l2tp: Add L2TPv3 IP encapsulation (no UDP) support

This patch adds a new L2TPIP socket family and modifies the core to
handle the case where there is no UDP header in the L2TP
packet. L2TP/IP uses IP protocol 115. Since L2TP/UDP and L2TP/IP
packets differ in layout, the datapath packet handling code needs
changes too. Userspace uses an L2TPIP socket instead of a UDP socket
when IP encapsulation is required.

We can't use raw sockets for this because the semantics of raw sockets
don't lend themselves to the socket-per-tunnel model - we need to

Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 17 insertions, 0 deletions

diff --git a/net/l2tp/Kconfig b/net/l2tp/Kconfig
index d60758d60478..0a11ccf2d95b 100644
--- a/net/l2tp/Kconfig
+++ b/net/l2tp/Kconfig
@@ -51,3 +51,20 @@ config L2TP_V3
           If you are connecting to L2TPv3 equipment, or you want to
           tunnel raw ethernet frames using L2TP, say Y here. If
           unsure, say N.
+
+config L2TP_IP
+        tristate "L2TP IP encapsulation for L2TPv3"
+        depends on L2TP_V3
+        help
+          Support for L2TP-over-IP socket family.
+
+          The L2TPv3 protocol defines two possible encapsulations for
+          L2TP frames, namely UDP and plain IP (without UDP). This
+          driver provides a new L2TPIP socket family with which
+          userspace L2TPv3 daemons may create L2TP/IP tunnel sockets
+          when UDP encapsulation is not required. When L2TP is carried
+          in IP packets, it used IP protocol number 115, so this port
+          must be enabled in firewalls.
+
+          To compile this driver as a module, choose M here. The module
+          will be called l2tp_ip.