func icmpRecvMsg()

in client.go [881:956]
72 lines of code
21 McCabe index (conditional complexity)

func icmpRecvMsg(c net.PacketConn, proto EthrProtocol, timeout time.Duration, neededPeer string, neededSig []byte, neededIcmpBody []byte, neededIcmpSeq int) (string, bool, error) {
	peerAddr := ""
	isLast := false
	err := c.SetDeadline(time.Now().Add(timeout))
	if err != nil {
		ui.printErr("Failed to set Deadline. Error: %v", err)
		return peerAddr, isLast, err
	}
	for {
		peerAddr = ""
		b := make([]byte, 1500)
		n, peer, err := c.ReadFrom(b)
		if err != nil {
			if proto == ICMP {
				// In case of non-ICMP TraceRoute, it is expected that no packet is received
				// in some case, e.g. when packet reach final hop and TCP connection establishes.
				ui.printDbg("Failed to receive ICMP packet. Error: %v", err)
			}
			return peerAddr, isLast, err
		}
		if n == 0 {
			continue
		}
		ui.printDbg("Packet:\n%s", hex.Dump(b[:n]))
		ui.printDbg("Finding Pattern\n%v", hex.Dump(neededSig[:4]))
		peerAddr = peer.String()
		if neededPeer != "" && peerAddr != neededPeer {
			ui.printDbg("Matching peer is not found.")
			continue
		}
		icmpMsg, err := icmp.ParseMessage(IcmpProto(), b[:n])
		if err != nil {
			ui.printDbg("Failed to parse ICMP message: %v", err)
			continue
		}
		if icmpMsg.Type == ipv4.ICMPTypeTimeExceeded || icmpMsg.Type == ipv6.ICMPTypeTimeExceeded {
			body := icmpMsg.Body.(*icmp.TimeExceeded).Data
			index := bytes.Index(body, neededSig[:4])
			if index > 0 {
				if proto == TCP {
					ui.printDbg("Found correct ICMP error message. PeerAddr: %v", peerAddr)
					return peerAddr, isLast, nil
				} else if proto == ICMP {
					if index < 4 {
						ui.printDbg("Incorrect length of ICMP message.")
						continue
					}
					innerIcmpMsg, _ := icmp.ParseMessage(IcmpProto(), body[index-4:])
					switch innerIcmpMsg.Body.(type) {
					case *icmp.Echo:
						seq := innerIcmpMsg.Body.(*icmp.Echo).Seq
						if seq == neededIcmpSeq {
							return peerAddr, isLast, nil
						}
					default:
						// Ignore as this is not the right ICMP packet.
						ui.printDbg("Unable to recognize packet.")
					}
				}
			} else {
				ui.printDbg("Pattern %v not found.", hex.Dump(neededSig[:4]))
			}
		}

		if proto == ICMP && (icmpMsg.Type == ipv4.ICMPTypeEchoReply || icmpMsg.Type == ipv6.ICMPTypeEchoReply) {
			echo := icmpMsg.Body.(*icmp.Echo)
			ethrUnused(echo)
			b, _ := icmpMsg.Body.Marshal(1)
			if string(b[4:]) != string(neededIcmpBody) {
				continue
			}
			isLast = true
			return peerAddr, isLast, nil
		}
	}
}