let’s learn tcpdump
When I worked on IPv6 implementations I used tcpdump(1) on a daily basis. — Those times are long gone, but even today it is an extremely helpful tool. Just last week it helped me to debug a database connection problem.
To quote the great Rachel: “tcpdump -e can resolve a great many mysteries.”
If you never used tcpdump (or Wireshark) before then Julia Evan’s zine “let’s learn tcpdump” is a great place to start and to learn the 3-7 important command line parameters.