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// Copyright 2012 Google, Inc. All rights reserved.
//
// Use of this source code is governed by a BSD-style license
// that can be found in the LICENSE file in the root of the source
// tree.
// arpscan implements ARP scanning of all interfaces' local networks using
// gopacket and its subpackages. This example shows, among other things:
// * Generating and sending packet data
// * Reading in packet data and interpreting it
// * Use of the 'pcap' subpackage for reading/writing
package main
import (
"bytes"
"encoding/binary"
"errors"
"log"
"net"
"sync"
"time"
"github.com/google/gopacket"
"github.com/google/gopacket/layers"
"github.com/google/gopacket/pcap"
)
func main() {
// Get a list of all interfaces.
ifaces, err := net.Interfaces()
if err != nil {
panic(err)
}
var wg sync.WaitGroup
for _, iface := range ifaces {
wg.Add(1)
// Start up a scan on each interface.
go func(iface net.Interface) {
defer wg.Done()
if err := scan(&iface); err != nil {
log.Printf("interface %v: %v", iface.Name, err)
}
}(iface)
}
// Wait for all interfaces' scans to complete. They'll try to run
// forever, but will stop on an error, so if we get past this Wait
// it means all attempts to write have failed.
wg.Wait()
}
// scan scans an individual interface's local network for machines using ARP requests/replies.
//
// scan loops forever, sending packets out regularly. It returns an error if
// it's ever unable to write a packet.
func scan(iface *net.Interface) error {
// We just look for IPv4 addresses, so try to find if the interface has one.
var addr *net.IPNet
if addrs, err := iface.Addrs(); err != nil {
return err
} else {
for _, a := range addrs {
if ipnet, ok := a.(*net.IPNet); ok {
if ip4 := ipnet.IP.To4(); ip4 != nil {
addr = &net.IPNet{
IP: ip4,
Mask: ipnet.Mask[len(ipnet.Mask)-4:],
}
break
}
}
}
}
// Sanity-check that the interface has a good address.
if addr == nil {
return errors.New("no good IP network found")
} else if addr.IP[0] == 127 {
return errors.New("skipping localhost")
} else if addr.Mask[0] != 0xff || addr.Mask[1] != 0xff {
return errors.New("mask means network is too large")
}
log.Printf("Using network range %v for interface %v", addr, iface.Name)
// Open up a pcap handle for packet reads/writes.
handle, err := pcap.OpenLive(iface.Name, 65536, true, pcap.BlockForever)
if err != nil {
return err
}
defer handle.Close()
// Start up a goroutine to read in packet data.
stop := make(chan struct{})
go readARP(handle, iface, stop)
defer close(stop)
for {
// Write our scan packets out to the handle.
if err := writeARP(handle, iface, addr); err != nil {
log.Printf("error writing packets on %v: %v", iface.Name, err)
return err
}
// We don't know exactly how long it'll take for packets to be
// sent back to us, but 10 seconds should be more than enough
// time ;)
time.Sleep(10 * time.Second)
}
}
// readARP watches a handle for incoming ARP responses we might care about, and prints them.
//
// readARP loops until 'stop' is closed.
func readARP(handle *pcap.Handle, iface *net.Interface, stop chan struct{}) {
src := gopacket.NewPacketSource(handle, layers.LayerTypeEthernet)
in := src.Packets()
for {
var packet gopacket.Packet
select {
case <-stop:
return
case packet = <-in:
arpLayer := packet.Layer(layers.LayerTypeARP)
if arpLayer == nil {
continue
}
arp := arpLayer.(*layers.ARP)
if arp.Operation != layers.ARPReply || bytes.Equal([]byte(iface.HardwareAddr), arp.SourceHwAddress) {
// This is a packet I sent.
continue
}
// Note: we might get some packets here that aren't responses to ones we've sent,
// if for example someone else sends US an ARP request. Doesn't much matter, though...
// all information is good information :)
log.Printf("IP %v is at %v", net.IP(arp.SourceProtAddress), net.HardwareAddr(arp.SourceHwAddress))
}
}
}
// 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
}
// ips is a simple and not very good method for getting all IPv4 addresses from a
// net.IPNet. It returns all IPs it can over the channel it sends back, closing
// the channel when done.
func ips(n *net.IPNet) (out []net.IP) {
num := binary.BigEndian.Uint32([]byte(n.IP))
mask := binary.BigEndian.Uint32([]byte(n.Mask))
num &= mask
for mask < 0xffffffff {
var buf [4]byte
binary.BigEndian.PutUint32(buf[:], num)
out = append(out, net.IP(buf[:]))
mask += 1
num += 1
}
return
}
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