Firewall

In a typical network, all traffic to the outside world has to pass one router/computer/cluster. On this router, you can prevent access to specific network ports. It is called the firewall.

Now every Linux kernel can play firewall by deciding which network traffic to forward and which not. Starting with Linux 2.4 the respective command is iptables.

Check if your firewall is running

To check if your firewall is running, use the command iptables --list. Here's an output that means your firewall is turned off:

iptables --list
Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

If you look at it, you will find that for all incoming packets ("Chain INPUT" entry above), the policy is set to ACCEPT with no exceptions. The same is true for FORWARD and OUTPUT.