello with version
393: "1".
394:
1.21 djm 395: 10. sftp: Extension request "fsync@openssh.com"
396:
397: This request asks the server to call fsync(2) on an open file handle.
398:
399: uint32 id
400: string "fsync@openssh.com"
401: string handle
402:
403: One receiving this request, a server will call fsync(handle_fd) and will
404: respond with a SSH_FXP_STATUS message.
405:
406: This extension is advertised in the SSH_FXP_VERSION hello with version
407: "1".
408:
1.24 ! millert 409: $OpenBSD: PROTOCOL,v 1.23 2013/12/01 23:19:05 djm Exp $