How to automatically switch ssh config based on local subnet?

Question

When I\'m on a certain network (subnet is 10.10.11.x) I need to jump through an intermediate host to reach my destination because of destination port I can\'t change and limited

Answer 1

Instead of checking the subnet CIDR, you can check what domain suffix DHCP has given you.

• If you're trying to use a jump box when outside the intranet, this approach is more robust than checking for IP ranges in the reserved private allocation (e.g. your home network or remote-work location uses the same block as example.com's intranet).

• This approach is not useful if your intranet uses the same DNS suffix everywhere and you're trying to traverse subnets within the intranet. If that's your situation, use Jakuje's solution.

Match Host web Exec "hostname -d | ! grep -q -E '^example\.com'"
    ForwardAgent yes
    ProxyCommand ssh -p 110 -q relay.example.com nc %h %p
Host web
    HostName web.example.com
    Port 1111

If you don't have a recent version of hostname that has the -d option (e.g. you're on MacOS), you can just query resolve.conf directly:

Match Host web Exec "! grep -q -E '^\s*search[ \t]+example\.com' /etc/resolv.conf"
    ...
    ...