SSH Config Precedence

Let’s consider the below example to understand more about the precedence in ssh options when multiple hosts are defined,

Host nancy
   HostName 10.21.43.150
   Compression no

Host jancy
   HostName 10.2.33.58
   PermitLocalCommand no

Host trial
   HostName 10.2.33.57
   PermitLocalCommand yes
   
Host *cy !jancy
   User woot
   Port 22
   
Host *
   User root
   Port 22
   Compression yes

• When a user types ssh nancy, the ssh client applies the options from the first match which is Host nancy. Then it checks for the next matching pattern which is Host *cy !jancy. Then the next matching pattern is Host *, here none of the options are considered as they got over-ridden in the earlier matched patterns. The full list of options are,

Host nancy
  HostName 10.21.43.150
  User woot
  Port 22
  Compression no

• When a user types ssh jancy, the matching patterns are Host jancy and Host *. The options used are,

Host jancy
  HostName 10.2.33.58
  Port 22
  User root
  Compression yes
  PermitLocalCommand no

• When the user types ssh trial, the matching patterns are Host trial and Host *,

Host trial
   HostName 10.2.33.57
   PermitLocalCommand yes
   User root
   Port 22
   Compression yes

• For all other remote connections, the ssh client will use Host *cy !jancy and Host *.

At times we may need to access multiple remote systems with different IP addresses/hostnames, usernames, non-standard-ports with various command-line options. One way is to create a bash alias for each remote connection. But we have an easy and much better solution to this problem. OpenSSH allows to create per-user configuration file to store different SSH options for each remote server. In this article, we will see the basics of SSH Client configuration with some examples.