We're very impressed by the collective smarts of the security community and we highly recommend reading the whole list and investigating any tools you are unfamiliar with. Click any tool name for more details on that particular application, including the chance to read (and write) reviews. Many site elements are explained by tool tips if you hover your mouse over them. Enjoy!
Netfilter is a powerful packet filter implemented in the standard Linux kernel. The userspace iptables tool is used for configuration. It now supports packet filtering (stateless or stateful), all kinds of network address and port translation (NAT/NAPT), and multiple API layers for 3rd party extensions. It includes many different modules for handling unruly protocols such as FTP.
Read 2 reviews.
Latest release: version 4.2 on Aug. 30, 2015 (2 years ago).
Like Netfilter and ipfilter on other platforms, OpenBSD users love PF, their firewall tool. It handles network address translation, normalizing TCP/IP traffic, providing bandwidth control, and packet prioritization. It also offers some eccentric features, such as passive OS detection. Coming from the same guys who created OpenBSD, you can trust that it has been well audited and coded to avoid the sort of security holes we have seen in otherpacketfilters.
Read 4 reviews.
Latest release: version 5.3 on May 1, 2012 (5 years, 4 months ago).