Learn SSH: Config File

In this article series, I will explain how to use a command-line terminal to connect to secure shell (SSH) servers, using the config file for quick access to frequently used servers, and setting up an SSH server on macOS, Ubuntu, or CentOS.

Table of Contents:

• Introduction
• SSH CLI Commands
• SSH Config File

Basic Setup

SSH config file makes it easier to connect to frequently used servers. Here, I start with a simple server setup and then explain how to setup proxy jumps.

SSH config file is located in home directory under .ssh folder:

~/.ssh/config

For each server, add a block of text starting with Host followed by a memorable name for that server.

Host favorite_ssh

Add information like the address of the host on the following line with indentation:

Host favorite_ssh
        HostName ip_or_address_of_server
        User username_at_the_server

Now save the file. To connect to the server, in a terminal window simply type ssh followed by the name you chose in the config file:

$ ssh favorite_ssh

This is equivalent of typing the following command:

$ ssh username_at_the_server@ip_or_address_of_server

Adding an Identity File

You can add the identity file location to the config file as well:

Host favorite_ssh
        HostName ip_or_address_of_server
        User username_at_the_server
        IdentityFile ~/.ssh/my_private_key

which is equivalent of the following command:

$ ssh username_at_the_server@ip_or_address_of_server -i ~/.ssh/my_private_key

Multiple Servers

You can add multiple servers to the config file:

Host favorite_ssh
        HostName ip_or_address_of_server
        User username_at_the_server
        IdentityFile ~/.ssh/my_private_key

Host favorite_ssh_2
        HostName ip_or_address_of_server_2
        User username_at_the_server_2
        IdentityFile ~/.ssh/my_private_key_2

Host favorite_ssh_3
        HostName ip_or_address_of_server_3
        User username_at_the_server_3
        IdentityFile ~/.ssh/my_private_key_3

Proxy / Jump Servers

You can add a proxy or jump server to the config file as well and use it to connect to your favorite_ssh:

Host proxy_1
        HostName ip_or_address_of_proxy_server
        User username_at_proxy_server
        IdentityFile ~/.ssh/proxy_server_private_key

Host favorite_ssh
        HostName ip_or_address_of_server
        User username_at_the_server
        IdentityFile ~/.ssh/my_private_key
        ProxyJump proxy_1

Using Wildcards

You can also use wildcards to apply the same setting to a range of IP addresses (10.0.* and 10.1.0.*):

Host proxy_1
        HostName ip_or_address_of_proxy_server
        User username_at_proxy_server
        IdentityFile ~/.ssh/proxy_server_private_key

Host 10.0.*
        User username_at_these_servers
        IdentityFile ~/.ssh/my_private_key_for_all_the_servers
        ProxyJump proxy_1

Host 10.1.0.*
        ProxyJump proxy_1

Now to connect to a server with IP address of 10.0.0.1, you can use the following command:

$ ssh 10.0.0.1

Or to connect to 10.1.0.2 with a username my_user, you can run:

$ ssh my_user@10.1.0.2

Let me know your questions and comments in the comment section.