// scan scans the dst IP address of this scanner. func (s *scanner) scan() error { // First off, get the MAC address we should be sending packets to. hwaddr, err := s.getHwAddr() if err != nil { return err } // Construct all the network layers we need. eth := layers.Ethernet{ SrcMAC: s.iface.HardwareAddr, DstMAC: hwaddr, EthernetType: layers.EthernetTypeIPv4, } ip4 := layers.IPv4{ SrcIP: s.src, DstIP: s.dst, Version: 4, TTL: 64, Protocol: layers.IPProtocolTCP, } tcp := layers.TCP{ SrcPort: 54321, DstPort: 0, // will be incremented during the scan SYN: true, } tcp.SetNetworkLayerForChecksum(&ip4) // Create the flow we expect returning packets to have, so we can check // against it and discard useless packets. ipFlow := gopacket.NewFlow(layers.EndpointIPv4, s.dst, s.src) start := time.Now() for { // Send one packet per loop iteration until we've sent packets // to all of ports [1, 65535]. if tcp.DstPort < 65535 { start = time.Now() tcp.DstPort++ if err := s.send(ð, &ip4, &tcp); err != nil { log.Printf("error sending to port %v: %v", tcp.DstPort, err) } } // Time out 5 seconds after the last packet we sent. if time.Since(start) > time.Second*5 { log.Printf("timed out for %v, assuming we've seen all we can", s.dst) return nil } // Read in the next packet. data, _, err := s.handle.ReadPacketData() if err == pcap.NextErrorTimeoutExpired { continue } else if err != nil { log.Printf("error reading packet: %v", err) continue } // Parse the packet. We'd use DecodingLayerParser here if we // wanted to be really fast. packet := gopacket.NewPacket(data, layers.LayerTypeEthernet, gopacket.NoCopy) // Find the packets we care about, and print out logging // information about them. All others are ignored. if net := packet.NetworkLayer(); net == nil { // log.Printf("packet has no network layer") } else if net.NetworkFlow() != ipFlow { // log.Printf("packet does not match our ip src/dst") } else if tcpLayer := packet.Layer(layers.LayerTypeTCP); tcpLayer == nil { // log.Printf("packet has not tcp layer") } else if tcp, ok := tcpLayer.(*layers.TCP); !ok { // We panic here because this is guaranteed to never // happen. panic("tcp layer is not tcp layer :-/") } else if tcp.DstPort != 54321 { // log.Printf("dst port %v does not match", tcp.DstPort) } else if tcp.RST { log.Printf(" port %v closed", tcp.SrcPort) } else if tcp.SYN && tcp.ACK { log.Printf(" port %v open", tcp.SrcPort) } else { // log.Printf("ignoring useless packet") } } }