Porting Kernel Code to Four BSDs and Linux

The U.S. Naval Research Laboratory develops and maintains a freely available IPv6 and IP Security distribution. All of the software builds and runs on BSD/OS, FreeBSD, NetBSD, and OpenBSD, and a growing portion of the software builds and runs on Linux. Each of the four BSDs has evolved significantly from their original 4.4BSD-Lite ancestor, and increasingly more of that evolution is along divergent paths. Linux shares no significant ancestry with the BSDs, but is still a POSIX system, which means that many of the same high-level facilities are available even though their implementation might be completely different. This paper discusses many of the differences and many of the similarities we encountered in the internals of these systems. It also discusses the techniques and glue software that we developed for isolating and abstracting the differences so that we could build a significant base of system code that is portable between all five systems.