// writeARP writes an ARP request for each address on our local network to the
// pcap handle.
func writeARP(handle *pcap.Handle, iface *net.Interface, addr *net.IPNet) error {
// Set up all the layers' fields we can.
eth := layers.Ethernet{
SrcMAC: iface.HardwareAddr,
DstMAC: net.HardwareAddr{0xff, 0xff, 0xff, 0xff, 0xff, 0xff},
EthernetType: layers.EthernetTypeARP,
}
arp := layers.ARP{
AddrType: layers.LinkTypeEthernet,
Protocol: layers.EthernetTypeIPv4,
HwAddressSize: 6,
ProtAddressSize: 4,
Operation: layers.ARPRequest,
SourceHwAddress: []byte(iface.HardwareAddr),
SourceProtAddress: []byte(addr.IP),
DstHwAddress: []byte{0, 0, 0, 0, 0, 0},
}
// Set up buffer and options for serialization.
buf := gopacket.NewSerializeBuffer()
opts := gopacket.SerializeOptions{
FixLengths: true,
ComputeChecksums: true,
}
// Send one packet for every address.
for _, ip := range ips(addr) {
arp.DstProtAddress = []byte(ip)
gopacket.SerializeLayers(buf, opts, ð, &arp)
if err := handle.WritePacketData(buf.Bytes()); err != nil {
return err
}
}
return nil
}
// newScanner creates a new scanner for a given destination IP address, using
// router to determine how to route packets to that IP.
func newScanner(ip net.IP, router routing.Router) (*scanner, error) {
s := &scanner{
dst: ip,
opts: gopacket.SerializeOptions{
FixLengths: true,
ComputeChecksums: true,
},
buf: gopacket.NewSerializeBuffer(),
}
// Figure out the route to the IP.
iface, gw, src, err := router.Route(ip)
if err != nil {
return nil, err
}
log.Printf("scanning ip %v with interface %v, gateway %v, src %v", ip, iface.Name, gw, src)
s.gw, s.src, s.iface = gw, src, iface
// Open the handle for reading/writing.
// Note we could very easily add some BPF filtering here to greatly
// decrease the number of packets we have to look at when getting back
// scan results.
handle, err := pcap.OpenLive(iface.Name, 65536, true, pcap.BlockForever)
if err != nil {
return nil, err
}
s.handle = handle
return s, nil
}
