Golang DHCP 中继程序：使用 gopacket 抓取和转发 DHCP 数据包

package main

import ( "fmt" "log" "net" "time" "github.com/google/gopacket" "github.com/google/gopacket/layers" "github.com/google/gopacket/pcap" )

func main() { if handle, err := pcap.OpenLive('eth0', 65536, true, pcap.BlockForever); err != nil { log.Fatal(err) } else { defer handle.Close()

	// 设置过滤条件，只关注目标端口为 67 或 68 的 UDP 数据包
	filter := 'udp and (dst port 67 or dst port 68)'
	if err := handle.SetBPFFilter(filter); err != nil {
		log.Fatal(err)
	}

	// 创建一个 DHCP 解析器
	parser := gopacket.NewDecodingLayerParser(layers.LayerTypeDHCPv4)
	decoded := []gopacket.LayerType{}

	// 开始捕获数据包
	packetSource := gopacket.NewPacketSource(handle, handle.LinkType())
	for packet := range packetSource.Packets() {
		// 解析数据包
		err := parser.DecodeLayers(packet.Data(), &decoded)
		if err != nil {
			log.Printf('Error decoding packet: %s', err)
		}

		// 处理 DHCP 请求包
		for _, layerType := range decoded {
			if layerType == layers.LayerTypeDHCPv4 {
				dhcpLayer := packet.Layer(layers.LayerTypeDHCPv4).(*layers.DHCPv4)
				if dhcpLayer.Operation == layers.DHCPOpRequest {
					fmt.Println('Received DHCP Request')

					// 构造 DHCP Offer 包
					offerPacket := gopacket.NewPacket(nil, layers.LayerTypeEthernet, gopacket.Default)
					ethLayer := &layers.Ethernet{
						SrcMAC:       net.HardwareAddr{0x00, 0x11, 0x22, 0x33, 0x44, 0x55},
						DstMAC:       packet.Layer(layers.LayerTypeEthernet).(*layers.Ethernet).SrcMAC,
						EthernetType: layers.EthernetTypeIPv4,
					}
					ipLayer := &layers.IPv4{
						SrcIP:    net.IP{192, 168, 199, 1},
						DstIP:    dhcpLayer.YourClientIP,
						Version:  4,
						TTL:      64,
						Protocol: layers.IPProtocolUDP,
					}
					udpLayer := &layers.UDP{
						SrcPort: layers.UDPPort(67),
						DstPort: layers.UDPPort(68),
					}
					dhcpOfferLayer := &layers.DHCPv4{
						Operation:   layers.DHCPOpOffer,
						TransactionID: dhcpLayer.TransactionID,
						ClientHWAddr: dhcpLayer.ClientHWAddr,
						YourClientIP: dhcpLayer.YourClientIP,
						ServerID:     net.IP{192, 168, 199, 1},
						Options: []layers.DHCPOption{
							layers.DHCPOption{Type: layers.DHCPOptSubnetMask, Data: []byte{255, 255, 255, 0}},
							layers.DHCPOption{Type: layers.DHCPOptRouter, Data: []byte{192, 168, 199, 1}},
							layers.DHCPOption{Type: layers.DHCPOptDNS, Data: []byte{8, 8, 8, 8}},
						},
					}

					// 将各个层添加到数据包中
					offerPacket.AddLayer(ethLayer)
					offerPacket.AddLayer(ipLayer)
					offerPacket.AddLayer(udpLayer)
					offerPacket.AddLayer(dhcpOfferLayer)
					offerPacket.SetNetworkLayerForChecksum(ipLayer)

					// 发送 DHCP Offer 包
					if err := handle.WritePacketData(offerPacket.Data()); err != nil {
						log.Printf('Error sending DHCP Offer packet: %s', err)
					} else {
						fmt.Println('Sent DHCP Offer')
					}

					// 转发 DHCP 请求包
					if err := handle.WritePacketData(packet.Data()); err != nil {
						log.Printf('Error forwarding DHCP Request packet: %s', err)
					} else {
						fmt.Println('Forwarded DHCP Request')
					}

					break
				}
			}
		}
	}
}