Very simple shell script for updating Duck DNS or Namecheap with your IP address from a given interface (or letting the remote side figure out your public IP).
./update-duckdns.sh domain token [interface]
./update-namecheap.sh host domain password [interface]
Both domain and token are required and interface is required if you'd
like to use a local interface's IP address (such as eth2 or wlan0). If
interface is omitted, the remote side will determine your public IP address
and use that.