Barbwire

Barbwire is an industrial-strength firewall that gives you control over your network traffic. It can either drop a packet or reject a request, depending on how you set it up.

Packet Filtering with Barbwire

This firewall lets you filter packets using four main actions: DROP, LOG, ACCEPT, or REJECT. It has three built-in chains to handle these actions.

The Three Chains Explained

The first chain is called FORWARD, and it's for all non-locally generated packets that aren't meant for your localhost (which is basically the firewall itself). Then, there's the INPUT chain, which deals with packets heading straight to your local host (the firewall). Lastly, we have the OUTPUT chain, used for packets created locally.

NAT Table Functions

The NAT table is mainly used for Network Address Translation. The PREROUTING chain alters packets as soon as they enter the firewall. The OUTPUT chain changes locally generated packets before they make routing decisions. Finally, the POSTROUTING chain tweaks packets just before they leave the firewall.

Mangle Table Usage

The mangle table is where you can modify packet headers and other details. It includes two built-in chains: PREROUTING and OUTPUT. The PREROUTING chain alters packets right when they enter the firewall, while OUTPUT changes locally generated ones before routing decisions are made. Keep in mind that mangle can't be used for any NAT operations; that's what the nat table is for!

A Practical Example with Barbwire

If we take an example where IP address 24.226.215.66 pings (ICMP) 24.226.215.67, you have two choices on how to respond through the firewall:

• You can configure it to DROPPED packets: In this case, no message goes back to the requester.


• You can choose to REJECT requests: Here, the requester receives a "network unreachable" response.

No matter which option you choose above, it will only take one hop for that request!

If you're interested in learning more about how to download Barbwire and make your network stronger, check out this link: [Download Barbwire].