Recommended way to setup an encrypted tunnel (a VPN)

[Stephane Bortzmeyer <bortzmeyer@netaktiv.com>]

I have to connect two networks together and the virtual link needs to
be safely encrypted (some users know SSH but some will just POP
blindly and LDAP in woody is not SSLized anyway).

I wonder what is the recommended way to setup an encrypted tunnel (to
make a VPN) between two Debian boxes:

- I tried pipsecd + userlink. The userlink module seems severely
  broken, at least with kernel 2.4. A simple ifconfig stays in D 'disk
  wait' forever!

- ssh + ppp seems interesting because I know both of them. But is
  there a trick when you combine them?
  http://www.linuxdoc.org/HOWTO/VPN-HOWTO.html does not seem to be
  maintained.

- GRE module in the kernel? (I use 2.4 on woody) Anyone has something
  to say about it?