netstat– to display TCP/UDP connections/interface statistics/routing table
By default, this command displays all open sockets only.
-r: to display routing table
-g: to display multicast group
-i : to display all network interfaces
-s: to display IP, ICMP, TCP, UDP statistics
-n: to show numeric address
-c: to display the output continuously
-p: to display which program opens the sockets
-l: to show only listening sockets
-a: to show all sockets
<number>: to display the interface after ‘number’ sec delay
ESTABLISHED – the TCP connection is established.
LISTEN – The socket is listening on that port#
CLOSED – the connections is closed
SYN_SENT, SYN_RECV,FIN_WAIT1,FIN_WAIT1 – States when SYN/FIN messages are sent to open/close the connection
- “cat /etc/protocols” displays all port number – protocol mapping.
- All outputs are taken from specific file in “/proc/net” folder