The last step in securing our server is to install a firewall. The idea behind a firewall is that every exposed port is a potential security vulnerability. Therefore, we want to expose as few ports as possible.
All Linux distributions come with a firewall called iptables, which, by default, allows all traffic to pass through. Configuring iptables by hand can be challenging as the format is not the most intuitive. For example, an inactive iptables configuration looks like this:
$ sudo iptables -L -n -vChain INPUT (policy ACCEPT 0 packets, 0 bytes) pkts bytes target prot opt in out source destination Chain FORWARD (policy ACCEPT 0 packets, 0 bytes) pkts bytes target prot opt in out source destination Chain OUTPUT (policy ACCEPT 0 packets, ...
