Regular Expression matching in ssh config

Solution 1:

This should do the trick on OpenSSH 5.5 and greater.

Host *.*
  Hostname %h

Host *
  Hostname %h.domain.com
  IdentityFile /path/to/keyfile.pem

The first rule matches any normal domain names and just passes the domain name through. The second rule handles single word hosts and appends domain.com to the end.

One side effect of this is that even for ssh calls to anotherdomain.com, ssh will try to use the IdentityFile for domain.com. I don't think that matters however.

Solution 2:

This sounds like a problem best solved by DNS. Add this to your /etc/resolv.conf:

search domain.com

If a DNS lookup contains no dots¹ or returns an NXDOMAIN >response then another DNS lookup will be made with that search value >appended.

Examples:

If you do ssh srv1, the DNS lookup will be made for srv1.domain.com.

If you do ssh srv1.dc1, the DNS lookup will be for srv1.dc1 which will return NXDOMAIN. The automatic followup DNS lookup will be for srv1.dc1.domain.com.

You can add multiple search domains to that line separated by whitespace and they will be tried in the order listed until one of them returns an A record².

1.) This value is configurable and refers to the number of dots the short name must have fewer than. The default value is 1 and it should be set higher than 1 for sites where the hosts are of the form srv1.dc1.domain.com. This avoids the useless request to the root servers for the dc1 top level domain.

2.) Or an AAAA record.

Updated answer 25th Nov 2020:

Although the outdated answer above is still a fully valid one, nowadays, I would rather suggest using the ssh_config built-in mechanism CanonicalDomains

That would mean, instead of changing /etc/resolv.conf or DNS records, you can simply add the following lines to the top of your ssh_config file:

CanonicalizeHostname yes
CanonicalDomains domain.com

Please refer to the official ssh_config documentation for details around these configuration statements.

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Solution 3:

You can use ProxyCommand to use regex on the host name specified on the command line.

Host srv*.domain.com
    User amac
    ProxyCommand nc $(sed -e "s/.domain.com//" <<< "%h") %p
    IdentityFile /home/amac/.ssh/id_rsa

Now ssh srv23.domain.com would connect to srv23.

Note, you don't need to specify HostName.