Linux in a Nutshell, 3rd Edition by Ellen Siever, Stephen Spainhour, Jessica P. Hekman and Stephen Figgins. Sebastopol, California: O'Reilly & Associates, 2000. An extremely useful book that includes a complete reference for the iptables command.
The Home Page for Netfilter (iptables): netfilter.samba.org
The Home of rsync: rsync.samba.org. One of my favorite file-transfer tools, which works particularly well over tunnels.
The Home Page for Stunnel, a Versatile and Strong Tunneling Utility: www.stunnel.org
