Comment 11 for bug 313218

Replace the ip with the one of your router.

This dumps dns traffic only, probably what you need:
sudo tcpdump -nls128 host 192.168.1.1 and port 53 | tee tcpdump.txt

This dumps everything going from/to your router:
sudo tcpdump -nls128 host 192.168.1.1 | tee tcpdump.txt