You want to change the default behavior of ssh.
Create a host alias named "*" in ~/.ssh/config:
Host * keyword value keyword value ...
If this is the first entry in the file, these values will override all others. If the last entry in the file, they are fallback values, i.e., defaults if nobody else has set them. You can make Host * both the first and last entry to achieve both behaviors.
We are just taking advantage of a few facts about host aliases in the configuration file:
Earlier values take precedence
The aliases may be patterns, and "*" matches anything
All matching aliases apply, not just the first one to match your ssh command
So if this is your ~/.ssh/config file:
Host * User smith Host server.example.com User jones PasswordAuthentication yes Host * PasswordAuthentication no
then your remote username will always be smith (even for server.example.com!), and password authentication will be disabled by default (except for server.example.com).
You can still override host aliases using command-line options:
$ ssh -l jane server.example.com The -l option overrides the User keyword
ssh_config(5) documents the client configuration keywords.