What Linux or Unix version underlies Mac OS, OS X 10.6?
Is there a good documentation for that version, specifically regarding security, networks and similar topics?
Darwin is at the core of Mac OS X. There is documentation of it scattered all over the net, but I’ve yet to find a good central location that has it all.
A good starting point is Apple’s Open Source pages.
MacOS X uses Apple’s Darwin system as a base. Darwin uses parts of FreeBSD but massively diverged in many areas so you can’t really give version numbers or such.
Mac OS is mainly based on FreeBSD.
But of course, there is a considerable amount of proprietary code base on top of it. Especially the kernel, besides the user-land, has been heavily modified.
As for documentation, you can check these two books:
Mac OS X Security and OS X for Hackers at Heart
I haven’t personally read these books, but I was told they were OK.
PS: The TCP/IP stack is one of the areas that have been modified to a lesser extent. The changes are mostly in the data link interfaces, so Mac OS practically inherits a lot from FreeBSD on the TCP/IP stack.