How to Use SSH Config

Connecting via SSH Config file

By default, the ssh config file is located inside ~/.ssh directory.

If the file is not present, you can create one using:

touch ~/.ssh/config

Now the format for writing a remote host configuration inside a config file is as follows:

Host <server-alias>
  HostName <server IP or url>
  User <username>
  IdentityFile <location of private key>

The space provided from the second line is not compulsory but helps in making the file more readable.

For our use case the configuration to connect to our AWS EC2 instance would be as follows:

Host nano-server
  HostName 174.129.141.81
  User ubuntu
  IdentityFile ~/t3_nano_ssh_aws_keys.pem

After saving the following configuration we can now ssh directly with the host name provided above.

ssh nano-server

Running the above command lets us connect to the EC2 instance directly.

SSH config file syntax

A single ssh config file can have multiple ssh configurations. For example:

Host HOST_NAME_1
  HostName IP_1
  User USER_1
  IdentityFile LOCATION_1Host HOST_NAME_2
  HostName IP_1Host HOST_NAME_3
  HostName Ifull list above the parameters like User IdentityFile are not mandatory and their presence can vary from one configuration to another.

The entire list of parameters can be found here

Along with having multiple configurations we can also use a lot of wildcards while creating out configuration files

• ( * ) Can be used as a substitute for one or more characters. For example, in case there is a common IdentityFile for all dev servers, we can add the following line in config file:

Host dev-*
  IdentityFile <location to identity file>

• ( ? ) Can be used as a substitute for a single character. For example, in case we want to write configuration for all servers, with same prefix we can write:

Host ????-server
  HostName 174.129.141.81
  User ubuntu

We can connect to this server via command like ssh nano-server tall-server omni-server but not via dev-server as dev only contains 3 characters.

• ( ! ) Can be used to negate the matches to the expression that is written after it

Host !prod-server
  User low-priority-user

The above configuration file would mean that until the host is prod-server set value of user field to low-priority-user

Based on these wildcards, we can write a sample configuration file as follows:

Host prod-server
  HostName xxx.xxx.xxx.xx
  User ubuntu
  IdentityFile ~/prod.pemHost stag-server
  HostName xxx.xxx.xxx.xx
  User ubuntu
  IdentityFile ~/stag.pemHost dev-server
  HostName xxx.xxx.xxx.xxHost !prod-server
  LogLevel DEBUGHost *-server
  IdentityFile ~/low-security.pem

In the above file we have defined separate configurations for prod-server and stag-server with their separate IdentityFile. While for dev-server and any other possible server, there is a default pemfile.

Also for all servers except prod-server the LogLevel is set to DEBUG: