Networking / Beginners

The netstat command

The netstat command lets you monitor just about every aspect of a Linux server's network functions. This command can generate page after page of interesting information - if you know what it all means.

The two most common reasons to use netstat are to display the routing table and to display open TCP/IP connections. The syntax for displaying the routing table is

netstat -r

This results in a display similar to this:

Kernel IP routing table
Destination 	Gateway Genmask 	Flags MSS Window irtt Iface
192.168.1.0 	* 	255.255.255.0 	U 	0 	0 0 	eth1
192.168.1.0 	* 	255.255.255.0 	U 	0 	0 0 	eth0
127.0.0.0 	* 	255.0.0.0 	U 	0 	0 0	lo

To display TCP/IP connections, use this syntax:

netstat -l

This results in a display similar to the following:

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address 		Foreign Address State
tcp 	0 	0 	*:1024 			*:* 		LISTEN
tcp 	0 	0 	LSERVER:1025 		*:* 		LISTEN
tcp 	0 	0 	*:netbios-ssn 		*:* 		LISTEN
tcp 	0 	0 	*:sunrpc 		*:* 		LISTEN
tcp 	0 	0 	*:http 			*:* 		LISTEN
tcp 	0 	0 	*:x11 			*:* 		LISTEN
tcp 	0 	0 	*:ssh 			*:* 		LISTEN
tcp 	0 	0 	LSERVER:ipp 		*:* 		LISTEN
tcp 	0 	0 	LSERVER:smtp 		*:* 		LISTEN
tcp 	0 	0 	*:https 		*:* 		LISTEN
udp 	0 	0 	*:1024 			*:*
udp 	0 	0 	LSERVER:1026 		*:*
udp 	0 	0  	192.168.1.20:netbios-ns *:*
udp 	0 	0 	192.168.1.20:netbios-ns *:*
udp 	0 	0 	*:netbios-ns 		*:*
udp 	0 	0 	192.168.1.2:netbios-dgm *:*
udp 	0 	0 	192.168.1.2:netbios-dgm *:*
udp 	0 	0 	*:netbios-dgm 		*:*
udp 	0 	0 	*:940 			*:*
udp 	0 	0 	*:sunrpc 		*:*
udp 	0 	0 	*:631 			*:*
.
.
.

From this display, you can tell which Linux services are actively listening on TCP/IP ports.

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In this tutorial:

  1. Linux Commands
  2. Command Shell Basics
  3. Editing commands
  4. Wildcards
  5. Redirection and piping
  6. Environment variables
  7. Shell scripts
  8. Directory and File Handling Commands
  9. The cd command
  10. The mkdir command
  11. The rmdir command
  12. The ls command
  13. The cp command
  14. The rm command
  15. The mv command
  16. The touch command
  17. The cat command
  18. Commands for Working with Packages and Services
  19. The rpm command
  20. Commands for Administering Users
  21. The usermod command
  22. The chage command
  23. The passwd command
  24. The newusers command
  25. The groupadd command
  26. The groupdel command
  27. Commands for Managing Ownership and Permissions
  28. The chgrp command
  29. The chmod command
  30. Networking Commands
  31. The ipconfig command
  32. The netstat command
  33. The ping command
  34. The route command
  35. The traceroute command