Linux Firewalls 
by Roberl L Ziegler
© 1999 New Riders Publishing
ISBN: 0735709009

Linux Routers 
A Primer for Network Administrators
by Tony Mancill
© 2001 by Prentice Hall
ISBN 0-13-086113-8

Building Linux and OpenBSD Firewalls
by Wes Sonnenrich and Tom Yates
© 2000 John Wily and Sons
ISBN 0-471-35366-3

Building Internet Firewalls 2nd Edition
by Elizabeth D. Zwicky, Simon Cooper and D. Brent Chapman
© 2000 O'Reilly and Associates