Function sendLLMNRProbe

respounder.go:157–201

Creates and sends a LLMNR request to the UDP multicast address.

(ip net.IP)

// Creates and sends a LLMNR request to the UDP multicast address.
func sendLLMNRProbe(ip net.IP) string {
	var cName string
	responderIP := ""
	// 2 byte random transaction id eg. 0x8e53
	randomTransactionID := fmt.Sprintf("%04x", rand.Intn(65535))

	switch hostnameType {
	case def, newHostname:
		cName = string(*hostnamePtr)
	case randHostname:
		cName = randomHostname()
	}

	cNameLen := fmt.Sprintf("%02x", len(cName))
	encCName := hex.EncodeToString([]byte(cName))
	// LLMNR request in raw bytes
	llmnrRequest := randomTransactionID +
		"00000001000000000000" + cNameLen + encCName + "0000010001"
	n, _ := hex.DecodeString(llmnrRequest)

	remoteAddr := net.UDPAddr{IP: net.ParseIP(BcastAddr), Port: LLMNRPort}

	conn, err := net.ListenUDP("udp", &net.UDPAddr{IP: ip})
	if err != nil {
		fmt.Printf("Could not bind to the interface. Is it disabled? ")
		logger.Printf("Bind error: %+v\nSource IP: %v\n", err, ip)
		return responderIP // return with IP = ''
	}

	defer conn.Close()
	_, _ = conn.WriteToUDP(n, &remoteAddr)

	conn.SetReadDeadline(time.Now().Add(TimeoutSec * time.Second))
	buffer := make([]byte, 1024)
	bytes, clientIP, err := conn.ReadFromUDP(buffer)
	if err == nil { // no timeout (or any other) error
		responderIP = strings.Split(clientIP.String(), ":")[0]
		logger.Printf("LLMNR request payload was: %x\n", n)
		logger.Printf("Data received on %s from responder IP %s: %x\n",
			ip, clientIP, buffer[:bytes])
	} else {
		logger.Printf("Error getting response:  %s\n", err)
	}
	return responderIP
}

// Calculate random hostname by taking random length
// of the SHA1 of current time.