Learn More from Books
    Design of 4.4BSD Operating System
        McKusick, Bostic, Carels, Quarterman
    Firewalls with Linux and OpenBSD
        Sonnenreich & Yates
    Building Internet Firewalls, 2e
        Elizabeth Zwicky, O'Reilly.
    Hacking Exposed
        McClure, Schambray, Kurtz
    UNIX System Administration, 2e
        Nemeth et al.
    See http://www.openbsd.org/books.html