Basic firewall using iptables

A firewall is a network service that is used to filter network traffic for unwanted traffic, block it, and allow the desired traffic to pass. The standard firewall tool for Linux is iptables, which is integrated into the kernel in recent versions.

How to do it...

iptables is present by default on all modern Linux distributions. It's easy to configure for common scenarios:

1.  If don't want to contact a given site (for example, a known malware site), you can block traffic to that IP address:

        #iptables -A OUTPUT -d 8.8.8.8 -j DROP

If you use PING 8.8.8.8 in another terminal, then by 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...