netfilter Operation: Part Five

netfilter operationSimulating the Windows Firewall

Now it’s time to configure the firewall. The built-in firewall on Windows XP/Vista/7/8 is enabled by default (on XP Service Pack 2 or better). The standard configuration is to allow outbound connections from the host system and deny inbound connections unless they are explicitly configured. The Windows firewall also allows any traffic that is a reply to traffic that the host originally generated. After you execute the iptables -F command to flush out all the previously configured rules, the following commands would configure the Linux host similarly:

iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -A INPUT -m state –state ESTABLISHED,RELATED -j ACCEPT

The –state extensions track the current status of the connections. By specifying ESTABLISHED or RELATED, the firewall allows packets that are starting a new session, but where the session is related to an existing session (such as an FTP data session). If you were hosting a service on this system, such as a Web server, you would need to configure the INPUT chain appropriately. This configuration would afford any Linux system a minimum level of firewall security with virtually no impact to its overall functionality.

Simulating a Consumer-Grade Home Router

With the basics of iptables configuration out of the way, let’s consider a more practical example. For a typical firewall, there is very little traffic destined to or from the firewall itself. In general, the only traffic that would fit this profile would be administrative sessions to configure the firewall itself. The vast majority of a firewall’s traffic is passing through the firewall, and will thus be checked against the FORWARD chain. The following examples would configure the Linux firewall with the same access controls as a typical home network router such as a Linksys or Netgear router/firewall. This example assumes that is the internal network on interface eth0 and the external interface is eth1.

iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -A INPUT -p tcp -s -i eth0 –dport 80 -j ACCEPT

iptables -A FORWARD -s -i eth0 -0 eth1 -j ACCEPT

iptables -A FORWARD -m state –state ESTABLISHED,RELATED -j ACCEPT

One important element is to always remember that if you have configured the default policy for a chain to DROP (for example, iptables -P FORWARD DROP) that you will need to include an explicit rule to permit the return traffic. This can be done by using the following command:

iptables -A -m state –state ESTABLISHED,RELATED -j ACCEPT

So if you wanted to permit the return traffic for a FORWARD chain, you would enter:

iptables -A FORWARD -m state –state ESTABLISHED,RELATED -j ACCEPT

Many hours of troubleshooting Linux firewalls have been spent by overlooking a rule that permits the return traffic.

We will continue our look at netfilter firewall configuration in the next article.

External Links:

Simulating windows firewall with iptables at

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