Some handy command I encountered while debugging networking related problems:

netstat -lnup

Show all process listening for UDP packets.

tcpdump -vv -X -i eth0 udp port 60001

Captures all UDP packets received on port 60001.

Testing connectivity using netcat (nc)

Server: netcat -lu 60001
Client: netcat -u 1.1.1.1 60001

See more here