Basic firewall using iptables

A firewall is a network service which is used to filter network traffic for unwanted traffic, block it, and allow the desired traffic to pass. The most powerful tool on Linux is iptables, which has kernel integration in recent versions of the kernels.

How to do it...

iptables is present, by default, on all modern Linux distributions. We will see how to configure iptables for common scenarios.

1. Block traffic to a specific IP address:

   #iptables -A OUTPUT -d 8.8.8.8 -j DROP
   

   If you run PING 8.8.8.8 in another terminal before running the iptables command, you will see this:

   PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
   64 bytes from 8.8.8.8: icmp_req=1 ttl=56 time=221 ms
   64 bytes from 8.8.8.8: icmp_req=2 ttl=56 time=221 ms
   ping: sendmsg: Operation not permitted
   ping: sendmsg: Operation not permitted
   

   Here, the ping fails the third time because we used the iptables command to drop all traffic to 8.8.8.8.

2. Block traffic to a specific port:

   #iptables -A OUTPUT -p tcp -dport...