This document was adapted from: http://cvsweb.openbsd.org/cgi-bin/cvsweb/~checkout~/src/usr.bin/ssh/PROTOCOL $OpenBSD: PROTOCOL,v 1.29 2015/07/17 03:09:19 djm Exp $ ---------------------------------------------------------------------------- This documents OpenSSH's deviations and extensions to the published SSH protocol. 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 "[email protected]" 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 "[email protected]" 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 "[email protected]", "[email protected]", "[email protected]", "[email protected]" and "[email protected]" OpenSSH introduces new public key algorithms to support certificate authentication for users and host keys. 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. 1.5 transport: Protocol 2 Encrypt-then-MAC MAC algorithms OpenSSH supports MAC algorithms, whose names contain "-etm", that perform the calculations in a different order to that defined in RFC 4253. These variants use the so-called "encrypt then MAC" ordering, calculating the MAC over the packet ciphertext rather than the plaintext. This ordering closes a security flaw in the SSH transport protocol, where decryption of unauthenticated ciphertext provided a "decryption oracle" that could, in conjunction with cipher flaws, reveal session plaintext. Specifically, the "-etm" MAC algorithms modify the transport protocol to calculate the MAC over the packet ciphertext and to send the packet length unencrypted. This is necessary for the transport to obtain the length of the packet and location of the MAC tag so that it may be verified without decrypting unauthenticated data. As such, the MAC covers: mac = MAC(key, sequence_number || packet_length || encrypted_packet) where "packet_length" is encoded as a uint32 and "encrypted_packet" contains: byte padding_length byte[n1] payload; n1 = packet_length - padding_length - 1 byte[n2] random padding; n2 = padding_length 1.6 transport: AES-GCM OpenSSH supports the AES-GCM algorithm as specified in RFC 5647. Because of problems with the specification of the key exchange the behaviour of OpenSSH differs from the RFC as follows: AES-GCM is only negotiated as the cipher algorithms "[email protected]" or "[email protected]" and never as an MAC algorithm. Additionally, if AES-GCM is selected as the cipher the exchanged MAC algorithms are ignored and there doesn't have to be a matching MAC. 1.7 transport: [email protected] authenticated encryption OpenSSH supports authenticated encryption using ChaCha20 and Poly1305 as described in PROTOCOL.chacha20poly1305. 1.8 transport: [email protected] key exchange algorithm OpenSSH supports the use of ECDH in Curve25519 for key exchange as described at: http://git.libssh.org/users/aris/libssh.git/plain/doc/[email protected]?h=curve25519 2. Connection protocol changes 2.1. connection: Channel write close extension "[email protected]" 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: "[email protected]" (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 "[email protected]" 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 "[email protected]" 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 "[email protected]" 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 "[email protected]" 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 "[email protected]" 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 "[email protected]" OpenSSH supports layer 2 and layer 3 tunnelling via the "[email protected]" 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 "[email protected]" 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. 2.4. connection: Unix domain socket forwarding OpenSSH supports local and remote Unix domain socket forwarding using the "streamlocal" extension. Forwarding is initiated as per TCP sockets but with a single path instead of a host and port. Similar to direct-tcpip, direct-streamlocal is sent by the client to request that the server make a connection to a Unix domain socket. byte SSH_MSG_CHANNEL_OPEN string "[email protected]" uint32 sender channel uint32 initial window size uint32 maximum packet size string socket path Similar to forwarded-tcpip, forwarded-streamlocal is sent by the server when the client has previously send the server a streamlocal-forward GLOBAL_REQUEST. byte SSH_MSG_CHANNEL_OPEN string "[email protected]" uint32 sender channel uint32 initial window size uint32 maximum packet size string socket path string reserved for future use The reserved field is not currently defined and is ignored on the remote end. It is intended to be used in the future to pass information about the socket file, such as ownership and mode. The client currently sends the empty string for this field. Similar to tcpip-forward, streamlocal-forward is sent by the client to request remote forwarding of a Unix domain socket. byte SSH2_MSG_GLOBAL_REQUEST string "[email protected]" boolean TRUE string socket path Similar to cancel-tcpip-forward, cancel-streamlocal-forward is sent by the client cancel the forwarding of a Unix domain socket. byte SSH2_MSG_GLOBAL_REQUEST string "[email protected]" boolean FALSE string socket path 2.5. connection: hostkey update and rotation "[email protected]" and "[email protected]" OpenSSH supports a protocol extension allowing a server to inform a client of all its protocol v.2 host keys after user-authentication has completed. byte SSH_MSG_GLOBAL_REQUEST string "[email protected]" string[] hostkeys Upon receiving this message, a client should check which of the supplied host keys are present in known_hosts. For keys that are not present, it should send a "[email protected]" message to request the server prove ownership of the private half of the key. byte SSH_MSG_GLOBAL_REQUEST string "[email protected]" char 1 /* want-reply */ string[] hostkeys When a server receives this message, it should generate a signature using each requested key over the following: string "[email protected]" string session identifier string hostkey These signatures should be included in the reply, in the order matching the hostkeys in the request: byte SSH_MSG_REQUEST_SUCCESS string[] signatures When the client receives this reply (and not a failure), it should validate the signatures and may update its known_hosts file, adding keys that it has not seen before and deleting keys for the server host that are no longer offered. These extensions let a client learn key types that it had not previously encountered, thereby allowing it to potentially upgrade from weaker key algorithms to better ones. It also supports graceful key rotation: a server may offer multiple keys of the same type for a period (to give clients an opportunity to learn them using this extension) before removing the deprecated key from those offered.