SSH Configuration File

How to save ssh configurations to a configuration file

When regularly connecting to multiple remote systems over SSH, you’ll find that remembering all the hosts and various command-line options becomes tedious. OpenSSH allows setting up a configuration file to store different SSH options for each remote machine you connect t.

SSH Config File Location

OpenSSH client-side (in this case your personal computer) configuration file is named config, and it is stored in the hidded .sshdirectory under your user’s home directory (i.e., ~/.ssh)

When you use the sshcommand for the first time. The ~/.ssh directory is automatically created. If the directory doesn’t exist on your system, create it using the command below:

mkdir -p ~/.ssh && chmod 700 ~/.ssh

By default, the SSH configuration file may not exist, so you may need to create it using the touch command :

touch ~/.ssh/config

This file must be readable and writable only by the user and not accessible by others:

chmod 600 ~/.ssh/config

SSH Config File Structure Basics

The SSH Config File takes the following structure:

Host hostname1
    SSH_OPTION value
    SSH_OPTION value

Host hostname2
    SSH_OPTION value

Host *
    SSH_OPTION value

The contents of the SSH config file is organized into sections. Each section starts with the Host directive and contains specific SSH options used when establishing a connection with the remote SSH server.

Oscar Hosts

Here we peovide a list of Oscar hosts and typical SSH configuration options. You have two options

1. Copy the list of hosts below directly into your SSH Config File (i.e., ~/.ssh/config)

2. Keep this content in a spearate file for Oscar hosts, lets say ~/.ssh/config.oscar and include that file in your main configuration file. In this case, the first line of ~/.ssh/config will be Include "~/.ssh/config.oscar"

Don't forget to replace <username> with your user. Also the configuration assumes your identity key is ~/.ssh/id_rsa - if you named it anything else, please update the value. If you need to generate a key got here

# Oscar Hosts. Any hosts with the -campus suffix can be accessed
# only whithin Brown network i.e. campus or vpn
# Hosts without -campus sufix can be accessed from outside Brown
# but will requiere 2FA

# Hosts to connect to login nodes
Host oscar
    HostName ssh.ccv.brown.edu
    User <username>
    IdentityFile ~/.ssh/id_rsa
    ForwardAgent yes
    ForwardX11 yes
    TCPKeepAlive yes
    ServerAliveCountMax 20
    ServerAliveInterval 15
Host oscar-campus
    HostName sshcampus.ccv.brown.edu
    User <username>
    IdentityFile ~/.ssh/id_rsa
    ForwardAgent yes
    ForwardX11 yes
    TCPKeepAlive yes
    ServerAliveCountMax 20
    ServerAliveInterval 15
    
# When connecting from VSCODE use the following hosts
Host vscode-oscar-campus
    HostName oscar2
    User <username>
    ProxyCommand ssh -q -W %h:%p desktop-oscar-campus
Host vscode-oscar
    HostName oscar2
    User <username>
    ProxyCommand ssh -q -W %h:%p desktop-oscar

Connecting to your preconfigured host

You may now connect using the shortchut notation provided by your configuration file. That is, all you need to type is:

ssh oscar-campus

According to the configuration above, this is equivalent to

ssh -X -A -o TCPKeepAlive=yes -o ServerAliveCountMax=20 -o ServerAliveInterval=15 [email protected]

Much shorter. Enjoy!