core: add pmtu_discovery=2 for IPv4 and IPv6 - set IP_PMTUDISC_WANT/I…

…PV6_PMTUDISC_WANT (GH #3141)

- add pmtu_discovery=2 for IPv4 and IPv6 - set IP_PMTUDISC_WANT/IPV6_PMTUDISC_WANT
- related to GH #3141
- for IPv4: will fragment a datagram if needed according to the path MTU,
  or will set the don't-fragment flag otherwise
- for IPv6: will fragment a datagram if needed according to the path MTU for IPv6

src/core/udp_server.c

@@ -349,19 +349,39 @@ int udp_init(struct socket_info* sock_info)
 #endif
 #if defined (__OS_linux)
 	if (addr->s.sa_family==AF_INET){
-		/* If pmtu_discovery=1 then set DF bit and do Path MTU discovery
-		 * disabled by default.  Specific to IPv4. */
-		optval= (pmtu_discovery) ? IP_PMTUDISC_DO : IP_PMTUDISC_DONT;
+		/* If pmtu_discovery=1 then set DF bit and do Path MTU discovery,
+		 * disabled by default. Specific to IPv4. If pmtu_discovery=2
+		 * then the datagram will be fragmented if needed according to
+		 * path MTU, or will set the don't-fragment flag otherwise */
+		switch (pmtu_discovery) {
+			case 1: optval=IP_PMTUDISC_DO;
+			break;
+			case 2: optval=IP_PMTUDISC_WANT;
+			break;
+			case 0:
+			default: optval=IP_PMTUDISC_DONT;
+			break;
+		}
 		if(setsockopt(sock_info->socket, IPPROTO_IP, IP_MTU_DISCOVER,
 				(void*)&optval, sizeof(optval)) ==-1){
 			LM_ERR("IPv4 setsockopt: %s\n", strerror(errno));
 			goto error;
 		}
 	} else if (addr->s.sa_family==AF_INET6){
 		/* IPv6 never fragments but sends ICMPv6 Packet too Big,
-		 * If pmtu_discovery=1 then set DF bit and do Path MTU discovery
-		 * disabled by default.  Specific to IPv6. */
-		optval= (pmtu_discovery) ? IPV6_PMTUDISC_DO : IPV6_PMTUDISC_DONT;
+		 * If pmtu_discovery=1 then set DF bit and do Path MTU discovery,
+		 * disabled by default. Specific to IPv6. If pmtu_discovery=2
+                 * then the datagram will be fragmented if needed according to
+                 * path MTU */
+                switch (pmtu_discovery) {
+			case 1: optval=IPV6_PMTUDISC_DO;
+			break;
+			case 2: optval=IPV6_PMTUDISC_WANT;
+			break;
+			case 0:
+			default: optval=IPV6_PMTUDISC_DONT;
+			break;
+		}
 		if(setsockopt(sock_info->socket, IPPROTO_IPV6,
 				IPV6_MTU_DISCOVER,
 				(void*)&optval, sizeof(optval)) ==-1){