xl2tpd is an implementation of the Layer 2 Tunnelling Protocol
(RFC 2661). L2TP allows you to tunnel PPP over UDP. Some ISPs use
L2TP to tunnel user sessions from dial-in servers (modem banks, ADSL
DSLAMs) to back-end PPP servers. Another important application is
Virtual Private Networks where the IPsec protocol is used to secure
the L2TP connection (L2TP/IPsec, RFC 3193). The L2TP/IPsec protocol
is mainly used by Windows and Mac OS X clients. On Linux, xl2tpd can
be used in combination with IPsec implementations such as Openswan.
Example configuration files for such a setup are included in this RPM.

xl2tpd works by opening a pseudo-tty for communicating with pppd.
It runs completely in userspace but supports kernel mode L2TP. 

xl2tpd supports IPsec SA Reference tracking to enable overlapping
internak NAT'ed IP's by different clients (eg all clients connecting
from their linksys internal IP 192.168.1.101) as well as multiple
clients behind the same NAT router.

xl2tpd supports the pppol2tp kernel mode operations on 2.6.23 or higher,
or via a patch in contrib for 2.4.x kernels. Note that kernel mode and
IPsec SA Reference tracking do not yet work together.