335 lines
12 KiB
Plaintext
335 lines
12 KiB
Plaintext
This documents OpenSSH's deviations and extensions to the published SSH
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protocol.
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Note that OpenSSH's sftp and sftp-server implement revision 3 of the SSH
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filexfer protocol described in:
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http://www.openssh.com/txt/draft-ietf-secsh-filexfer-02.txt
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Newer versions of the draft will not be supported, though some features
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are individually implemented as extensions described below.
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The protocol used by OpenSSH's ssh-agent is described in the file
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PROTOCOL.agent
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1. Transport protocol changes
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1.1. transport: Protocol 2 MAC algorithm "umac-64@openssh.com"
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This is a new transport-layer MAC method using the UMAC algorithm
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(rfc4418). This method is identical to the "umac-64" method documented
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in:
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http://www.openssh.com/txt/draft-miller-secsh-umac-01.txt
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1.2. transport: Protocol 2 compression algorithm "zlib@openssh.com"
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This transport-layer compression method uses the zlib compression
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algorithm (identical to the "zlib" method in rfc4253), but delays the
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start of compression until after authentication has completed. This
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avoids exposing compression code to attacks from unauthenticated users.
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The method is documented in:
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http://www.openssh.com/txt/draft-miller-secsh-compression-delayed-00.txt
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1.3. transport: New public key algorithms "ssh-rsa-cert-v00@openssh.com",
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"ssh-dsa-cert-v00@openssh.com",
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"ecdsa-sha2-nistp256-cert-v01@openssh.com",
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"ecdsa-sha2-nistp384-cert-v01@openssh.com" and
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"ecdsa-sha2-nistp521-cert-v01@openssh.com"
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OpenSSH introduces new public key algorithms to support certificate
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authentication for users and hostkeys. These methods are documented in
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the file PROTOCOL.certkeys
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1.4. transport: Elliptic Curve cryptography
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OpenSSH supports ECC key exchange and public key authentication as
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specified in RFC5656. Only the ecdsa-sha2-nistp256, ecdsa-sha2-nistp384
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and ecdsa-sha2-nistp521 curves over GF(p) are supported. Elliptic
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curve points encoded using point compression are NOT accepted or
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generated.
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1.5 transport: Protocol 2 Encrypt-then-MAC MAC algorithms
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OpenSSH supports MAC algorithms, whose names contain "-etm", that
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perform the calculations in a different order to that defined in RFC
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4253. These variants use the so-called "encrypt then MAC" ordering,
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calculating the MAC over the packet ciphertext rather than the
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plaintext. This ordering closes a security flaw in the SSH transport
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protocol, where decryption of unauthenticated ciphertext provided a
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"decryption oracle" that could, in conjunction with cipher flaws, reveal
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session plaintext.
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Specifically, the "-etm" MAC algorithms modify the transport protocol
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to calculate the MAC over the packet ciphertext and to send the packet
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length unencrypted. This is necessary for the transport to obtain the
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length of the packet and location of the MAC tag so that it may be
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verified without decrypting unauthenticated data.
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As such, the MAC covers:
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mac = MAC(key, sequence_number || packet_length || encrypted_packet)
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where "packet_length" is encoded as a uint32 and "encrypted_packet"
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contains:
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byte padding_length
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byte[n1] payload; n1 = packet_length - padding_length - 1
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byte[n2] random padding; n2 = padding_length
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1.6 transport: AES-GCM
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OpenSSH supports the AES-GCM algorithm as specified in RFC 5647.
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Because of problems with the specification of the key exchange
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the behaviour of OpenSSH differs from the RFC as follows:
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AES-GCM is only negotiated as the cipher algorithms
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"aes128-gcm@openssh.com" or "aes256-gcm@openssh.com" and never as
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an MAC algorithm. Additionally, if AES-GCM is selected as the cipher
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the exchanged MAC algorithms are ignored and there doesn't have to be
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a matching MAC.
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2. Connection protocol changes
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2.1. connection: Channel write close extension "eow@openssh.com"
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The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF
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message to allow an endpoint to signal its peer that it will send no
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more data over a channel. Unfortunately, there is no symmetric way for
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an endpoint to request that its peer should cease sending data to it
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while still keeping the channel open for the endpoint to send data to
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the peer.
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This is desirable, since it saves the transmission of data that would
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otherwise need to be discarded and it allows an endpoint to signal local
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processes of the condition, e.g. by closing the corresponding file
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descriptor.
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OpenSSH implements a channel extension message to perform this
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signalling: "eow@openssh.com" (End Of Write). This message is sent by
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an endpoint when the local output of a session channel is closed or
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experiences a write error. The message is formatted as follows:
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byte SSH_MSG_CHANNEL_REQUEST
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uint32 recipient channel
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string "eow@openssh.com"
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boolean FALSE
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On receiving this message, the peer SHOULD cease sending data of
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the channel and MAY signal the process from which the channel data
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originates (e.g. by closing its read file descriptor).
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As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does
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remain open after a "eow@openssh.com" has been sent and more data may
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still be sent in the other direction. This message does not consume
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window space and may be sent even if no window space is available.
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NB. due to certain broken SSH implementations aborting upon receipt
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of this message (in contravention of RFC4254 section 5.4), this
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message is only sent to OpenSSH peers (identified by banner).
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Other SSH implementations may be whitelisted to receive this message
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upon request.
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2.2. connection: disallow additional sessions extension
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"no-more-sessions@openssh.com"
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Most SSH connections will only ever request a single session, but a
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attacker may abuse a running ssh client to surreptitiously open
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additional sessions under their control. OpenSSH provides a global
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request "no-more-sessions@openssh.com" to mitigate this attack.
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When an OpenSSH client expects that it will never open another session
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(i.e. it has been started with connection multiplexing disabled), it
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will send the following global request:
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byte SSH_MSG_GLOBAL_REQUEST
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string "no-more-sessions@openssh.com"
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char want-reply
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On receipt of such a message, an OpenSSH server will refuse to open
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future channels of type "session" and instead immediately abort the
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connection.
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Note that this is not a general defence against compromised clients
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(that is impossible), but it thwarts a simple attack.
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NB. due to certain broken SSH implementations aborting upon receipt
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of this message, the no-more-sessions request is only sent to OpenSSH
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servers (identified by banner). Other SSH implementations may be
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whitelisted to receive this message upon request.
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2.3. connection: Tunnel forward extension "tun@openssh.com"
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OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com"
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channel type. This channel type supports forwarding of network packets
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with datagram boundaries intact between endpoints equipped with
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interfaces like the BSD tun(4) device. Tunnel forwarding channels are
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requested by the client with the following packet:
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byte SSH_MSG_CHANNEL_OPEN
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string "tun@openssh.com"
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uint32 sender channel
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uint32 initial window size
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uint32 maximum packet size
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uint32 tunnel mode
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uint32 remote unit number
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The "tunnel mode" parameter specifies whether the tunnel should forward
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layer 2 frames or layer 3 packets. It may take one of the following values:
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SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */
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SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */
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The "tunnel unit number" specifies the remote interface number, or may
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be 0x7fffffff to allow the server to automatically chose an interface. A
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server that is not willing to open a client-specified unit should refuse
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the request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful
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open, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS.
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Once established the client and server may exchange packet or frames
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over the tunnel channel by encapsulating them in SSH protocol strings
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and sending them as channel data. This ensures that packet boundaries
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are kept intact. Specifically, packets are transmitted using normal
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SSH_MSG_CHANNEL_DATA packets:
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byte SSH_MSG_CHANNEL_DATA
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uint32 recipient channel
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string data
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The contents of the "data" field for layer 3 packets is:
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uint32 packet length
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uint32 address family
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byte[packet length - 4] packet data
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The "address family" field identifies the type of packet in the message.
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It may be one of:
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SSH_TUN_AF_INET 2 /* IPv4 */
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SSH_TUN_AF_INET6 24 /* IPv6 */
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The "packet data" field consists of the IPv4/IPv6 datagram itself
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without any link layer header.
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The contents of the "data" field for layer 2 packets is:
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uint32 packet length
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byte[packet length] frame
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The "frame" field contains an IEEE 802.3 Ethernet frame, including
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header.
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3. SFTP protocol changes
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3.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK
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When OpenSSH's sftp-server was implemented, the order of the arguments
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to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
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the reversal was not noticed until the server was widely deployed. Since
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fixing this to follow the specification would cause incompatibility, the
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current order was retained. For correct operation, clients should send
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SSH_FXP_SYMLINK as follows:
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uint32 id
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string targetpath
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string linkpath
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3.2. sftp: Server extension announcement in SSH_FXP_VERSION
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OpenSSH's sftp-server lists the extensions it supports using the
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standard extension announcement mechanism in the SSH_FXP_VERSION server
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hello packet:
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uint32 3 /* protocol version */
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string ext1-name
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string ext1-version
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string ext2-name
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string ext2-version
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...
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string extN-name
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string extN-version
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Each extension reports its integer version number as an ASCII encoded
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string, e.g. "1". The version will be incremented if the extension is
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ever changed in an incompatible way. The server MAY advertise the same
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extension with multiple versions (though this is unlikely). Clients MUST
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check the version number before attempting to use the extension.
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3.3. sftp: Extension request "posix-rename@openssh.com"
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This operation provides a rename operation with POSIX semantics, which
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are different to those provided by the standard SSH_FXP_RENAME in
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draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
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SSH_FXP_EXTENDED request with the following format:
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uint32 id
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string "posix-rename@openssh.com"
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string oldpath
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string newpath
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On receiving this request the server will perform the POSIX operation
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rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
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This extension is advertised in the SSH_FXP_VERSION hello with version
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"1".
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3.4. sftp: Extension requests "statvfs@openssh.com" and
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"fstatvfs@openssh.com"
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These requests correspond to the statvfs and fstatvfs POSIX system
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interfaces. The "statvfs@openssh.com" request operates on an explicit
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pathname, and is formatted as follows:
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uint32 id
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string "statvfs@openssh.com"
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string path
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The "fstatvfs@openssh.com" operates on an open file handle:
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uint32 id
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string "fstatvfs@openssh.com"
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string handle
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These requests return a SSH_FXP_STATUS reply on failure. On success they
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return the following SSH_FXP_EXTENDED_REPLY reply:
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uint32 id
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uint64 f_bsize /* file system block size */
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uint64 f_frsize /* fundamental fs block size */
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uint64 f_blocks /* number of blocks (unit f_frsize) */
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uint64 f_bfree /* free blocks in file system */
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uint64 f_bavail /* free blocks for non-root */
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uint64 f_files /* total file inodes */
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uint64 f_ffree /* free file inodes */
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uint64 f_favail /* free file inodes for to non-root */
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uint64 f_fsid /* file system id */
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uint64 f_flag /* bit mask of f_flag values */
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uint64 f_namemax /* maximum filename length */
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The values of the f_flag bitmask are as follows:
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#define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
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#define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
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Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are
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advertised in the SSH_FXP_VERSION hello with version "2".
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10. sftp: Extension request "hardlink@openssh.com"
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This request is for creating a hard link to a regular file. This
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request is implemented as a SSH_FXP_EXTENDED request with the
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following format:
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uint32 id
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string "hardlink@openssh.com"
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string oldpath
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string newpath
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On receiving this request the server will perform the operation
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link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
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This extension is advertised in the SSH_FXP_VERSION hello with version
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"1".
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$OpenBSD: PROTOCOL,v 1.20 2013/01/08 18:49:04 markus Exp $
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