localsshconfig

# ssh config to connect to a host via local network if available.
#
# For example, if the host is 192.168.1.10 on your local network,
# first run ssh-keyscan 192.168.1.10 to see what keys are available,
# then substitute that key for your_host_key (-t selects the key type).

Match exec "ssh-keyscan -T 1 -t ed25519 local.ip.for.host 2>/dev/null | grep -F your_host_key" host yourhostname
    Hostname local.ip.for.host
    Port 22

Host yourhostname
    Hostname external.ip.for.host
    Port 1234
    # Additional shared config options here...