Hi, i would like to develop a firewall box that have a list of function :- 1. Packet filtering (stateful)
2. VPN
3. Routing
4. Logging

May i know which references and source should i refer.

Search for like Linux router distro's?

Errmmm unspawn, i'm not to implement a linux router, but linux security box that have a functionality as a firewall. This box should have a features like VPN, routing and logging. So that i would like to know where is the source or reference should i refer.

Well, basically you're looking at your basic Netfilter tutorials, SANS' Firewalls & Perimeter Protection, maybe Firewall, VPN, Intrusion Detection, and Router Tips, some Advanced Networking Overview, the Linux Advanced Routing & Traffic Control HOWTO, and looking for the right firewall/VPN (management) tools.

Then, if you combine
netfilter = ingress/egress logging
netfilter = access restriction
netfilter + ip2route = routing capabilities
then you could argument a router distro like Astaro, Smoothwall, Wolverine/Coyote, Keeper and whatnot could provide either a good basis to decide on what you're looking for or provide you with a headstart wrt ease of configuration, time and whatnot.

That's why I said that (having a real problem with Occam's Razor thingie) and of course I should have been clearer.