Fixing loops for mDNS packets when mDNS plugin is misconfigured

If two mDNS plug-in enabled OLSR routers are attached to the same HNA network, then mDNS packets will loop back and forth between the two, also flooding the whole OLSR network. For the moment the temporary solution to this problem was to enable the plug-in on one router at a time on the same HNA. A real implemented here is  the following:

    usually mDNS packets have a TTL != 1, so:
    capture only mDNS packets with TTL != 1
    when decapsulating mDNS packets, set TTL = 1
    where TTL is the IP Time-to-live field or the IPv6 HopLimit field

 * Modified code to forward mdns packet from HNA into OLSRd network changing TTL/Hlim to 1
 * Now the mDns plugin discard all packet from HNA that have TTL/Hlim equals to 1

lib/mdns/src/mdns.c

@@ -112,10 +112,12 @@ PacketReceivedFromOLSR(unsigned char *encapsulationUdpData, int len)
       if ((encapsulationUdpData[0] & 0xf0) == 0x40) {
         dest.sll_protocol = htons(ETH_P_IP);
 	stripped_len = ntohs(ipHeader->ip_len);
+	ipHeader->ip_ttl = (u_int8_t) 1; //setting up TTL to 1 to avoid mdns packets flood 
 	}
       if ((encapsulationUdpData[0] & 0xf0) == 0x60) {
         dest.sll_protocol = htons(ETH_P_IPV6);
         stripped_len = 40 + ntohs(ip6Header->ip6_plen); //IPv6 Header size (40) + payload_len 
+        ip6Header->ip6_hops = (uint8_t) 1; //setting up Hop Limit to 1 to avoid mdns packets flood
         }
       // Sven-Ola: Don't know how to handle the "stripped_len is uninitialized" condition, maybe exit(1) is better...?
       if (0 == stripped_len) return;
@@ -134,7 +136,7 @@ PacketReceivedFromOLSR(unsigned char *encapsulationUdpData, int len)
        * seem to matter when the destination MAC address is set to all-ones
        * in that case. */
       memset(dest.sll_addr, 0xFF, IFHWADDRLEN);
-
+      
       nBytesWritten = sendto(walker->capturingSkfd, encapsulationUdpData, stripped_len, 0, (struct sockaddr *)&dest, sizeof(dest));
       if (nBytesWritten != stripped_len) {
         BmfPError("sendto() error forwarding unpacked encapsulated pkt on \"%s\"", walker->ifName);
@@ -359,6 +361,9 @@ BmfPacketCaptured(
     if (destPort != 5353) {
       return;
     }
+    if(((u_int8_t) ipHeader->ip_ttl) <= ((u_int8_t) 1))    // Discard mdns packet with TTL limit 1 or less
+      return;
+
   }                             //END IPV4
 
   else if ((encapsulationUdpData[0] & 0xf0) == 0x60) {  //IPv6
@@ -380,6 +385,9 @@ BmfPacketCaptured(
     if (destPort != 5353) {
       return;
     }
+    if(((uint8_t) ipHeader6->ip6_hops) <= ((uint8_t) 1))  // Discard mdns packet with hop limit 1 or less
+      return;
+
   }                             //END IPV6
   else
     return;                     //Is not IP packet