SSH

Secure Shell (SSH) is a cryptographic network protocol for operating network services securely over an unsecured network.[1] 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

Enabling SSH only requires you to add service ssh port NN, where ‘NN’ is the port you want SSH to listen on. By default, SSH runs on port 22.

set service ssh port 22

Options

  • Listening address - Specify the IPv4/IPv6 listening address for connection requests. Multiple listen-address nodes can be defined.

    set service ssh listen-address <address>

  • Allow root login, this can be set to allow root logins on SSH connections, however it is not advisable to use this setting as this bears serious security risks. The default system user possesses all required privileges.

    set service ssh allow-root

  • Allowed ciphers - A number of allowed ciphers can be specified, use multiple occurrences to allow multiple ciphers.

    set service ssh ciphers <cipher>

    Available ciphers:

  • 3des-cbc
  • aes128-cbc
  • aes192-cbc
  • aes256-cbc
  • aes128-ctr
  • aes192-ctr
  • aes256-ctr
  • arcfour128
  • arcfour256
  • arcfour
  • blowfish-cbc
  • cast128-cbc
  • Disable password authentication - If SSH key authentication is set up, password-based user authentication can be disabled. This hardens security!

    set service ssh disable-password-authentication

  • Disable host validation - Disable the host validation through reverse DNS lookups.

    set service ssh disable-host-validation

  • MAC algorithms - Specifies the available MAC (message authentication code) algorithms. The MAC algorithm is used in protocol version 2 for data integrity protection. Multiple algorithms can be entered.

    set service ssh macs <macs>

    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

Key Authentication

It is highly recommended to use SSH Key authentication. By default there is only one user (vyos), and you can assign any number of keys to that user. You can generate a ssh key with the ssh-keygen command on your local machine, which will (by default) save it as ~/.ssh/id_rsa.pub which is in three parts:

ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAA...VByBD5lKwEWB username@host.example.com

Only the type (ssh-rsa) and the key (AAAB3N...) are used. Note that the key will usually be several hundred characters long, and you will need to copy and paste it. Some terminal emulators may accidentally split this over several lines. Be attentive when you paste it that it only pastes as a single line. The third part is simply an identifier, and is for your own reference.

Assign SSH Key to user

Under the user (in this example, vyos), add the public key and the type. The identifier is simply a string that is relevant to you.

set system login user vyos authentication public-keys 'identifier' key "AAAAB3Nz...."
set system login user vyos authentication public-keys 'identifier' type ssh-rsa"

You can assign multiple keys to the same user by changing the identifier. In the following example, both Unicron and xrobau will be able to SSH into VyOS as the vyos user using their own keys.

set system login user vyos authentication public-keys 'Unicron' key "AAAAB3Nz...."
set system login user vyos authentication public-keys 'Unicron' type ssh-rsa
set system login user vyos authentication public-keys 'xrobau' key "AAAAQ39x...."
set system login user vyos authentication public-keys 'xrobau' type ssh-rsa