Using Netstat

Introduction

The netstat command is a powerful networking tool used to display network connections, routing tables, interface statistics, masquerade connections, and multicast memberships. It's available on Unix-like operating systems, including Linux, and Windows.

Basic netstat Command

To see the basic usage of netstat, simply type the command without any options:

netstat

This will display a list of active connections and their states.

Viewing Listening Ports

To display all listening ports, use the -l option:

netstat -l

Proto Recv-Q Send-Q Local Address           Foreign Address         State      
tcp        0      0 *:http                  *:*                     LISTEN     
tcp        0      0 *:ssh                   *:*                     LISTEN

Displaying Numerical Addresses

To display numerical addresses instead of resolving hostnames, use the -n option:

netstat -n

Proto Recv-Q Send-Q Local Address           Foreign Address         State      
tcp        0      0 192.168.1.2:80          192.168.1.1:34567       ESTABLISHED
tcp        0      0 192.168.1.2:22          192.168.1.3:12345       ESTABLISHED

Displaying Routing Tables

To display the kernel routing tables, use the -r option:

netstat -r

Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
default         192.168.1.1     0.0.0.0         UG        0 0          0 eth0
192.168.1.0     *               255.255.255.0   U         0 0          0 eth0

Displaying Interface Statistics

To display statistics for each network interface, use the -i option:

netstat -i

Kernel Interface table
Iface       MTU  RX-OK RX-ERR RX-DRP RX-OVR TX-OK TX-ERR TX-DRP TX-OVR Flg
eth0       1500  12345      0      0      0  6789      0      0      0 BMRU
lo        65536   6789      0      0      0  6789      0      0      0 LRU

Displaying All Connections

To display all connections including TCP, UDP, and Unix socket connections, use the -a option:

netstat -a

Proto Recv-Q Send-Q Local Address           Foreign Address         State      
tcp        0      0 *:http                  *:*                     LISTEN     
tcp        0      0 192.168.1.2:80          192.168.1.1:34567       ESTABLISHED
udp        0      0 *:domain                *:*                                 
unix  2      [ ACC ]     STREAM     LISTENING     12345 /run/systemd/journal/stdout

Conclusion

The netstat command is an essential tool for network troubleshooting and monitoring. By understanding and using its various options, you can gain valuable insights into your network's performance and behavior.