Network Connection Monitoring in Linux

ss             list out all connections.
ss -t          tcp only.  
ss -u          udp only.
ss -x          unix only.
ss -a -A udp   report both "CONNECTED" and "LISTENING" sockets udp
ss -nt         tcp with hostnames not resolved.
ss -ltn        listening sockets only, tcp with hostnames not resolved.
ss -pnt        tcp with hostnames not resolved, associated process id's using connection. <- very useful
ss -s          summary statistics

netstat -natp  show active Internet connections
netstat -tupn

tcpdump -X -i eth0