diff -urN linux-2.6.21.1.old/include/net/xfrmudp.h linux-2.6.21.1.dev/include/net/xfrmudp.h
--- linux-2.6.21.1.old/include/net/xfrmudp.h	1970-01-01 01:00:00.000000000 +0100
+++ linux-2.6.21.1.dev/include/net/xfrmudp.h	2007-05-26 20:24:53.933599448 +0200
@@ -0,0 +1,10 @@
+/*
+ * pointer to function for type that xfrm4_input wants, to permit
+ * decoupling of XFRM from udp.c
+ */
+#define HAVE_XFRM4_UDP_REGISTER
+
+typedef int (*xfrm4_rcv_encap_t)(struct sk_buff *skb, __u16 encap_type);
+extern int udp4_register_esp_rcvencap(xfrm4_rcv_encap_t func
+				      , xfrm4_rcv_encap_t *oldfunc);
+extern int udp4_unregister_esp_rcvencap(xfrm4_rcv_encap_t func);
diff -urN linux-2.6.21.1.old/net/ipv4/Kconfig linux-2.6.21.1.dev/net/ipv4/Kconfig
--- linux-2.6.21.1.old/net/ipv4/Kconfig	2007-04-27 23:49:26.000000000 +0200
+++ linux-2.6.21.1.dev/net/ipv4/Kconfig	2007-05-26 20:24:53.965594584 +0200
@@ -266,6 +266,12 @@
 	  Network), but can be distributed all over the Internet. If you want
 	  to do that, say Y here and to "IP multicast routing" below.
 
+config IPSEC_NAT_TRAVERSAL
+	bool "IPSEC NAT-Traversal (KLIPS compatible)"
+	depends on INET
+	---help---
+          Includes support for RFC3947/RFC3948 NAT-Traversal of ESP over UDP.
+
 config IP_MROUTE
 	bool "IP: multicast routing"
 	depends on IP_MULTICAST
diff -urN linux-2.6.21.1.old/net/ipv4/udp.c linux-2.6.21.1.dev/net/ipv4/udp.c
--- linux-2.6.21.1.old/net/ipv4/udp.c	2007-04-27 23:49:26.000000000 +0200
+++ linux-2.6.21.1.dev/net/ipv4/udp.c	2007-05-26 20:24:53.966594432 +0200
@@ -101,12 +101,15 @@
 #include <net/route.h>
 #include <net/checksum.h>
 #include <net/xfrm.h>
+#include <net/xfrmudp.h>
 #include "udp_impl.h"
 
 /*
  *	Snmp MIB for the UDP layer
  */
 
+static xfrm4_rcv_encap_t xfrm4_rcv_encap_func;
+
 DEFINE_SNMP_STAT(struct udp_mib, udp_statistics) __read_mostly;
 
 struct hlist_head udp_hash[UDP_HTABLE_SIZE];
@@ -1008,6 +1011,42 @@
 	return 0;
 }
 
+#if defined(CONFIG_XFRM) || defined(CONFIG_IPSEC_NAT_TRAVERSAL)
+
+/* if XFRM isn't a module, then register it directly. */
+#if 0 && !defined(CONFIG_XFRM_MODULE) && !defined(CONFIG_IPSEC_NAT_TRAVERSAL)
+static xfrm4_rcv_encap_t xfrm4_rcv_encap_func = xfrm4_rcv_encap;
+#else
+static xfrm4_rcv_encap_t xfrm4_rcv_encap_func = NULL;
+#endif
+
+int udp4_register_esp_rcvencap(xfrm4_rcv_encap_t func
+			       , xfrm4_rcv_encap_t *oldfunc)
+{
+  if(oldfunc != NULL) {
+    *oldfunc = xfrm4_rcv_encap_func;
+  }
+
+#if 0
+  if(xfrm4_rcv_encap_func != NULL)
+    return -1;
+#endif
+
+  xfrm4_rcv_encap_func = func;
+  return 0;
+}
+
+int udp4_unregister_esp_rcvencap(xfrm4_rcv_encap_t func)
+{
+  if(xfrm4_rcv_encap_func != func)
+    return -1;
+
+  xfrm4_rcv_encap_func = NULL;
+  return 0;
+}
+#endif /* CONFIG_XFRM_MODULE || CONFIG_IPSEC_NAT_TRAVERSAL */
+
+
 /* return:
  * 	1  if the UDP system should process it
  *	0  if we should drop this packet
@@ -1015,7 +1054,7 @@
  */
 static int udp_encap_rcv(struct sock * sk, struct sk_buff *skb)
 {
-#ifndef CONFIG_XFRM
+#if !defined(CONFIG_XFRM) && !defined(CONFIG_IPSEC_NAT_TRAVERSAL)
 	return 1;
 #else
 	struct udp_sock *up = udp_sk(sk);
@@ -1030,11 +1069,11 @@
 	/* if we're overly short, let UDP handle it */
 	len = skb->len - sizeof(struct udphdr);
 	if (len <= 0)
-		return 1;
+		return 2;
 
 	/* if this is not encapsulated socket, then just return now */
 	if (!encap_type)
-		return 1;
+		return 3;
 
 	/* If this is a paged skb, make sure we pull up
 	 * whatever data we need to look at. */
@@ -1057,7 +1096,7 @@
 			len = sizeof(struct udphdr);
 		} else
 			/* Must be an IKE packet.. pass it through */
-			return 1;
+			return 4;
 		break;
 	case UDP_ENCAP_ESPINUDP_NON_IKE:
 		/* Check if this is a keepalive packet.  If so, eat it. */
@@ -1070,7 +1109,7 @@
 			len = sizeof(struct udphdr) + 2 * sizeof(u32);
 		} else
 			/* Must be an IKE packet.. pass it through */
-			return 1;
+			return 5;
 		break;
 	}
 
@@ -1081,6 +1120,8 @@
 	 */
 	if (skb_cloned(skb) && pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
 		return 0;
+	if (skb_cloned(skb) && pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
+		return 0;
 
 	/* Now we can update and verify the packet length... */
 	iph = ip_hdr(skb);
@@ -1145,9 +1186,13 @@
 			return 0;
 		}
 		if (ret < 0) {
-			/* process the ESP packet */
-			ret = xfrm4_rcv_encap(skb, up->encap_type);
-			UDP_INC_STATS_BH(UDP_MIB_INDATAGRAMS, up->pcflag);
+			if(xfrm4_rcv_encap_func != NULL) {
+			  ret = (*xfrm4_rcv_encap_func)(skb, up->encap_type);
+			  UDP_INC_STATS_BH(UDP_MIB_INDATAGRAMS, up->pcflag);
+			} else {
+			  UDP_INC_STATS_BH(UDP_MIB_INERRORS, up->pcflag);
+			  ret = 1;
+			}
 			return -ret;
 		}
 		/* FALLTHROUGH -- it's a UDP Packet */
@@ -1847,3 +1892,9 @@
 EXPORT_SYMBOL(udp_proc_register);
 EXPORT_SYMBOL(udp_proc_unregister);
 #endif
+
+#if defined(CONFIG_IPSEC_NAT_TRAVERSAL)
+EXPORT_SYMBOL(udp4_register_esp_rcvencap);
+EXPORT_SYMBOL(udp4_unregister_esp_rcvencap);
+#endif
+
