How to block an IP in Linux

I often find that my server is being attacked by other computers. Brute force SSH attacks, viruses scanning for the ability to spread, things like that. I’ll go into the SSH brute force defenses in a later post, but for now I’ll cover how to easily block an IP address.

First, I’ll assume you are already using iptables on Debian.


I have a small script called “block” which looks like this:

#!/bin/bash
sudo iptables -I INPUT -s $1 -j DROP
sudo bash -c "iptables-save > /etc/network/iptables.save"

Whenever I find a “bad” IP in my logs or notifications, I just run:

block bad.ip.add.18

Substituting the bad ip for that nonesense above. This adds it to the list of IP address which iptables will simply drop any incoming packets from, and saves the in memory iptables configuration, so that it is preserved through reboots.

Then in your /etc/network/interfaces file, just add this at the bottom:

post-up iptables-restore /etc/network/iptables.save