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+.. _ssh:
+
+###
+SSH
+###
+
+:abbr:`SSH (Secure Shell)` is a cryptographic network protocol for operating
+network services securely over an unsecured network. The standard TCP port for
+SSH is 22. The best known example application is for remote login to computer
+systems by users.
+
+SSH provides a secure channel over an unsecured network in a client-server
+architecture, connecting an SSH client application with an SSH server. Common
+applications include remote command-line login and remote command execution,
+but any network service can be secured with SSH. The protocol specification
+distinguishes between two major versions, referred to as SSH-1 and SSH-2.
+
+The most visible application of the protocol is for access to shell accounts
+on Unix-like operating systems, but it sees some limited use on Windows as
+well. In 2015, Microsoft announced that they would include native support for
+SSH in a future release.
+
+SSH was designed as a replacement for Telnet and for unsecured remote shell
+protocols such as the Berkeley rlogin, rsh, and rexec protocols.
+Those protocols send information, notably passwords, in plaintext,
+rendering them susceptible to interception and disclosure using packet
+analysis. The encryption used by SSH is intended to provide confidentiality
+and integrity of data over an unsecured network, such as the Internet.
+
+Configuration
+=============
+
+.. cfgcmd:: set service ssh port <port>
+
+Enabling SSH only requires you to specify the port ``<port>`` you want SSH to
+listen on. By default, SSH runs on port 22.
+
+.. cfgcmd:: set service ssh listen-address <address>
+
+Specify IPv4/IPv6 listen address of SSH server. Multiple addresses can be
+defined.
+
+.. cfgcmd:: set service ssh ciphers <cipher>
+
+Define allowed ciphers used for the SSH connection. A number of allowed ciphers
+can be specified, use multiple occurrences to allow multiple ciphers. You can
+choose from the following ciphers: ``3des-cbc``, ``aes128-cbc``, ``aes192-cbc``,
+``aes256-cbc``, ``aes128-ctr``, ``aes192-ctr``, ``aes256-ctr``, ``arcfour128``,
+``arcfour256``, ``arcfour``, ``blowfish-cbc``, ``cast128-cbc``
+
+.. cfgcmd:: set service ssh disable-password-authentication
+
+Disable password based authentication. Login via SSH keys only. This hardens
+security!
+
+.. cfgcmd:: set service ssh disable-host-validation
+
+Disable the host validation through reverse DNS lookups - can speedup login
+time when reverse lookup is not possible.
+
+.. cfgcmd:: set service ssh macs <mac>
+
+Specifies the available :abbr:`MAC (Message Authentication Code)` algorithms.
+The MAC algorithm is used in protocol version 2 for data integrity protection.
+Multiple algorithms can be provided. Supported MACs: ``hmac-md5``,
+``hmac-md5-96``, ``hmac-ripemd160``, ``hmac-sha1``, ``hmac-sha1-96``,
+``hmac-sha2-256``, ``hmac-sha2-512``, ``umac-64@openssh.com``,
+``umac-128@openssh.com``, ``hmac-md5-etm@openssh.com``,
+``hmac-md5-96-etm@openssh.com``, ``hmac-ripemd160-etm@openssh.com``,
+``hmac-sha1-etm@openssh.com``, ``hmac-sha1-96-etm@openssh.com``,
+``hmac-sha2-256-etm@openssh.com``, ``hmac-sha2-512-etm@openssh.com``,
+``umac-64-etm@openssh.com``, ``umac-128-etm@openssh.com``
+
+.. note:: VyOS 1.1 supported login as user ``root``. This has been removed due
+   to tighter security in VyOS 1.2.
+
+.. cfgcmd:: set service ssh access-control <allow | deny> <group | user> <name>
+
+Add access-control directive to allow or deny users and groups. Directives are
+processed in the following order of precedence: ``deny-users``, ``allow-users``,
+``deny-groups`` and ``allow-groups``.
+
+.. cfgcmd:: set service ssh client-keepalive-interval <interval>
+
+Specify timeout interval for keepalive message in seconds.
+
+.. cfgcmd:: set service ssh key-exchange <kex>
+
+Specify allowed :abbr:`KEX (Key Exchange)` algorithms.
+Supported algorithms: ``diffie-hellman-group1-sha1``,
+``diffie-hellman-group14-sha1``, ``diffie-hellman-group14-sha256``,
+``diffie-hellman-group16-sha512``, ``diffie-hellman-group18-sha512``,
+``diffie-hellman-group-exchange-sha1``,
+``diffie-hellman-group-exchange-sha256``, ``ecdh-sha2-nistp256``,
+``ecdh-sha2-nistp384``, ``ecdh-sha2-nistp521``, ``curve25519-sha256`` and
+``curve25519-sha256@libssh.org``.
+
+.. cfgcmd:: set service ssh loglevel <quiet | fatal | error | info | verbose>
+
+Set the ``sshd`` log level. The default is ``info``.
+
+.. cfgcmd:: set service ssh vrf <name>
+
+Specify name of the :abbr:`VRF (Virtual Routing and Forwarding)` instance.
+
+.. seealso:: SSH :ref:`ssh_key_based_authentication`