online resource for firewalls

is there a document or book preferably downloadable or online to teach me about Ideas and principles of firewalls

How deep would you like to go…

This deep:
http://www.ipcortex.co.uk/wp/fw.rhtm

This deep:
http://ornellas.apanela.com/dokuwiki/pub:firewall_and_adv_routing

Or maybe this deep:
http://www.linuxhomenetworking.com/wiki/index.php/Quick_HOWTO_:_Ch14_:_Linux_Firewalls_Using_iptables

And when you consider the last link considers itself to be a “Quick how to”… you can see the “network routing and security” rabbit hole goes down and down and down and down and…

Or perhaps the Cisco route, where you can spend a few years of your life, and £10,000+ becoming a CCNA Associate, and CCNA Security Associate, before even being allowed on this course:

:slight_smile:

the middle level