what to study

[Cyber]

What should i be studing to write my own firewall software, and is this complex as it sounds

Thanks

[OBonio]

The ins and outs of the TCP/IP stack and how network traffic gets from A to B. Is it a hardware or software firewall? If it's hardware it's just a case of intercepting and rebroadcasting packets. If it's a software firewall you will need to know how to hook into the OS's TCP/IP stack (I think there are APIs for this in Windows). Network topology and the 7 layers of networking.

So basically know ALL there is to know about networking. Then there's the easy part of filtering packets (ie, this packet is destined for computer X on port Y, is this allowed in the rules?).

Yes, it's complicated but very interesting stuff.

[fo3nix]

If you are asking this sort of question on this sort of site, I would argue that you need to learn considerably more things before you can even attempt to create a firewall.