This series aims to highlight the project’s signature security features and development practices — razor-sharp focus on correct and secure code coupled with continuing code audit — as well as the project’s role as a source of innovation in security practices and an ‘upstream’ source for numerous widely-used components such as OpenSSH, PF, LibreSSL and others. Part 1 focused on the history, Part 2 focused on usage and user experience, and this final part will looks at that packet filter (PF).