pcap handles are not thread safe

they contain an errbuf; there might be other issues too.
So we must use different handles in the sniffer and udplistener
goroutines.

pcap.go

@@ -8,16 +8,26 @@ type PcapIO struct {
 	handle *pcap.Handle
 }
 
-func NewPcapIO(ifName string, bufSz int) (handle *PcapIO, err error) {
+func NewPcapIO(ifName string, bufSz int) (pio PacketSourceSink, err error) {
+	pio, err = newPcapIO(ifName, true, 65535, bufSz)
+	return
+}
+
+func NewPcapO(ifName string) (po PacketSink, err error) {
+	po, err = newPcapIO(ifName, false, 0, 0)
+	return
+}
+
+func newPcapIO(ifName string, promisc bool, snaplen int, bufSz int) (handle *PcapIO, err error) {
 	inactive, err := pcap.NewInactiveHandle(ifName)
 	if err != nil {
 		return
 	}
 	defer inactive.CleanUp()
-	if inactive.SetPromisc(true) != nil {
+	if inactive.SetPromisc(promisc) != nil {
 		return
 	}
-	if inactive.SetSnapLen(65535) != nil {
+	if inactive.SetSnapLen(snaplen) != nil {
 		return
 	}
 	if inactive.SetTimeout(-1) != nil {

router.go

@@ -38,13 +38,11 @@ func (router *Router) UsingPassword() bool {
 }
 
 func (router *Router) Start() {
-	pio, err := NewPcapIO(router.Iface.Name, router.BufSz)
-	checkFatal(err)
 	router.ConnectionMaker = StartConnectionMaker(router)
 	router.Topology = StartTopology(router)
-	router.UDPListener = router.listenUDP(Port, pio)
+	router.UDPListener = router.listenUDP(Port)
 	router.listenTCP(Port)
-	router.sniff(pio)
+	router.sniff()
 }
 
 func (router *Router) Status() string {
@@ -58,7 +56,9 @@ func (router *Router) Status() string {
 	return buf.String()
 }
 
-func (router *Router) sniff(pio PacketSourceSink) {
+func (router *Router) sniff() {
+	pio, err := NewPcapIO(router.Iface.Name, router.BufSz)
+	checkFatal(err)
 	log.Println("Sniffing traffic on", router.Iface)
 
 	dec := NewEthernetDecoder()
@@ -141,7 +141,7 @@ func (router *Router) acceptTCP(tcpConn *net.TCPConn) {
 	NewLocalConnection(connRemote, UnknownPeerName, tcpConn, nil, router)
 }
 
-func (router *Router) listenUDP(localPort int, po PacketSink) *net.UDPConn {
+func (router *Router) listenUDP(localPort int) *net.UDPConn {
 	localAddr, err := net.ResolveUDPAddr("udp4", fmt.Sprint(":", localPort))
 	checkFatal(err)
 	conn, err := net.ListenUDP("udp4", localAddr)
@@ -153,6 +153,8 @@ func (router *Router) listenUDP(localPort int, po PacketSink) *net.UDPConn {
 	// This one makes sure all packets we send out do not have DF set on them.
 	err = syscall.SetsockoptInt(fd, syscall.IPPROTO_IP, syscall.IP_MTU_DISCOVER, syscall.IP_PMTUDISC_DONT)
 	checkFatal(err)
+	po, err := NewPcapO(router.Iface.Name)
+	checkFatal(err)
 	go router.udpReader(conn, po)
 	return conn
 }