OpenSSH Specifications

OpenSSH implements the following specifications. Where versions are noted, support for the corresponding specification was added or removed in that OpenSSH version.

SSH protocol version 2 Core RFCs

Source: secsh working group
Specification Description
RFC4250 SSH Protocol Assigned Numbers
RFC4251 SSH Protocol Architecture
RFC4252 (e) SSH Authentication Protocol
RFC4253 (e) SSH Transport Layer Protocol
RFC4254 (e) SSH Connection Protocol

SSH protocol version 2 Extension RFCs

Specification Versions Description
RFC4255 (e) Using DNS to Securely Publish SSH Key Fingerprints (SSHFP)
RFC4256 (e) Generic Message Exchange Authentication (aka keyboard-interactive)
RFC4335 (e) SSH Session Channel Break Extension
RFC4344 SSH Transport Layer Encryption Modes (aes128-ctr, aes192-ctr, aes256-ctr)
RFC4345 (e) 4.1-7.6 Improved Arcfour Modes for the SSH Transport Layer Protocol
RFC4419 (e) Diffie-Hellman Group Exchange
RFC4462 (e) GSS-API Authentication and Key Exchange (only authentication implemented)
RFC4716 SSH Public Key File Format (import and export via ssh-keygen only).
RFC5647 6.2- AES Galois Counter Mode (GCM) packet format (as aes128-gcm@openssh.com and aes256-gcm@openssh.com). Algorithm negotation differs as per draft-miller-sshm-aes-gcm due to problems with the original spec.
RFC5656 (e) 5.6- Elliptic Curve Algorithm Integration in SSH
RFC6594 (e) 6.1- SHA-256 SSHFP Resource Records
RFC6668 5.9- SHA-2 Data Integrity Algorithms (hmac-sha2-256, hmac-sha2-512)
RFC7479 (e) 6.5- ED25519 SSHFP Resource Records
RFC8160 7.3- IUTF8 Terminal Mode
RFC8270 (e) 7.1- Increase Diffie-Hellman Modulus Size
RFC8308 7.2-, 9.6- Extension Negotiation in the Secure Shell (SSH) Protocol (ext-info-c added in 7.2, ext-info-s added in 9.6)
RFC8332 7.2- Use of RSA Keys with SHA-2 (rsa-sha2-256, rsa-sha2-512)
RFC8709 (e) 6.5- Ed25519 and Ed448 Public Key Algorithms (ssh-ed25519 only)
RFC8731 7.4- Key Exchange Method Using Curve25519 and Curve448 (curve25519-sha256 only). Previously implemented as curve25519-sha256@libssh.org in 6.5
RFC9941 8.9-, 9.9- sntrup761x25519-sha512 key exchange method. Added as sntrup761x25519-sha512@openssh.com in 8.9.

SSH protocol version 2 IETF draft specifications

Specification Versions Description
draft-ietf-secsh-filexfer-02 SSH File Transfer Protocol version 3
draft-ietf-secsh-filexfer-extensions-00 9.0- SFTP extension copy-data
draft-ietf-secsh-filexfer-extensions-00 9.1- SFTP extension home-directory
draft-ietf-sshm-chacha20-poly1305 6.5- chacha20-poly1305@openssh.com authenticated encryption mode.
draft-ietf-curdle-ssh-kex-sha2-03 7.3- Key Exchange (KEX) Method Updates and Recommendations
draft-ietf-secsh-scp-sftp-ssh-uri-04 7.6- Uniform Resource Identifier (URI) Scheme for SSH and SFTP (with the exception of fingerprint)

SSH protocol version 2 vendor extensions

Specification Versions Description
draft-miller-ssh-agent-04 ssh-agent protocol (auth-agent@openssh.com)
draft-miller-secsh-compression-delayed-00 4.2 Delayed compression until after authentication (zlib@openssh.com)
draft-miller-secsh-umac-01 6.2- Use of UMAC in SSH (umac-64@openssh.com, umac-128@openssh.com)
draft-miller-sshm-hostkey-update 6.8- Allows clients to learn additional host keys supported by hosts where one or more keys is already known (hostkeys-00@openssh.com, hostkeys-prove-00@openssh.com).
draft-kampanakis-curdle-pq-ssh-00 8.0-8.5 Post-quantum public key algorithms (sntrup4591761x25519-sha512@tinyssh.org)
draft-miller-sshm-strict-kex 9.6- "Strict KEX" to improve the integrity of the initial key exchange.
PROTOCOL An overview of all vendor extensions detailed below, and the specifications of the following protocol extensions:
  • SSH2 connection:
    • eow@openssh.com, no-more-sessions@openssh.com
    • tun@openssh.com (layer 2 and 3 tunnelling)
    • direct-streamlocal@openssh.com, forwarded-streamlocal@openssh.com, streamlocal-forward@openssh.com, cancel-streamlocal-forward@openssh.com (Unix domain socket forwarding)
    • INFO@openssh.com (BSD SIGINFO)
    • publickey-hostbound-v00@openssh.com (host-bound public key authentication)
  • SSH2 transport MACs: hmac-sha1-etm@openssh.com, hmac-sha1-96-etm@openssh.com, hmac-sha2-256-etm@openssh.com, hmac-sha2-512-etm@openssh.com, hmac-md5-etm@openssh.com, hmac-md5-96-etm@openssh.com, umac-64-etm@openssh.com, umac-128-etm@openssh.com
  • SFTP: posix-rename@openssh.com, statvfs@openssh.com, fstatvfs@openssh.com, hardlink@openssh.com, fsync@openssh.com, lesetstat@openssh.com, limits@openssh.com, expand-path@openssh.com
PROTOCOL.certkeys ssh-rsa-cert-v01@openssh.com, ssh-dsa-cert-v01@openssh.com, ecdsa-sha2-nistp256-cert-v01@openssh.com, ecdsa-sha2-nistp384-cert-v01@openssh.com, ecdsa-sha2-nistp521-cert-v01@openssh.com, ssh-ed25519-cert-v01@openssh.com, rsa-sha2-256-cert-v01@openssh.com, rsa-sha2-512-cert-v01@openssh.com : new public key algorithms supporting certificates.
PROTOCOL.key OpenSSH private key format (openssh-key-v1).
PROTOCOL.krl Key Revocation Lists for OpenSSH keys and certificates.
PROTOCOL.mux Multiplexing protocol used by ssh(1) ControlMaster connection-sharing.

Other specifications

Specification Description
socks4.protocol SOCKS protocol version 4. Used for ssh(1) DynamicForward.
socks4a.protocol SOCKS protocol version 4a. Used for ssh(1) DynamicForward.
RFC1928 SOCKS protocol version 5. Used for ssh(1) DynamicForward.
RFC1349 RFC8325 IP Type of Service (ToS) and Differentiated Services. OpenSSH will automatically set the IP Type of Service according to RFC8325 unless otherwise specified via the IPQoS keyword in ssh_config and sshd_config. Versions 7.7 and earlier will set it per RFC1349 unless otherwise specified.