SSH - Advanced Usage

Introduction

This page is dedicated to advanced SSH usage examples. We will discuss the following concepts:

Config files

Config files allow you to specify certain rules for all or chosen hosts. The file has a really simple structure. It is divided into sections which begin with the Host keyword. Sections are read one by one and the first matching section takes precedence over the remaining sections - you write more specific sections at the top and the more general sections below.

Why even bother?

You might say that SSH client doesn't need any special configuration - you just type the user@host and that's it. Well, what happens when you manage multiple servers? Maybe you want to use a different pair of keys for each servers? Maybe the server uses a port other than the default 22 to avoid automated bots trying to log in?

That's where config files come in handy!

Example scenario

Let's assume that you manage 3 servers, with the following access info:

  1. very.long.hostname.example1.com
    • user: admin
    • port: 22
    • key name: id_rsa
  2. example2.com
    • user: billthemaster
    • port: 2222
    • key name: example2_ecdsa
  3. 192.168.133.7
    • user: management
    • port: 22
    • key name: id_rsa

You got tired having to always specify the identity file location with the -i option and the port with -p option for example2.com. Don't even mention admin@very.long.hostname.example1.com!

In the given example, the config file could look like this: 

Host server1
  HostName very.long.hostname.example1.com
  User admin
  IdentityFile ~/.ssh/id_rsa

Host server2
  HostName example2.com
  Port 2222
  User billthemaster
  IdentityFile ~/.ssh/example2_ecdsa

Host server3
  HostName 192.168.133.7
  User management
  IdentityFile ~/.ssh/id_rsa

Host *
  IdentityFile /path/to/some/other/key

You can see here usage of Host *. Options specified in this section will affect all other hosts.

But where do I put this file?

SSH looks for the options in the following order:

  1. command line arguments
  2. ~/.ssh/config
  3. /etc/ssh/ssh_config

You can also specify a custom path with the -F argument, for example: 

ssh -F ~/Documents/projects/someproject/config/ssh production

...or discard any config file: 

ssh -F /dev/null username@hostname

There's more to ssh config files, but I direct you to man ssh_config for more information