Put ssh back into the repository
Change-Id: I23324372188fa6ed3f93a32b84365f5df6367590
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+This documents OpenSSH's deviations and extensions to the published SSH
+protocol.
+
+Note that OpenSSH's sftp and sftp-server implement revision 3 of the SSH
+filexfer protocol described in:
+
+http://www.openssh.com/txt/draft-ietf-secsh-filexfer-02.txt
+
+Newer versions of the draft will not be supported, though some features
+are individually implemented as extensions described below.
+
+The protocol used by OpenSSH's ssh-agent is described in the file
+PROTOCOL.agent
+
+1. Transport protocol changes
+
+1.1. transport: Protocol 2 MAC algorithm "umac-64@openssh.com"
+
+This is a new transport-layer MAC method using the UMAC algorithm
+(rfc4418). This method is identical to the "umac-64" method documented
+in:
+
+http://www.openssh.com/txt/draft-miller-secsh-umac-01.txt
+
+1.2. transport: Protocol 2 compression algorithm "zlib@openssh.com"
+
+This transport-layer compression method uses the zlib compression
+algorithm (identical to the "zlib" method in rfc4253), but delays the
+start of compression until after authentication has completed. This
+avoids exposing compression code to attacks from unauthenticated users.
+
+The method is documented in:
+
+http://www.openssh.com/txt/draft-miller-secsh-compression-delayed-00.txt
+
+1.3. transport: New public key algorithms "ssh-rsa-cert-v00@openssh.com",
+ "ssh-dsa-cert-v00@openssh.com",
+ "ecdsa-sha2-nistp256-cert-v01@openssh.com",
+ "ecdsa-sha2-nistp384-cert-v01@openssh.com" and
+ "ecdsa-sha2-nistp521-cert-v01@openssh.com"
+
+OpenSSH introduces new public key algorithms to support certificate
+authentication for users and hostkeys. These methods are documented in
+the file PROTOCOL.certkeys
+
+1.4. transport: Elliptic Curve cryptography
+
+OpenSSH supports ECC key exchange and public key authentication as
+specified in RFC5656. Only the ecdsa-sha2-nistp256, ecdsa-sha2-nistp384
+and ecdsa-sha2-nistp521 curves over GF(p) are supported. Elliptic
+curve points encoded using point compression are NOT accepted or
+generated.
+
+2. Connection protocol changes
+
+2.1. connection: Channel write close extension "eow@openssh.com"
+
+The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF
+message to allow an endpoint to signal its peer that it will send no
+more data over a channel. Unfortunately, there is no symmetric way for
+an endpoint to request that its peer should cease sending data to it
+while still keeping the channel open for the endpoint to send data to
+the peer.
+
+This is desirable, since it saves the transmission of data that would
+otherwise need to be discarded and it allows an endpoint to signal local
+processes of the condition, e.g. by closing the corresponding file
+descriptor.
+
+OpenSSH implements a channel extension message to perform this
+signalling: "eow@openssh.com" (End Of Write). This message is sent by
+an endpoint when the local output of a session channel is closed or
+experiences a write error. The message is formatted as follows:
+
+ byte SSH_MSG_CHANNEL_REQUEST
+ uint32 recipient channel
+ string "eow@openssh.com"
+ boolean FALSE
+
+On receiving this message, the peer SHOULD cease sending data of
+the channel and MAY signal the process from which the channel data
+originates (e.g. by closing its read file descriptor).
+
+As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does
+remain open after a "eow@openssh.com" has been sent and more data may
+still be sent in the other direction. This message does not consume
+window space and may be sent even if no window space is available.
+
+NB. due to certain broken SSH implementations aborting upon receipt
+of this message (in contravention of RFC4254 section 5.4), this
+message is only sent to OpenSSH peers (identified by banner).
+Other SSH implementations may be whitelisted to receive this message
+upon request.
+
+2.2. connection: disallow additional sessions extension
+ "no-more-sessions@openssh.com"
+
+Most SSH connections will only ever request a single session, but a
+attacker may abuse a running ssh client to surreptitiously open
+additional sessions under their control. OpenSSH provides a global
+request "no-more-sessions@openssh.com" to mitigate this attack.
+
+When an OpenSSH client expects that it will never open another session
+(i.e. it has been started with connection multiplexing disabled), it
+will send the following global request:
+
+ byte SSH_MSG_GLOBAL_REQUEST
+ string "no-more-sessions@openssh.com"
+ char want-reply
+
+On receipt of such a message, an OpenSSH server will refuse to open
+future channels of type "session" and instead immediately abort the
+connection.
+
+Note that this is not a general defence against compromised clients
+(that is impossible), but it thwarts a simple attack.
+
+NB. due to certain broken SSH implementations aborting upon receipt
+of this message, the no-more-sessions request is only sent to OpenSSH
+servers (identified by banner). Other SSH implementations may be
+whitelisted to receive this message upon request.
+
+2.3. connection: Tunnel forward extension "tun@openssh.com"
+
+OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com"
+channel type. This channel type supports forwarding of network packets
+with datagram boundaries intact between endpoints equipped with
+interfaces like the BSD tun(4) device. Tunnel forwarding channels are
+requested by the client with the following packet:
+
+ byte SSH_MSG_CHANNEL_OPEN
+ string "tun@openssh.com"
+ uint32 sender channel
+ uint32 initial window size
+ uint32 maximum packet size
+ uint32 tunnel mode
+ uint32 remote unit number
+
+The "tunnel mode" parameter specifies whether the tunnel should forward
+layer 2 frames or layer 3 packets. It may take one of the following values:
+
+ SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */
+ SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */
+
+The "tunnel unit number" specifies the remote interface number, or may
+be 0x7fffffff to allow the server to automatically chose an interface. A
+server that is not willing to open a client-specified unit should refuse
+the request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful
+open, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS.
+
+Once established the client and server may exchange packet or frames
+over the tunnel channel by encapsulating them in SSH protocol strings
+and sending them as channel data. This ensures that packet boundaries
+are kept intact. Specifically, packets are transmitted using normal
+SSH_MSG_CHANNEL_DATA packets:
+
+ byte SSH_MSG_CHANNEL_DATA
+ uint32 recipient channel
+ string data
+
+The contents of the "data" field for layer 3 packets is:
+
+ uint32 packet length
+ uint32 address family
+ byte[packet length - 4] packet data
+
+The "address family" field identifies the type of packet in the message.
+It may be one of:
+
+ SSH_TUN_AF_INET 2 /* IPv4 */
+ SSH_TUN_AF_INET6 24 /* IPv6 */
+
+The "packet data" field consists of the IPv4/IPv6 datagram itself
+without any link layer header.
+
+The contents of the "data" field for layer 2 packets is:
+
+ uint32 packet length
+ byte[packet length] frame
+
+The "frame" field contains an IEEE 802.3 Ethernet frame, including
+header.
+
+3. SFTP protocol changes
+
+3.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK
+
+When OpenSSH's sftp-server was implemented, the order of the arguments
+to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
+the reversal was not noticed until the server was widely deployed. Since
+fixing this to follow the specification would cause incompatibility, the
+current order was retained. For correct operation, clients should send
+SSH_FXP_SYMLINK as follows:
+
+ uint32 id
+ string targetpath
+ string linkpath
+
+3.2. sftp: Server extension announcement in SSH_FXP_VERSION
+
+OpenSSH's sftp-server lists the extensions it supports using the
+standard extension announcement mechanism in the SSH_FXP_VERSION server
+hello packet:
+
+ uint32 3 /* protocol version */
+ string ext1-name
+ string ext1-version
+ string ext2-name
+ string ext2-version
+ ...
+ string extN-name
+ string extN-version
+
+Each extension reports its integer version number as an ASCII encoded
+string, e.g. "1". The version will be incremented if the extension is
+ever changed in an incompatible way. The server MAY advertise the same
+extension with multiple versions (though this is unlikely). Clients MUST
+check the version number before attempting to use the extension.
+
+3.3. sftp: Extension request "posix-rename@openssh.com"
+
+This operation provides a rename operation with POSIX semantics, which
+are different to those provided by the standard SSH_FXP_RENAME in
+draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
+SSH_FXP_EXTENDED request with the following format:
+
+ uint32 id
+ string "posix-rename@openssh.com"
+ string oldpath
+ string newpath
+
+On receiving this request the server will perform the POSIX operation
+rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
+This extension is advertised in the SSH_FXP_VERSION hello with version
+"1".
+
+3.4. sftp: Extension requests "statvfs@openssh.com" and
+ "fstatvfs@openssh.com"
+
+These requests correspond to the statvfs and fstatvfs POSIX system
+interfaces. The "statvfs@openssh.com" request operates on an explicit
+pathname, and is formatted as follows:
+
+ uint32 id
+ string "statvfs@openssh.com"
+ string path
+
+The "fstatvfs@openssh.com" operates on an open file handle:
+
+ uint32 id
+ string "fstatvfs@openssh.com"
+ string handle
+
+These requests return a SSH_FXP_STATUS reply on failure. On success they
+return the following SSH_FXP_EXTENDED_REPLY reply:
+
+ uint32 id
+ uint64 f_bsize /* file system block size */
+ uint64 f_frsize /* fundamental fs block size */
+ uint64 f_blocks /* number of blocks (unit f_frsize) */
+ uint64 f_bfree /* free blocks in file system */
+ uint64 f_bavail /* free blocks for non-root */
+ uint64 f_files /* total file inodes */
+ uint64 f_ffree /* free file inodes */
+ uint64 f_favail /* free file inodes for to non-root */
+ uint64 f_fsid /* file system id */
+ uint64 f_flag /* bit mask of f_flag values */
+ uint64 f_namemax /* maximum filename length */
+
+The values of the f_flag bitmask are as follows:
+
+ #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
+ #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
+
+Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are
+advertised in the SSH_FXP_VERSION hello with version "2".
+
+10. sftp: Extension request "hardlink@openssh.com"
+
+This request is for creating a hard link to a regular file. This
+request is implemented as a SSH_FXP_EXTENDED request with the
+following format:
+
+ uint32 id
+ string "hardlink@openssh.com"
+ string oldpath
+ string newpath
+
+On receiving this request the server will perform the operation
+link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
+This extension is advertised in the SSH_FXP_VERSION hello with version
+"1".
+
+$OpenBSD: PROTOCOL,v 1.17 2010/12/04 00:18:01 djm Exp $