probe/internal/protocol/dhcp/dhcp.go

package dhcp

import (
	"bytes"
	"encoding/binary"
	"fmt"
	"net"
	"strings"
	"time"

	"github.com/google/gopacket/layers"

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

func doDiscoverV4(address string) (map[string]string, error) {

	localAddr, err := net.ResolveUDPAddr("udp", address+":68")
	if err != nil {
		return nil, err
	}
	conn, err := net.ListenUDP("udp", localAddr)
	if err != nil {
		return nil, err
	}
	conn.SetDeadline(time.Now().Add(time.Second * 3))
	defer func() {
		conn.Close()
	}()

	sendDiscover(conn) // broadcast 255.255.255.255:67
	return readOffer(conn)

}

func sendDiscover(conn *net.UDPConn) error {

	server := fmt.Sprintf("%s:%d", "255.255.255.255", 67)
	serverAddr, err := net.ResolveUDPAddr("udp", server)
	if err != nil {
		return err
	}

	dhcp := &gl.DHCPv4{Operation: gl.DHCPOpRequest, HardwareType: gl.LinkTypeEthernet, Xid: 0x12345678,
		ClientIP: net.IP{0, 0, 0, 0}, YourClientIP: net.IP{0, 0, 0, 0}, NextServerIP: net.IP{0, 0, 0, 0}, RelayAgentIP: net.IP{0, 0, 0, 0},
		ClientHWAddr: net.HardwareAddr{0x12, 0x34, 0x56, 0x78, 0x9a, 0xbc},
		ServerName:   make([]byte, 64), File: make([]byte, 128)}

	dhcp.Options = append(dhcp.Options, gl.NewDHCPOption(gl.DHCPOptMessageType, []byte{byte(gl.DHCPMsgTypeDiscover)}))
	dhcp.Options = append(dhcp.Options, gl.NewDHCPOption(gl.DHCPOptHostname, []byte{'o', 'v', 'e', 'r', 'f', 'l', 'o', 'w', '.', 'c', 'o', 'm'}))
	dhcp.Options = append(dhcp.Options, gl.NewDHCPOption(gl.DHCPOptPad, nil))
	dhcp.Options = append(dhcp.Options, gl.NewDHCPOption(gl.DHCPOptParamsRequest,
		[]byte{byte(gl.DHCPOptSubnetMask), byte(gl.DHCPOptBroadcastAddr), byte(gl.DHCPOptTimeOffset),
			byte(gl.DHCPOptRouter), byte(gl.DHCPOptDomainName), byte(gl.DHCPOptDNS), byte(gl.DHCPOptDomainSearch),
			byte(gl.DHCPOptHostname), byte(gl.DHCPOptNetBIOSTCPNS), byte(gl.DHCPOptInterfaceMTU), byte(gl.DHCPOptClasslessStaticRoute),
			byte(gl.DHCPOptNTPServers)}))

	buf := gopacket.NewSerializeBuffer()
	opts := gopacket.SerializeOptions{FixLengths: true}
	err = gopacket.SerializeLayers(buf, opts, dhcp)
	if err != nil {
		return err
	}
	_, err = conn.WriteToUDP(buf.Bytes(), serverAddr)
	if err != nil {
		return err
	}

	return nil
}

func readOffer(conn *net.UDPConn) (map[string]string, error) {
	offerOptions := make(map[string]string)

	buf := make([]byte, 1024)
	n, _, err := conn.ReadFromUDP(buf)
	if err != nil {
		return nil, err
	}
	if n <= 0 {
		return nil, fmt.Errorf("%s", "Emptry response.")
	}

	var dhcp gl.DHCPv4
	decoded := []gopacket.LayerType{}
	parser := gopacket.NewDecodingLayerParser(gl.LayerTypeDHCPv4, &dhcp)
	if err = parser.DecodeLayers(buf, &decoded); err != nil {
		return nil, err
	}
	for _, layerType := range decoded {
		switch layerType {
		case layers.LayerTypeDHCPv4:
			for _, opt := range dhcp.Options {
				switch opt.Type {
				case gl.DHCPOptServerID:
					offerOptions["DHCP_SERVER"] = optDataToIPString(opt.Data)
					break
				case gl.DHCPOptLeaseTime:
					var sec int32
					if err = binary.Read(bytes.NewReader(opt.Data), binary.BigEndian, &sec); err == nil {
						offerOptions["LEASE_TIME"] = fmt.Sprintf("%d", sec)
					}
					break
				case gl.DHCPOptSubnetMask:
					offerOptions["SUBNET_MASK"] = optDataToIPString(opt.Data)
					break
				case gl.DHCPOptRouter:
					offerOptions["ROUTER"] = optDataToIPString(opt.Data)
					break
				case gl.DHCPOptDNS:
					cnt := len(opt.Data) / 4
					var dns []string
					ipStr := optDataToIPString(opt.Data[0:4])
					dns = append(dns, ipStr)
					if cnt > 1 {
						ipStr := optDataToIPString(opt.Data[4:8])
						dns = append(dns, ipStr)
					}
					offerOptions["DOMAIN_NAME_SERVER"] = strings.Join(dns, ",")
					break
				}
			}
		}
	}

	return offerOptions, nil
}

func optDataToIPString(b []byte) string {
	var ipStr string
	len := len(b)
	for i := 0; i < len; i++ {
		v := b[i]
		ipStr += fmt.Sprintf("%d", v)
		if i < len-1 {
			ipStr += "."
		}
	}

	return ipStr
}