Network Connection Monitoring in Linux

ss

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

netstat -natp  show active Internet connections
netstat -tupn

View established connections and associated programs

sudo netstat -atupen | grep ESTABLISHED
netstat -nputw

continuously updated

netstat -nputwc

tcpdump

tcpdump -X -i eth0

lsof

View established connections and associated programs type TCP only

sudo lsof -nP -iTCP -sTCP:ESTABLISHED

ethtool

To get information about your Ethernet interface, you can use:

ethtool eth0

Replace eth0 with your actual interface name.

iftop

iftop is a real-time console-based network bandwidth monitoring tool.

sudo iftop