howtouselinux

Understanding SSH config file with Examples

Table of Contents

 

If you’ve ever worked with ssh, you know that it can be a bit of a pain to configure. There are so many options and parameters! One way to make working with ssh a bit easier is by using an ssh config file. In this blog post, we will discuss what ssh config files are, how to use them, and some of the benefits they offer.

What is an ssh config file?

An ssh config file is a text file that contains all of your ssh connection information. This includes the hostname of the server you’re connecting to, the username you’re using to connect, the port number, and the protocol you want to use. You can also specify a key file to use for authentication, as well as other options.

Why should I use an SSH config file?

There are many benefits to using an SSH config file. Some of the most common benefits include:

  • Consistent settings across all servers – This makes it easy to keep your settings consistent across all of your servers. You can create separate configurations for each server you connect to, or you can create global configurations that apply to all servers.
  • Easy configuration for multiple connections – You can create separate configurations for each situation, allowing you to easily specify different settings for different situations. For example, you could create a configuration that uses a specific key file when you’re connecting from your office, and another configuration that uses a different key file when you’re connecting from home.

 

How do I create an SSH config file?

There are a few different ways to create an ssh config file. Here is one example.

1. Launch the terminal application on your local computer and create your config file in your home directory:
touch ~/.ssh/config
NOTE:
Your .ssh directory is automatically created when you use the ssh command for the first time. If you have never used ssh before under this user account please create the directory first using:
mkdir ~/.ssh/ && chmod 700 touch ~/.ssh/config
2. Edit the file using vi or any Unix text editor you are comfortable with. We will use vi in this example:
vi ~/.ssh/config

Where should I store my SSH config file?

The location of your SSH config file will vary depending on your operating system and the software you’re using to edit it. The most common locations are:

  • macOS: /etc/ssh/ssh_config or ~/.ssh/config
  • Linux: /etc/ssh/ssh_config or ~/.ssh/config
  • Windows: C:\Program Files (x86)\PuTTY\Configuration\ or %APPDATA%\Roaming\.putty\Configuration

 

Example of SSH config file

Host server
HostName howtouselinux.com
User howtouselinux
Port 4242
IdentityFile ~/.ssh/id_rsa

When we run ssh server command, it will try  to login server howtouselinux.com with user howtouselinux, port 4242 and private key file ~/.ssh/id_rsa.

can I use host pattern in an SSH config file?

A pattern for Host directive is nothing but IP address, DNS hostname, or combination of special wildcard characters. For example, ? wildcard that matches exactly one character. * wildcard matches zero or more characters. The following example matches the host a.howtouselinux.com and b.howtouselinux.com.

Host *.howtouselinux.com
User howtouselinux
IdentityFile ~/.ssh/id_ed25519.pub

What options are available in an SSH config file?

The ssh config file contains all of your ssh connection information, including the hostname of the server you’re connecting to, the username you’re using to connect, the port number, and the protocol you want to use. You can also specify a key file to use for authentication, as well as other options.

Can I use an SSH config file with all types of ssh connections?

Yes, you can use an SSH config file with all types of ssh connections. The configuration options will vary depending on the type of connection being used. The following is an example.

### default for all ##
Host *
ForwardAgent no
ForwardX11 no
ForwardX11Trusted yes
User howtouselinux
Port 22
Protocol 2
ServerAliveInterval 60
ServerAliveCountMax 30

Can I store my private key in an SSH config file?

Yes, you can store your private key in an ssh config file. This will allow you to use your private key for authentication without having to enter it each time.

Host host
HostName 192.168.1.100
User root
IdentityFile ~/.ssh/host.key

What if I don’t have an SSH config file?

If you don’t have an ssh config file, then you’ll need to enter your connection information each time you connect to a server. This can be tedious and error-prone, so it’s a good idea to create a configuration file for your most common connections.

How do I change the settings in my SSH config file?

The settings in your SSH config file can be changed by editing the text file itself or by using a graphical interface like PuTTY. On Linux and macOS, we can use vi command. On Windows, PuTTY provides its own graphical interface for editing ssh config files.

How do I connect to a server using my specified SSH config file?

To use your specified SSH config file, you’ll need to specify the location of the file when you connect to a server. The command will vary depending on your operating system and software. For example, on macOS you would use “ssh -F /etc/ssh/ssh_config username@server”. On Linux, the command is usually “ssh -F ~/.ssh/config username@server”.

-F configfile

Specifies an alternative per-user configuration file. If a configuration file is given on the command line, the system-wide configuration file (/etc/ssh/ssh_config) will be ignored. The default for the per-user configuration file is ~/.ssh/config.

What if I have multiple SSH keys?

If you have multiple SSH keys, you can specify which key to use for authentication by adding the “IdentityFile” option to your ssh config file. For example, “IdentityFile ~/.ssh/id_rsa”. This will tell ssh to use the id_rsa key file for authentication of this host.

What if I need to connect to a server that doesn’t have an ssh config file?

If you need to connect to a server that doesn’t have an ssh config file, then you’ll need to enter your connection information manually. You can also create a global SSH config file that will apply to all servers.

What is the difference between a System-wide and User-specific SSH config file?

  • System-wide OpenSSH config file client configuration – /etc/ssh/ssh_config : This files set the default configuration for all users of OpenSSH clients on that desktop/laptop and it must be readable by all users on the system.
  • User-specific OpenSSH file client configuration – ~/.ssh/config or $HOME/.ssh/config : This is user’s own configuration file which, overrides the settings in the global client configuration file, /etc/ssh/ssh_config.

 

Can I use both a System-wide and User-specific SSH config file?

Yes, you can use both a System-wide and User-specific SSH config file. The ssh command reads its configuration in the following order:

  • ssh command line-option
  • ~/.ssh/config option
  • /etc/ssh/ssh_config options
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