feat: Initial revision

2022-01-27 11:28:51 +01:00
commit ed6198f71a
4 changed files with 249 additions and 0 deletions
--- a/main.go
+++ b/main.go
@@ -0,0 +1,166 @@
+package main
+
+import (
+	"bytes"
+	"encoding/binary"
+	"net"
+	"unsafe"
+
+	"go.uber.org/zap"
+)
+
+type DhcpOp uint8
+
+const (
+	BOOTREQUEST DhcpOp = iota + 1
+	BOOTREPLY
+)
+
+type DhcpMessage struct {
+	Op     DhcpOp
+	Htype  uint8
+	Hlen   uint8
+	Hops   uint8
+	Xid    uint32
+	Secs   uint16
+	Flags  uint16
+	Ciaddr uint32
+	Yiaddr uint32
+	Siaddr uint32
+	Giaddr uint32
+	Chaddr [16]byte
+	Sname  [64]byte
+	File   [128]byte
+	Magic  [4]byte
+}
+
+func (dhcp DhcpMessage) Debug(s *zap.SugaredLogger) {
+	s.Debugf("op=%x, htype=%x, hlen=%x, hops=%x", dhcp.Op, dhcp.Htype, dhcp.Hlen, dhcp.Hops)
+	s.Debugf("xid=%x", dhcp.Xid)
+	s.Debugf("secs=%d, flags=%x", dhcp.Secs, dhcp.Flags)
+
+	s.Debugf("ciaddr=%x", dhcp.Ciaddr)
+	s.Debugf("siaddr=%x", dhcp.Siaddr)
+	s.Debugf("giaddr=%x", dhcp.Giaddr)
+	s.Debugf("chaddr=%x", dhcp.Chaddr)
+
+	s.Debugf("sname=%s", string(dhcp.Sname[:]))
+	s.Debugf("file=%s", string(dhcp.File[:]))
+	s.Debugf("magic=%x", dhcp.Magic)
+}
+
+func parseOptions(data []byte) {
+	s := zap.S()
+	i := 0
+	for i < len(data) {
+		code := data[i]
+		if code == 255 {
+			s.Debug("Found END option at offset ", i)
+			break
+		}
+
+		if code == 0 {
+			s.Debug("Found padding option at offset ", i)
+			i += 1
+			continue
+		}
+
+		size := int(data[i+1])
+		payload := data[i+2 : i+2+size]
+
+		s.Debugf("code=%d, size=%d, payload=%x", code, size, payload)
+		i += size + 2
+	}
+}
+
+func prepareOffer(request DhcpMessage) DhcpMessage {
+	var sname [64]byte
+	var file [128]byte
+	copy(sname[:], "go-dhcp-server")
+
+	dhcp := DhcpMessage{
+		Op:     BOOTREPLY,
+		Htype:  1, // Ethernet
+		Hlen:   6, // Ethernet address length
+		Hops:   0,
+		Secs:   0,
+		Flags:  request.Flags,
+		Xid:    request.Xid,
+		Siaddr: binary.BigEndian.Uint32(net.IPv4(10, 0, 0, 1).To4()),
+		Ciaddr: 0,
+		Yiaddr: binary.BigEndian.Uint32(net.IPv4(10, 0, 0, 2).To4()),
+		Giaddr: request.Giaddr,
+		Chaddr: request.Chaddr,
+		Magic:  [4]byte{0x63, 0x82, 0x53, 0x63},
+		Sname:  sname,
+		File:   file,
+	}
+	return dhcp
+}
+
+func handleMsg(data []byte, remote *net.UDPAddr) {
+	s := zap.S()
+
+	s.Debugf("Connection from client %v", remote.IP)
+
+	var dhcp DhcpMessage
+	reader := bytes.NewReader(data)
+	binary.Read(reader, binary.BigEndian, &dhcp)
+	dhcp.Debug(s)
+
+	if dhcp.Magic != [4]byte{0x63, 0x82, 0x53, 0x63} {
+		panic("Invalid DHCP magic field")
+	}
+
+	optDataOffset := int(unsafe.Sizeof(dhcp))
+	optData := data[optDataOffset:]
+	s.Debug("Raw options data:", optData)
+
+	parseOptions(optData)
+
+	offer := prepareOffer(dhcp)
+	localAddr, _ := net.ResolveUDPAddr("udp", "172.17.0.1:68")
+	clientAddr, _ := net.ResolveUDPAddr("udp", "255.255.255.255:68")
+	conn, err := net.DialUDP("udp", localAddr, clientAddr)
+	if err != nil {
+		panic(err)
+	}
+	defer conn.Close()
+
+	s.Info("Sending DHCPOFFER...")
+	buf := make([]byte, 0)
+	w := bytes.NewBuffer(buf)
+	binary.Write(w, binary.BigEndian, offer)
+	w.Write([]byte{53, 1, 2})
+	w.WriteByte(255)
+	msg := w.Bytes()
+	s.Debug("Msg", msg)
+	conn.Write(msg)
+}
+
+func main() {
+	// Set up logging
+	logger, _ := zap.NewDevelopment()
+	defer logger.Sync() // flushes buffer, if any
+	zap.ReplaceGlobals(logger)
+
+	addr, _ := net.ResolveUDPAddr("udp4", ":67")
+	conn, err := net.ListenUDP("udp4", addr)
+	if err != nil {
+		panic(err)
+	}
+	defer conn.Close()
+
+	zap.L().Info("Listening for incoming connections")
+
+	for {
+		buf := make([]byte, 1024)
+		rlen, remote, err := conn.ReadFromUDP(buf[:])
+		if err != nil {
+			panic(err)
+		}
+
+		// Do stuff with the read bytes
+		handleMsg(buf[0:rlen], remote)
+	}
+}