FreeBSD Handbook
(FreeBSD Documentation Portal)
referenceofficial manual for installing and administering FreeBSD base systems
OpenBSD Frequently Asked Questions
(openbsd.org)
referencesupplemental admin guide to the man pages: console and serial setup, pf example rulesets, current release
The NetBSD Guide
(netbsd.org)
referenceOfficial NetBSD handbook: installation and day-to-day administration of the BSD base system
OpenSMTPD
(opensmtpd.org)
toolOpenBSD's server-side SMTP implementation of RFC 5321, small and auditable, ISC licensed