http://metalab.unc.edu/pub/Linux/docs/HOWTO/mini/VPN/: required reading for this article.
http://www.cs.hut.fi/ssh/: follow links to download ssh files, to be unpacked and compiled. I used ssh v1.2.26, the high-water mark of v1. v2 differs in internal code and licensing.
http://www.replay.com/redhat/ssh.html: download ssh in RPM form, installable pre-compiled binaries. Easier to install, but you forego making certain configuration choices that can be set only at compile time.
http://www.uni-karlsruhe.de/~ig25/ssh-faq/: ssh (Secure Shell) FAQ: frequently asked questions.
http://www.tac.nyc.ny.us/~kim/ssh/: getting started with ssh.
http://www.net.lut.ac.uk/psst/: Win32 links, among others. MS machines need their own client to interact with sshd on a Linux server.
ssh man page, sshd man page, ssh-keygen man page, README.SSH all install with ssh.
http://rlz.ne.mediaone.net/linux/faq/index3.shtml: discussion and concrete example of firewall building with Linux ipfwadm.
http://metalab.unc.edu/pub/Linux/docs/HOWTO/PPP-HOWTO/