use dst address as source of icmp 3.4, instead of router address

For some reason, packets injected with a src ip that isn't in the
sub-net of the dst ip get dropped.

fixes #1

ethernet_decoder.go

@@ -19,47 +19,23 @@ func (dec *EthernetDecoder) DecodeLayers(data []byte) error {
 	return dec.parser.DecodeLayers(data, &dec.decoded)
 }
 
-func (dec *EthernetDecoder) CheckFrameTooBigFunc(srcMac net.HardwareAddr, localAddrs []net.Addr, handle *pcap.Handle) func(error) error {
-	networks := make([]LocalAddress, len(localAddrs))
-	idx := 0
-	for _, addr := range localAddrs {
-		ip, ipnet, err := net.ParseCIDR(addr.String())
-		if err == nil && ip.To4() != nil {
-			networks[idx] = LocalAddress{ip: ip.To4(), network: ipnet}
-			idx++
-		}
-	}
-	networks = networks[:idx]
-	if len(networks) == 0 {
-		log.Fatal("Interface has no addresses assigned. Cannot continue")
-	}
-	findIP := func(ip net.IP) net.IP {
-		for _, la := range networks {
-			if la.network.Contains(ip) {
-				return la.ip
-			}
-		}
-		// can't find anything, we'll have to rely on default gateway.
-		return networks[0].ip
-	}
-
+func (dec *EthernetDecoder) CheckFrameTooBigFunc(handle *pcap.Handle) func(error) error {
 	return func(err error) error {
 		if ftbe, ok := err.(FrameTooBigError); ok {
 			// we know: 1. ip is valid, 2. it was ip and DF was set
-			srcIP := findIP(dec.ip.SrcIP)
-			icmpFrame, err := dec.formICMPMTUPacket(srcMac, srcIP, ftbe.PMTU)
+			icmpFrame, err := dec.formICMPMTUPacket(ftbe.PMTU)
 			if err != nil {
 				return err
 			}
-			log.Println("Injecting ICMP 3,4. PMTU:", ftbe.PMTU)
+			log.Printf("Injecting ICMP 3,4 (%v -> %v): PMTU= %v\n", dec.ip.DstIP, dec.ip.SrcIP, ftbe.PMTU)
 			return handle.WritePacketData(icmpFrame)
 		} else {
 			return err
 		}
 	}
 }
 
-func (dec *EthernetDecoder) formICMPMTUPacket(srcMac net.HardwareAddr, srcIP net.IP, mtu int) ([]byte, error) {
+func (dec *EthernetDecoder) formICMPMTUPacket(mtu int) ([]byte, error) {
 	buf := gopacket.NewSerializeBuffer()
 	opts := gopacket.SerializeOptions{
 		FixLengths:       true,
@@ -68,7 +44,7 @@ func (dec *EthernetDecoder) formICMPMTUPacket(srcMac net.HardwareAddr, srcIP net
 	payload := gopacket.Payload(dec.ip.BaseLayer.Contents[:ipHeaderSize+8])
 	err := gopacket.SerializeLayers(buf, opts,
 		&layers.Ethernet{
-			SrcMAC:       srcMac,
+			SrcMAC:       dec.eth.DstMAC,
 			DstMAC:       dec.eth.SrcMAC,
 			EthernetType: dec.eth.EthernetType},
 		&layers.IPv4{
@@ -80,7 +56,7 @@ func (dec *EthernetDecoder) formICMPMTUPacket(srcMac net.HardwareAddr, srcIP net
 			TTL:        64,
 			Protocol:   layers.IPProtocolICMPv4,
 			DstIP:      dec.ip.SrcIP,
-			SrcIP:      srcIP},
+			SrcIP:      dec.ip.DstIP},
 		&layers.ICMPv4{
 			TypeCode: 0x304,
 			Id:       0,

router.go

@@ -62,11 +62,7 @@ func (router *Router) sniff(handle *pcap.Handle) {
 	log.Println("Sniffing traffic on", router.Iface)
 
 	dec := NewEthernetDecoder()
-
-	localAddrs, err := router.Iface.Addrs()
-	checkFatal(err)
-	checkFrameTooBig := dec.CheckFrameTooBigFunc(router.Iface.HardwareAddr, localAddrs, handle)
-
+	checkFrameTooBig := dec.CheckFrameTooBigFunc(handle)
 	mac := router.Iface.HardwareAddr
 	if router.Macs.Enter(mac, router.Ourself) {
 		log.Println("Discovered our MAC", mac)
@@ -198,9 +194,7 @@ func (router *Router) udpReader(conn *net.UDPConn, handle *pcap.Handle) {
 }
 
 func (router *Router) handleUDPPacketFunc(dec *EthernetDecoder, handle *pcap.Handle) FrameConsumer {
-	localAddrs, err := router.Iface.Addrs()
-	checkFatal(err)
-	checkFrameTooBig := dec.CheckFrameTooBigFunc(router.Iface.HardwareAddr, localAddrs, handle)
+	checkFrameTooBig := dec.CheckFrameTooBigFunc(handle)
 
 	return func(relayConn *LocalConnection, sender *net.UDPAddr, srcNameByte, dstNameByte []byte, frameLen uint16, frame []byte) error {
 		srcName := PeerNameFromBin(srcNameByte)