Security Portal: Using Linux 2.4 Firewalling – Building a Firewall with Netfilter

“In my last column, I introduced Netfilter, Linux 2.4’s new
stateful firewalling package. Now, we’ll get to actually building a
firewall with Netfilter. While we’ll build a Small Office/Home
Office (SOHO) firewall script here, this article should be very
useful for any environment. This article should be useful for
network and system administrators, but also to the home user who
wants to secure a network or a single machine. CIO types might
find this article too detail-oriented — they should profit most
from the first part of this series, the aforementioned last column.
For the rest of you, go get yourself a cup of coffee or a can of
Mountain Dew: even a small firewall can be fairly involved.”

“This article assumes a basic knowledge of firewalls and
protocols. If you don’t yet have one, don’t despair. I’ve made a
“prerequisite” article on firewall placement, default deny/allow
policies, and very basic TCP/IP fundamentals:
http://securityportal.com/articles/prereq20010219.html.”

“Remember, to filter a service, you usually just need to filter
packets with the appropriate source/destination port. If you wanted
to block incoming telnet connections, you might block all incoming
TCP packets with destination port 23. If you wanted to block your
network from making external web connections, you’d block outgoing
TCP packets with destination ports 80 (HTTP) and 443 (HTTPS). Other
protocols, like FTP, are more complicated – in those cases, you
just need to learn a little about the protocol. I’ll discuss FTP
messiness later in this article. For now, let’s get going.”

Complete
Story