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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: https://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: https://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: https://www.openssh.com/txt/draft-miller-secsh-compression-delayed-00.txt 1.3. transport: Certificate key algorithms OpenSSH introduces new public key algorithms to support certificate authentication for users and host keys. These methods are documented in at https://datatracker.ietf.org/doc/draft-miller-ssh-cert/ 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 design of the algorithm negotiation in this RFC, OpenSSH (and other SSH implementations) use different rules as described in: https://datatracker.ietf.org/doc/draft-miller-sshm-aes-gcm/ 1.7 transport: chacha20-poly1305@openssh.com authenticated encryption OpenSSH supports authenticated encryption using ChaCha20 and Poly1305 as described in: https://datatracker.ietf.org/doc/draft-ietf-sshm-chacha20-poly1305/ 1.8 transport: ping facility OpenSSH implements a transport level ping message SSH2_MSG_PING and a corresponding SSH2_MSG_PONG reply. #define SSH2_MSG_PING 192 #define SSH2_MSG_PONG 193 The ping message is simply: byte SSH_MSG_PING string data The reply copies the data (which may be the empty string) from the ping: byte SSH_MSG_PONG string data Replies are sent in order. They are sent immediately except when rekeying is in progress, in which case they are queued until rekeying completes. The server advertises support for these messages using the SSH2_MSG_EXT_INFO mechanism (RFC8308), with the following message: string "ping@openssh.com" string "0" (version) The ping/reply message is implemented at the transport layer rather than as a named global or channel request to allow pings with very short packet lengths, which would not be possible with other approaches. 1.9 transport: strict key exchange extension OpenSSH supports a number of transport-layer hardening measures designed to thwart the so-called "Terrapin" attack against the early SSH protocol. These are collectively referred to as "strict KEX" and documented in an Internet-Draft: https://datatracker.ietf.org/doc/draft-miller-sshm-strict-kex/ 1.10 transport: SSH2_MSG_EXT_INFO during user authentication This protocol extension allows the SSH2_MSG_EXT_INFO to be sent during user authentication. RFC8308 does allow a second SSH2_MSG_EXT_INFO notification, but it may only be sent at the end of user authentication and this is too late to signal per-user server signature algorithms. Support for receiving the SSH2_MSG_EXT_INFO message during user authentication is signalled by the client including a "ext-info-in-auth@openssh.com" key via its initial SSH2_MSG_EXT_INFO set after the SSH2_MSG_NEWKEYS message. A server that supports this extension MAY send a second SSH2_MSG_EXT_INFO message any time after the client's first SSH2_MSG_USERAUTH_REQUEST, regardless of whether it succeed or fails. The client SHOULD be prepared to update the server-sig-algs that it received during an earlier SSH2_MSG_EXT_INFO with the later one. 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 listed 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 listed 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 choose 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 "direct-streamlocal@openssh.com" uint32 sender channel uint32 initial window size uint32 maximum packet size string socket path string reserved uint32 reserved 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 "forwarded-streamlocal@openssh.com" 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 "streamlocal-forward@openssh.com" 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 "cancel-streamlocal-forward@openssh.com" boolean FALSE string socket path 2.5. connection: hostkey update and rotation "hostkeys-00@openssh.com" and "hostkeys-prove-00@openssh.com" 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. This is documented in an Internet-Draft https://datatracker.ietf.org/doc/draft-miller-sshm-hostkey-update/ 2.6. connection: SIGINFO support for "signal" channel request The SSH channels protocol (RFC4254 section 6.9) supports sending a signal to a session attached to a channel. OpenSSH supports one extension signal "INFO@openssh.com" that allows sending SIGINFO on BSD-derived systems. 3. Authentication protocol changes 3.1. Host-bound public key authentication This is trivial change to the traditional "publickey" authentication method. The authentication request is identical to the original method but for the name and one additional field: byte SSH2_MSG_USERAUTH_REQUEST string username string "ssh-connection" string "publickey-hostbound-v00@openssh.com" bool has_signature string pkalg string public key string server host key Because the entire SSH2_MSG_USERAUTH_REQUEST message is included in the signed data, this ensures that a binding between the destination user, the server identity and the session identifier is visible to the signer. OpenSSH uses this binding via signed data to implement per-key restrictions in ssh-agent. A server may advertise this method using the SSH2_MSG_EXT_INFO mechanism (RFC8308), with the following message: string "publickey-hostbound@openssh.com" string "0" (version) Clients should prefer host-bound authentication when advertised by server. 4. SFTP protocol changes 4.1. 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 4.2. 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. 4.3. 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". 4.4. 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". 4.5. 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". 4.6. sftp: Extension request "fsync@openssh.com" This request asks the server to call fsync(2) on an open file handle. uint32 id string "fsync@openssh.com" string handle On receiving this request, a server will call fsync(handle_fd) and will respond with a SSH_FXP_STATUS message. This extension is advertised in the SSH_FXP_VERSION hello with version "1". 4.7. sftp: Extension request "lsetstat@openssh.com" This request is like the "setstat" command, but sets file attributes on symlinks. It is implemented as a SSH_FXP_EXTENDED request with the following format: uint32 id string "lsetstat@openssh.com" string path ATTRS attrs See the "setstat" command for more details. This extension is advertised in the SSH_FXP_VERSION hello with version "1". 4.8. sftp: Extension request "limits@openssh.com" This request is used to determine various limits the server might impose. Clients should not attempt to exceed these limits as the server might sever the connection immediately. uint32 id string "limits@openssh.com" The server will respond with a SSH_FXP_EXTENDED_REPLY reply: uint32 id uint64 max-packet-length uint64 max-read-length uint64 max-write-length uint64 max-open-handles The 'max-packet-length' applies to the total number of bytes in a single SFTP packet. Servers SHOULD set this at least to 34000. The 'max-read-length' is the largest length in a SSH_FXP_READ packet. Even if the client requests a larger size, servers will usually respond with a shorter SSH_FXP_DATA packet. Servers SHOULD set this at least to 32768. The 'max-write-length' is the largest length in a SSH_FXP_WRITE packet the server will accept. Servers SHOULD set this at least to 32768. The 'max-open-handles' is the maximum number of active handles that the server allows (e.g. handles created by SSH_FXP_OPEN and SSH_FXP_OPENDIR packets). Servers MAY count internal file handles against this limit (e.g. system logging or stdout/stderr), so clients SHOULD NOT expect to open this many handles in practice. If the server doesn't enforce a specific limit, then the field may be set to 0. This implies the server relies on the OS to enforce limits (e.g. available memory or file handles), and such limits might be dynamic. The client SHOULD take care to not try to exceed reasonable limits. This extension is advertised in the SSH_FXP_VERSION hello with version "1". 4.9. sftp: Extension request "expand-path@openssh.com" This request supports canonicalisation of relative paths and those that need tilde-expansion, i.e. "~", "~/..." and "~user/..." These paths are expanded using shell-like rules and the resultant path is canonicalised similarly to SSH2_FXP_REALPATH. It is implemented as a SSH_FXP_EXTENDED request with the following format: uint32 id string "expand-path@openssh.com" string path Its reply is the same format as that of SSH2_FXP_REALPATH. This extension is advertised in the SSH_FXP_VERSION hello with version "1". 4.10. sftp: Extension request "copy-data" This request asks the server to copy data from one open file handle and write it to a different open file handle. This avoids needing to transfer the data across the network twice (a download followed by an upload). byte SSH_FXP_EXTENDED uint32 id string "copy-data" string read-from-handle uint64 read-from-offset uint64 read-data-length string write-to-handle uint64 write-to-offset The server will copy read-data-length bytes starting from read-from-offset from the read-from-handle and write them to write-to-handle starting from write-to-offset, and then respond with a SSH_FXP_STATUS message. It's equivalent to issuing a series of SSH_FXP_READ requests on read-from-handle and a series of requests of SSH_FXP_WRITE on write-to-handle. If read-from-handle and write-to-handle are the same, the server will fail the request and respond with a SSH_FX_INVALID_PARAMETER message. If read-data-length is 0, then the server will read data from the read-from-handle until EOF is reached. This extension is advertised in the SSH_FXP_VERSION hello with version "1". This request is identical to the "copy-data" request documented in: https://tools.ietf.org/html/draft-ietf-secsh-filexfer-extensions-00#section-7 4.11. sftp: Extension request "home-directory" This request asks the server to expand the specified user's home directory. An empty username implies the current user. This can be used by the client to expand ~/ type paths locally. byte SSH_FXP_EXTENDED uint32 id string "home-directory" string username This extension is advertised in the SSH_FXP_VERSION hello with version "1". This provides similar information as the "expand-path@openssh.com" extension. This request is identical to the "home-directory" request documented in: https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-extensions-00#section-5 4.12. sftp: Extension request "users-groups-by-id@openssh.com" This request asks the server to return user and/or group names that correspond to one or more IDs (e.g. as returned from a SSH_FXP_STAT request). This may be used by the client to provide usernames in directory listings. byte SSH_FXP_EXTENDED uint32 id string "users-groups-by-id@openssh.com" string uids string gids Where "uids" and "gids" consists of one or more integer user or group identifiers: uint32 id-0 ... The server will reply with a SSH_FXP_EXTENDED_REPLY: byte SSH_FXP_EXTENDED_REPLY uint32 id string usernames string groupnames Where "username" and "groupnames" consists of names in identical request order to "uids" and "gids" respectively: string name-0 ... If a name cannot be identified for a given user or group ID, an empty string will be returned in its place. It is acceptable for either "uids" or "gids" to be an empty set, in which case the respective "usernames" or "groupnames" list will also be empty. This extension is advertised in the SSH_FXP_VERSION hello with version "1". 5. Miscellaneous changes 5.1 Public key format OpenSSH public keys, as generated by ssh-keygen(1) and appearing in authorized_keys files, are formatted as a single line of text consisting of the public key algorithm name followed by a base64-encoded key blob. The public key blob (before base64 encoding) is the same format used for the encoding of public keys sent on the wire: as described in RFC4253 section 6.6 for RSA keys, RFC5656 section 3.1 for ECDSA keys and https://datatracker.ietf.org/doc/draft-miller-ssh-cert/ for the OpenSSH certificate formats. 5.2 Private key format OpenSSH private keys, as generated by ssh-keygen(1) use the format described in PROTOCOL.key by default. As a legacy option, PEM format (RFC7468) private keys are also supported for RSA and ECDSA keys and were the default format before OpenSSH 7.8. 5.3 KRL format OpenSSH supports a compact format for Key Revocation Lists (KRLs). This format is described in the PROTOCOL.krl file. 5.4 Connection multiplexing OpenSSH's connection multiplexing uses messages as described in PROTOCOL.mux over a Unix domain socket for communications between a master instance and later clients. 5.5. Agent protocol extensions OpenSSH extends the usual agent protocol. These changes are documented in the PROTOCOL.agent file. $OpenBSD: PROTOCOL,v 1.60 2026/02/09 22:09:48 dtucker Exp $