How to start writing a firewall for linux? [closed]

Question

I want to develope a firewall for Linux. I prefer C/C++ language.

Is there any simple sample code for writing a firewall?

Which libraries should I use?

Update: There are some firewalls for Linux, but I want develope a simple firewall for learning.

Answer 1

You can start by using the Netfilters API (http://www.netfilter.org/). I think it is a good starting point for packet filtering. I've worked a lot with this API in kernel space. I'm not sure if there is a library for user land, but I must tell you that it's pretty easy to develop something with netfilters in the kernel.

As an example, iptables use the netfilters API.