kdhcp/dhcp/packet.go

package dhcp

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

	log "git.wntrmute.dev/kyle/goutils/syslog"
)

const (
	maxHardwareAddrLen = 16
	maxServerName      = 64
	maxFileName        = 128
)

var anyAddr = net.IP([]byte{0, 0, 0, 0})

func formatMAC(mac []byte) string {
	s := []string{}
	for i := 0; i < len(mac); i++ {
		s = append(s, fmt.Sprintf("%0x", mac[i:i+1]))
	}

	return strings.Join(s, ":")
}

type BootRequest struct {
	MessageType     uint8
	HardwareType    uint8
	HardwareAddress []byte
	Hops            uint8
	TransactionID   uint32
	SecondsElapsed  uint16
	Flags           uint16
	ServerName      string
	FileName        string

	ClientIP net.IP
	YourIP   net.IP
	NextIP   net.IP
	RelayIP  net.IP

	DHCPType          uint8  // option 53
	HostName          string // option 12
	ParameterRequests []string
}

func (req *BootRequest) Read(packet []byte) error {
	buf := bytes.NewBuffer(packet)
	read := func(v any) error {
		return binary.Read(buf, binary.BigEndian, v)
	}

	if err := read(&req.MessageType); err != nil {
		return err
	}

	if err := read(&req.HardwareType); err != nil {
		return err
	}

	var hwaLength uint8
	if err := read(&hwaLength); err != nil {
		return err
	}

	if err := read(&req.Hops); err != nil {
		return err
	}

	if err := read(&req.TransactionID); err != nil {
		return err
	}

	if err := read(&req.SecondsElapsed); err != nil {
		return err
	}

	if err := read(&req.Flags); err != nil {
		return err
	}

	req.ClientIP = anyAddr[:]
	if _, err := buf.Read(req.ClientIP); err != nil {
		return err
	}

	req.YourIP = anyAddr[:]
	if _, err := buf.Read(req.YourIP); err != nil {
		return err
	}

	req.NextIP = anyAddr[:]
	if _, err := buf.Read(req.NextIP); err != nil {
		return err
	}

	req.RelayIP = anyAddr[:]
	if _, err := buf.Read(req.RelayIP); err != nil {
		return err
	}

	req.HardwareAddress = make([]byte, int(hwaLength))
	if _, err := buf.Read(req.HardwareAddress); err != nil {
		return err
	}

	hwaPad := make([]byte, maxHardwareAddrLen-hwaLength)
	if _, err := buf.Read(hwaPad); err != nil {
		return err
	}
	log.Debugf("padding: %x", hwaPad)

	tempBuf := make([]byte, maxServerName)
	if _, err := buf.Read(tempBuf); err != nil {
		return err
	}
	req.ServerName = string(bytes.Trim(tempBuf, "\x00"))

	tempBuf = make([]byte, maxFileName)
	if _, err := buf.Read(tempBuf); err != nil {
		return err
	}
	req.FileName = string(bytes.Trim(tempBuf, "\x00"))

	for {
		tag, err := buf.ReadByte()
		if err != nil {
			if err == io.EOF {
				break
			}
			return err
		}

		err = ReadOption(req, tag, buf)
		if err != nil {
			return err
		}
	}
	return nil
}

func ReadRequest(pkt []byte) (*BootRequest, error) {
	req := &BootRequest{}
	err := req.Read(pkt)
	if err != nil {
		return nil, err
	}

	log.Debugf("dhcp: BOOTP request with txid %d for %s", req.TransactionID, formatMAC(req.HardwareAddress))
	return req, nil
}