IPSec-VPN on Stock Kernel
Attention: What ever you do, you do it on your own risk
You have to have firmware 1.34 already installed and opened for telnet access. If not, you can find a guide here: Open Stock Firmware LS-XHL
For Firmwares before 1.34 this also may work, but it's unknown if the IPSEC-XL2TP Packages are also in there.
What's the aim ?
The aim is to realize a VPN-Server that uses L2TP-IPSec as tunneling technology.
Why this, and not PPTP ? The issue with PPTP is, that it needs MPPE support within the kernel, which is simply not there.
Therefore we are going to use IPSec & L2TP, as they are more secury in most scenarios in any way.
What is needed ?
The good news are: everything is already on the box, you don't have to install any external software-package at all.
The bad news: The packages are configured to be used for a service called PocketU (only in Japan). As a matter of fact all boxes outside of Japan are not using those things at all.
As a general guidline for an IPSec-L2TP Server we need: IPSec - Package (here OpenSwan with pluto), an IPSec-Configuration and a tunnel-configuration L2TP - Package (here xl2tp), and xl2tp-Configuration and ppp.xl2tp options
How does it work
The VPN works as follows:
1) An IPSec tunnel will be opened (using a preshared-key or certificates)
2) Within the tunnel L2TP is used to authenticate a user and do IP-adressing with PPP in there