Using the SSH Config File

If you’re regularly connecting to multiple remote systems via SSH, remembering various IP addresses, usernames, non-standard ports, and command-line options can be challenging or even impossible.

One approach is to create a bash alias for each remote server connection. However, there’s a better, more straightforward solution to this problem. OpenSSH allows you to set up a per-user configuration file where you can store different SSH options for each remote machine you connect to.

This guide covers the basics of the SSH client configuration file and highlights some of the most common options.

Prerequisites #

We assume you are using Linux or macOS with an OpenSSH client installed.

SSH Config File Location #

The OpenSSH client-side configuration file is named config, and resides in the .ssh directory under the user’s home directory.

The ~/.ssh directory is created automatically when the user runs the ssh command for the first time. If the directory doesn’t exist on your system, create it with:

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

For security, the file must be readable and writable only by the user and not accessible by others:

chmod 600 ~/.ssh/config

SSH Config File Structure and Patterns #

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 client config file are organised into stanzas (sections).

Each stanza begins with the Host directive and contains specific SSH options that apply when connecting to matching hosts.

Indentation is not required, but it is recommended because it makes the file easier to read.

The Host value can be a single hostname, IP address, or pattern, or a space-separated list of patterns. Each pattern can contain zero or more non-whitespace characters or one of the following pattern specifiers:

• * - Matches zero or more characters. (e.g, Host * matches all hosts, while 192.168.0.* matches hosts in the 192.168.0.0/24 subnet.)
• ? - Matches exactly one character. (e.g The pattern, Host 10.10.0.? matches all hosts in 10.10.0.[0-9] range.)
• ! - When used at the start of a pattern, it negates the match. (e.g Host 10.10.0.* !10.10.0.5 matches any host in the 10.10.0.0/24 subnet except 10.10.0.5.)

The SSH client processes the file from top to bottom. If more than one patterns match, the options from the first matching stanza take precedence. Therefore more host-specific declarations should be given at the beginning of the file, and more general overrides (like Host *) at the end of the file.

You can find a full list of available SSH options by typing man ssh_config in your terminal or by visiting the ssh_config man page .

The SSH config file is also read by other programs such as scp , sftp , and rsync .

Basic Example #

Now that we’ve covered the basics of the SSH configuration file let’s look at the following example.

Normally, when connecting to a remote server via SSH, you would specify the remote user name, hostname, and port. For example, to log in as a user named john to a host called dev.example.com on port 2322 from the command line, you would type:

ssh john@dev.example.com -p 2322

To connect to the server using the same options as provided in the command above by typing ssh dev, add the following lines in your "~/.ssh/config file:

Host dev
    HostName dev.example.com
    User john
    Port 2322

Now, when you type ssh dev, the ssh client will read the configuration file and use the connection details that are specified for the dev host:

ssh dev

Advanced Example: Patterns and Precedence #

This example gives more detailed information about the host patterns and option precedence.

Consider the following example file:

Host targaryen
    HostName 192.168.1.10
    User daenerys
    Port 7654
    IdentityFile ~/.ssh/targaryen.key

Host tyrell
    HostName 192.168.10.20

Host martell
    HostName 192.168.10.50

Host *ell
    user oberyn

Host * !martell
    LogLevel INFO

Host *
    User root
    Compression yes

• When you type ssh targaryen, the ssh client reads the file and applies the options from the first match, which is Host targaryen. Then it checks the next stanzas one by one for a matching pattern. The next matching one is Host * !martell (meaning all hosts except martell), and it will apply the connection option from this stanza. The last definition Host * also matches, but the ssh client will take only the Compression option because the User option is already defined in the Host targaryen stanza.

  The full list of options used when you type ssh targaryen is as follows:

  HostName 192.168.1.10
  User daenerys
  Port 7654
  IdentityFile ~/.ssh/targaryen.key
  LogLevel INFO
  Compression yes

• When running ssh tyrell the matching host patterns are: Host tyrell, Host *ell, Host * !martell and Host *. The options used in this case are:

  HostName 192.168.10.20
  User oberyn
  LogLevel INFO
  Compression yes

• If you run ssh martell, the matching host patterns are: Host martell, Host *ell and Host *. The options used in this case are:

  HostName 192.168.10.50
  User oberyn
  Compression yes

• For all other connections, the ssh client will use the options specified in the Host * !martell and Host * sections.

Overriding Options #

The SSH client reads its configuration in the following precedence order:

1. Options specified from the command line.
2. Options defined in the ~/.ssh/config.
3. Options defined in the system-wide /etc/ssh/ssh_config file.

To override a single option, specify it on the command line. For example, if you have the following definition:

Host dev
    HostName dev.example.com
    User john
    Port 2322

and you want to use all other options but to connect as user root instead of john simply specify the user on the command line:

ssh -o "User=root" dev

The -F (configfile) option allows you to specify an alternative per-user configuration file.

To tell the SSH client to ignore all of the options specified in the SSH configuration file, use:

ssh -F /dev/null user@example.com

Conclusion #

We’ve shown you how to configure your user’s SSH config file to simplify managing multiple remote connections.

You may also want to set up SSH key-based authentication so you can connect to your Linux servers without entering a password.

By default, SSH listens on port 22. Changing the default SSH port adds an extra layer of security to your server by reducing the risk of automated attacks.

If you have any questions, please leave a comment below.