Crafting an UDP Packet in Go

udp.go

package main

import (
	"context"
	"fmt"
	"log"
	"net"
	"syscall"

	"github.com/google/gopacket"
	"github.com/google/gopacket/layers"
)

type PacketConfig struct {
	SrcMAC      net.HardwareAddr
	DstMAC      net.HardwareAddr
	SrcIP       net.IP
	DstIP       net.IP
	SrcPort     layers.UDPPort
	DstPort     layers.UDPPort
	PayloadSize int
}

func send(ctx context.Context, s *PacketConfig) error {
	fd, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_RAW, syscall.IPPROTO_RAW)
	if err != nil {
		log.Fatalf("Failed to create raw socket: %v", err)
	}
	defer syscall.Close(fd)

	// To manually include the IP header enable IP_HDRINCL
	if err := syscall.SetsockoptInt(fd, syscall.IPPROTO_IP, syscall.IP_HDRINCL, 1); err != nil {
		log.Fatalf("Failed to set IP_HDRINCL: %v", err)
	}

	ipv4DstIP := s.DstIP.To4()
	if ipv4DstIP == nil {
		log.Fatalf("Destination IP address is not an IPv4 address: %s", s.DstIP)
	}

	dstAddr := &syscall.SockaddrInet4{
		Port: int(s.DstPort),
		Addr: [4]byte{ipv4DstIP[0], ipv4DstIP[1], ipv4DstIP[2], ipv4DstIP[3]},
	}

	packet, err := buildPacket(s)
	fmt.Println(packet)
	if err != nil {
		return fmt.Errorf("failed to build packet: %w", err)
	}

	fmt.Println("Packet is created!")

	select {
	case <-ctx.Done():
		return nil
	default:
		if err := syscall.Sendto(fd, packet, 0, dstAddr); err != nil {
			return fmt.Errorf("failed to send packet: %v", err)
		}
		fmt.Println("Packet sent!")
	}
	return nil
}

func buildPacket(c *PacketConfig) ([]byte, error) {
	buf := gopacket.NewSerializeBuffer()
	var layersToSerialize []gopacket.SerializableLayer

	ethLayer := &layers.Ethernet{
		SrcMAC:       c.SrcMAC,
		DstMAC:       c.DstMAC,
		EthernetType: layers.EthernetTypeIPv4,
	}
	layersToSerialize = append(layersToSerialize, ethLayer)

	ipLayer := &layers.IPv4{
		Version:  4,
		TTL:      64,
		SrcIP:    c.SrcIP,
		DstIP:    c.DstIP,
		Protocol: layers.IPProtocolUDP,
	}
	layersToSerialize = append(layersToSerialize, ipLayer)

	udpLayer := &layers.UDP{
		SrcPort: c.SrcPort,
		DstPort: c.DstPort,
	}
	udpLayer.SetNetworkLayerForChecksum(ipLayer)
	layersToSerialize = append(layersToSerialize, udpLayer)

	payload := make([]byte, c.PayloadSize)
	layersToSerialize = append(layersToSerialize, gopacket.Payload(payload))

	if err := gopacket.SerializeLayers(buf, gopacket.SerializeOptions{ComputeChecksums: true, FixLengths: true}, layersToSerialize...); err != nil {
		return nil, fmt.Errorf("error serializing packet: %w", err)
	}

	return buf.Bytes(), nil
}

func main() {
	fmt.Println("netctl v0.0.1. Born 7:04 pm Friday, 29 November 2024 Indian Standard Time (IST)")

	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()

	srcMAC, err := net.ParseMAC("de:ad:be:ef:ca:fe")
	if err != nil {
		fmt.Println("Error parsing MAC address:", err)
		return
	}

	dstMAC, err := net.ParseMAC("de:ad:be:ef:ca:ff")
	if err != nil {
		fmt.Println("Error parsing MAC address:", err)
		return
	}

	// now create a packet configuration
	config := PacketConfig{
		SrcMAC:      srcMAC,
		DstMAC:      dstMAC,
		SrcIP:       net.ParseIP("191.191.191.191"),
		DstIP:       net.ParseIP("192.192.192.192"),
		SrcPort:     12345,
		DstPort:     12346,
		PayloadSize: 100,
	}

	err = send(ctx, &config)
	if err != nil {
		fmt.Println(err)
	}
}