func SendGratuitousARPReply(ip string, iface string) error {
// Set up ARP client with socket
ifi, err := net.InterfaceByName(iface)
if err != nil {
return err
}
c, err := arp.NewClient(ifi)
if err != nil {
return err
}
// Set request deadline from flag
if err := c.SetDeadline(time.Now().Add(1 * time.Second)); err != nil {
return err
}
srcIp := net.ParseIP(ip).To4()
packet, err := arp.NewPacket(arp.OperationReply, ifi.HardwareAddr, srcIp, ethernetBroadcast, net.IPv4bcast)
if err != nil {
return err
}
if err := c.WriteTo(packet, ethernetBroadcast); err != nil {
return err
}
// Clean up ARP client socket
_ = c.Close()
return nil
}
func main() {
flag.Parse()
// Ensure valid interface and IPv4 address
ifi, err := net.InterfaceByName(*ifaceFlag)
if err != nil {
log.Fatal(err)
}
ip := net.ParseIP(*ipFlag).To4()
if ip == nil {
log.Fatalf("invalid IPv4 address: %q", *ipFlag)
}
// Handle ARP requests bound for designated IPv4 address, using proxy ARP
// to indicate that the address belongs to this machine
arp.HandleFunc(arp.OperationRequest, func(w arp.ResponseSender, r *arp.Request) {
// Ignore ARP requests which are not broadcast or bound directly for
// this machine
if !bytes.Equal(r.TargetHardwareAddr, ethernet.Broadcast) && !bytes.Equal(r.TargetHardwareAddr, ifi.HardwareAddr) {
return
}
log.Printf("request: who-has %s? tell %s (%s)", r.TargetIP, r.SenderIP, r.SenderHardwareAddr)
// Ignore ARP requests which do not indicate the target IP
if !bytes.Equal(r.TargetIP, ip) {
return
}
// Send reply indicating that this machine has the requested
// IP address
p, err := arp.NewPacket(
arp.OperationReply,
ifi.HardwareAddr,
ip,
r.SenderHardwareAddr,
r.SenderIP,
)
if err != nil {
log.Fatal(err)
}
log.Printf(" reply: %s is-at %s", ip, ifi.HardwareAddr)
if _, err := w.Send(p); err != nil {
log.Fatal(err)
}
})
if err := arp.ListenAndServe(*ifaceFlag, nil); err != nil {
log.Fatal(err)
}
}