Using the SSH Config File (original) (raw)

SSH Config File

If you are regularly connecting to multiple remote systems over SSH, you’ll find that remembering all of the remote IP addresses, different usernames, non-standard ports, and various command-line options is difficult, if not impossible.

One option would be to create a bash aliasfor each remote server connection. However, there is another, much better, and 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 article covers the basics of the SSH client configuration file and explains some of the most common configuration options.

Prerequisites

We are assuming that you are using a Linux or a macOS system with an OpenSSH client installed.

SSH Config File Location

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

The ~/.ssh directory is automatically created when the user runs the sshcommand for the first time. 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 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 is organized into stanzas (sections). Each stanza starts with the Host directive and contains specific SSH options used when establishing a connection with the remote SSH server.

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

The Host directive can contain one pattern or a whitespace-separated list of patterns. Each pattern can contain zero or more non-whitespace character or one of the following pattern specifiers:

* - Matches zero or more characters. For example, Host * matches all hosts, while 192.168.0.* matches hosts in the 192.168.0.0/24 subnet.
? - Matches exactly one character. 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. For example, 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 reads the configuration file stanza by stanza, and 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 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 visiting the ssh_config man page.

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

SSH Config File Example

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

Typically, 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, simply by typing ssh dev, put the following lines to your "~/.ssh/config file:

~/.ssh/config

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

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

Let’s take 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 apply 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.

Override SSH Config File Option

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

Options specified from the command line.
Options defined in the ~/.ssh/config.
Options defined in the /etc/ssh/ssh_config.

If you want to override a single option, you can 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 how to configure your user ssh config file. You may also want to set up an SSH key-based authenticationand connect to your Linux servers without entering a password.

By default, SSH listens on port 22. Changing the default SSH portadds 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.

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