IPSEC: secure IP over the Internet [48]
IPsec/L2TP Tunneling with Linux-2.6 and Racoon [49]
Using a Linux L2TP/IPsec VPN server [50]
Using a Linux server with the Microsoft L2TP/IPsec VPN Client [51]
FreeBSD vs. WIN2K IPsec road warrior HOWTO [52]
Basic L2TP/IPsec Troubleshooting in Windows XP [53]
Netfilter Extensions HOWTO [54]