kernel-netlink: Enable UDP GRO

This enables GRO offload for inbound ESP-in-UDP packets if the
esp4|6_offload modules are loaded.  Note that inbound ESP or ESP-in-UDP
packets won't be visible on layer 3 in Netfilter or tcpdump.

src/include/linux/udp.h

@@ -29,6 +29,7 @@ struct udphdr {
 /* UDP socket options */
 #define UDP_CORK	1	/* Never send partially complete segments */
 #define UDP_ENCAP	100	/* Set the socket to accept encapsulated packets */
+#define UDP_GRO		104	/* This socket can receive UDP GRO packets */
 
 /* UDP encapsulation types */
 #define UDP_ENCAP_ESPINUDP_NON_IKE	1 /* draft-ietf-ipsec-nat-t-ike-00/01 */

src/libcharon/plugins/kernel_netlink/kernel_netlink_ipsec.c

@@ -3884,6 +3884,12 @@ METHOD(kernel_ipsec_t, enable_udp_decap, bool,
 		DBG1(DBG_KNL, "unable to set UDP_ENCAP: %s", strerror(errno));
 		return FALSE;
 	}
+	type = 1;
+	if (setsockopt(fd, SOL_UDP, UDP_GRO, &type, sizeof(type)) < 0)
+	{
+		DBG1(DBG_KNL, "unable to set UDP_GRO: %s", strerror(errno));
+		return FALSE;
+	}
 	return TRUE;
 }