vxlan: nag user to set port value

This change shifts burden onto the users to choose the UDP port value.
Kernel default value is incorrect UDP port 5287 but now there is
an official assigned port for VXLAN.

The kernel can't change because of legacy compatibility
but new deployments should not use the legacy port value.

ip/iplink_vxlan.c

@@ -53,7 +53,8 @@ static int vxlan_parse_opt(struct link_util *lu, int argc, char **argv,
 	__u8 noage = 0;
 	__u32 age = 0;
 	__u32 maxaddr = 0;
-	__u16 dstport = 4789;
+	__u16 dstport = 0;
+	int dst_port_set = 0;
 	struct ifla_vxlan_port_range range = { 0, 0 };
 
 	while (argc > 0) {
@@ -131,6 +132,7 @@ static int vxlan_parse_opt(struct link_util *lu, int argc, char **argv,
 			NEXT_ARG();
 			if (get_u16(&dstport, *argv, 0))
 				invarg("dst port", *argv);
+			dst_port_set = 1;
 		} else if (!matches(*argv, "nolearning")) {
 			learning = 0;
 		} else if (!matches(*argv, "learning")) {
@@ -161,10 +163,20 @@ static int vxlan_parse_opt(struct link_util *lu, int argc, char **argv,
 		}
 		argc--, argv++;
 	}
+
 	if (!vni_set) {
 		fprintf(stderr, "vxlan: missing virtual network identifier\n");
 		return -1;
 	}
+
+	if (!dst_port_set) {
+		fprintf(stderr, "vxlan: destination port not specified\n"
+			"Will use Linux kernel default (non-standard value)\n");
+		fprintf(stderr, 
+			"Use 'dstport 4789' to get the IANA assigned value\n"
+			"Use 'dstport 0' to get default and quiet this message\n");
+	}
+
 	addattr32(n, 1024, IFLA_VXLAN_ID, vni);
 	if (gaddr)
 		addattr_l(n, 1024, IFLA_VXLAN_GROUP, &gaddr, 4);
@@ -179,6 +191,7 @@ static int vxlan_parse_opt(struct link_util *lu, int argc, char **argv,
 	addattr8(n, 1024, IFLA_VXLAN_RSC, rsc);
 	addattr8(n, 1024, IFLA_VXLAN_L2MISS, l2miss);
 	addattr8(n, 1024, IFLA_VXLAN_L3MISS, l3miss);
+
 	if (noage)
 		addattr32(n, 1024, IFLA_VXLAN_AGEING, 0);
 	else if (age)