OpenSSH Specifications

OpenSSH implements the following specifications.

SSH protocol version 2 Core RFCs

Source: secsh working group

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

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
RFC4345 (e) 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).
RFC5656 (e) Elliptic Curve Algorithm Integration in SSH
RFC6594 (e) SHA-256 SSHFP Resource Records (new in OpenSSH 6.1).
RFC6668 SHA-2 Data Integrity Algorithms (new in OpenSSH 5.9)
RFC7479 (e) ED25519 SSHFP Resource Records (new in OpenSSH 6.5).
RFC8160 IUTF8 Terminal Mode (new in OpenSSH 7.3).
RFC8270 (e) Increase Diffie-Hellman Modulus Size (in OpenSSH 7.1).
RFC8308 Extension Negotiation in the Secure Shell (SSH) Protocol (ext-info-s and ext-info-c, new in OpenSSH 7.2).
RFC8332 RSA Keys with SHA-2 256 and 512 (new in OpenSSH 7.2).
RFC8709 (e) Public Key Algorithms (Ed25519 only, new in OpenSSH 6.5).
RFC8731 curve25519-sha256 only (new in OpenSSH 7.3).

SSH protocol version 2 draft specifications

draft-ietf-secsh-filexfer-02 SSH File Transfer Protocol version 3
draft-ietf-curdle-ssh-kex-sha2-03 Key Exchange (KEX) Method Updates and Recommendations (new in OpenSSH 7.3).
draft-ietf-secsh-scp-sftp-ssh-uri-04 Uniform Resource Identifier (URI) Scheme for SSH and SFTP (with the exception of fingerprint, new in OpenSSH 7.6).

SSH protocol version 2 vendor extensions

PROTOCOL An overview of all vendor extensions detailed below, and the specifications of the SSH2 extensions,, and the sftp extensions,
draft-miller-ssh-agent-04 ssh-agent protocol (
PROTOCOL.certkeys,,,, : new public key algorithms supporting certificates.
PROTOCOL.chacha20poly1305 authenticated encryption mode.
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.
draft-miller-secsh-umac-01 a new transport-layer MAC.
draft-miller-secsh-compression-delayed-00 Delayed compression until after authentication. key exchange method.

Other specifications

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.