Firewall Overview - Overview, add-ons and alternatives. (Kurt Seifried)
Firewalls - It's Time to Evolve or Die - Introduction, Securing the firewall itself, The network filter and proxy, The application filter and proxy, Examining and filtering encrypted data. (Kurt Seifried)
Foiling DNS Attacks - Discusses how people attack DNS servers and what can be done to better the security. Slightly more specifically, it shows: how crackers exploit a DNS servers; how can a DNS server be hardened via configuration; how to make it a pain to hack a DNS server.
Build a Flexible VPN with FreeS/WAN and Linux, Part 1 - For secure links between your corporate LAN and branch offices, telecommuters, and road warriors, FreeS/WAN and Linux provide security and flexibility for the cost of an old Pentium. (April 17, 2002)
Iptables Tutorial - Offers a look at iptables, explaining the theory and offering some examples of practice in setting up a simple firewall. (May 28, 2001)
Reject vs. Deny, Default-open vs. Default-closed - Kurt discusses a number of issues considered all too rarely by firewall administrators. Should you silently discard packets, or reject them with an ICMP error message? Should you take a default-open or a default-closed approach to traffic passing? (May 23, 2001)
Encrypting with Stunnel - Stunnel is a program that allows both programmers and system administrators to easily add encryption to arbitrary TCP sessions. You can SSL-enable clients and servers with ease -- and you can do so without interfering with program source. (May, 2001)
Linux Kernel 2.4 Firewalling Matures: netfilter - In yet another set of advancements to the kernel IP packet filtering code, netfilter allows users to set up, maintain, and inspect the packet filtering rules in the new 2.4 kernel. (February 14, 2001)
Linux Gets Stateful Firewalling: Introducing Netfilter - The 2.4 kernel's packet filtering system, Netfilter, is Linux's first stateful firewall. Stateful firewalls represent a major technological jump in the intelligence of a firewall and are present in all serious Enterprise firewalling products. (January 22, 2001)