netfilter: introduce l2tp match extension

Introduce an xtables add-on for matching L2TP packets. Supports L2TPv2
and L2TPv3 over IPv4 and IPv6. As well as filtering on L2TP tunnel-id
and session-id, the filtering decision can also include the L2TP
packet type (control or data), protocol version (2 or 3) and
encapsulation type (UDP or IP).

The most common use for this will likely be to filter L2TP data
packets of individual L2TP tunnels or sessions. While a u32 match can
be used, the L2TP protocol headers are such that field offsets differ
depending on bits set in the header, making rules for matching generic
L2TP connections cumbersome. This match extension takes care of all
that.

Signed-off-by: James Chapman <[email protected]>
Signed-off-by: Pablo Neira Ayuso <[email protected]>

include/uapi/linux/netfilter/Kbuild

@@ -58,6 +58,7 @@ header-y += xt_helper.h
 header-y += xt_ipcomp.h
 header-y += xt_iprange.h
 header-y += xt_ipvs.h
+header-y += xt_l2tp.h
 header-y += xt_length.h
 header-y += xt_limit.h
 header-y += xt_mac.h

include/uapi/linux/netfilter/xt_l2tp.h

@@ -0,0 +1,27 @@
+#ifndef _LINUX_NETFILTER_XT_L2TP_H
+#define _LINUX_NETFILTER_XT_L2TP_H
+
+#include <linux/types.h>
+
+enum xt_l2tp_type {
+	XT_L2TP_TYPE_CONTROL,
+	XT_L2TP_TYPE_DATA,
+};
+
+/* L2TP matching stuff */
+struct xt_l2tp_info {
+	__u32 tid;			/* tunnel id */
+	__u32 sid;			/* session id */
+	__u8 version;			/* L2TP protocol version */
+	__u8 type;			/* L2TP packet type */
+	__u8 flags;			/* which fields to match */
+};
+
+enum {
+	XT_L2TP_TID	= (1 << 0),	/* match L2TP tunnel id */
+	XT_L2TP_SID	= (1 << 1),	/* match L2TP session id */
+	XT_L2TP_VERSION	= (1 << 2),	/* match L2TP protocol version */
+	XT_L2TP_TYPE	= (1 << 3),	/* match L2TP packet type */
+};
+
+#endif /* _LINUX_NETFILTER_XT_L2TP_H */

net/netfilter/Kconfig

@@ -1131,6 +1131,16 @@ config NETFILTER_XT_MATCH_IPVS
 
 	  If unsure, say N.
 
+config NETFILTER_XT_MATCH_L2TP
+	tristate '"l2tp" match support'
+	depends on NETFILTER_ADVANCED
+	default L2TP
+	---help---
+	This option adds an "L2TP" match, which allows you to match against
+	L2TP protocol header fields.
+
+	To compile it as a module, choose M here. If unsure, say N.
+
 config NETFILTER_XT_MATCH_LENGTH
 	tristate '"length" match support'
 	depends on NETFILTER_ADVANCED

net/netfilter/Makefile

@@ -137,6 +137,7 @@ obj-$(CONFIG_NETFILTER_XT_MATCH_HL) += xt_hl.o
 obj-$(CONFIG_NETFILTER_XT_MATCH_IPCOMP) += xt_ipcomp.o
 obj-$(CONFIG_NETFILTER_XT_MATCH_IPRANGE) += xt_iprange.o
 obj-$(CONFIG_NETFILTER_XT_MATCH_IPVS) += xt_ipvs.o
+obj-$(CONFIG_NETFILTER_XT_MATCH_L2TP) += xt_l2tp.o
 obj-$(CONFIG_NETFILTER_XT_MATCH_LENGTH) += xt_length.o
 obj-$(CONFIG_NETFILTER_XT_MATCH_LIMIT) += xt_limit.o
 obj-$(CONFIG_NETFILTER_XT_MATCH_MAC) += xt_mac.o