Linux : Check Network Connection Command

1. ss command: It dump socket (network connection) statistics such as all TCP / UDP connections, established connection per protocol (e.g., display all established ssh connections), display all the tcp sockets in various state such as ESTABLISHED or FIN-WAIT-1 and so on.
2. netstat command: It can display network connections, routing tables, interfaces and much more.
3. tcptrack and iftop commands: Displays information about TCP connections it sees on a network interface and display bandwidth usage on an interface by host respectively.

Display Currently Established, Closed, Orphaned and Waiting TCP sockets, enter:

# ss -s
Or you can use the netstat command as follows:
# netstat -s

Display All Open Network Ports

Use the ss command as follows:
# ss -l

OR Use the netstat command as follows:
# netstat -tulpn

Display All TCP Sockets

Type the ss command as follows:
# ss -t -a
Or use the netstat command as follows:
# netstat -nat

Display All UDP Sockets

Type the ss command as follows:
# ss -u -a
Or use the netstat command as follows:
# netstat -nau

lsof Command

# lsof -i :portNumber
# lsof -i tcp:portNumber
# lsof -i udp:portNumber
# lsof -i :80 | grep LISTEN

View Established Connections Only

Use the netstat command as follows:
# netstat -natu | grep 'ESTABLISHED'

Say Hello To tcptrack

The tcptrack command displays the status of TCP connections that it sees on a given network interface. tcptrack monitors their state and displays information such as state, source/destination addresses and bandwidth usage in a sorted, updated list very much like the top command.
# tcptrack -i eth0

iftop command

The iftop command listens to network traffic on a given network interface such as eth0, and displays a table of current bandwidth usage by pairs of hosts:
# iftop -i eth1
It can display or analyses packet flowing in and out of the 192.168.1.0/24 network:
# iftop -F 192.168.1.0/24