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Gratuitous ARP on Windows (ARPING)

When a new IP address is added to a machine, the IP address may not be immediately visible to other machines in the LAN.

It may eventually become visible after ARP entries expire on all other machines, and the IP address is re-requested. To make it visible before that time, the machine must send a Gratuitous ARP (GARP). This is an unsolicited ARP response, even though no ARP request was sent by any other machines.

This functionality is possible using the arping command on linux. However, on Windows there is no such tool. Windows only sends a gratuitous ARP (GARP) whenever a new IP is added to an adaptor. In Windows Server 2003, only a single GARP is sent when one or more IP addresses are added.

The following technique allows a gratuitous ARP (GARP) to be sent for multiple IP addresses:

  • If the IP is already added, remove it first. If it is the only IP, add a temporary fake IP.
  • Close all property windows
  • Right click the network connection icon in the system tray, and select the Repair option (if the network connection does not show an icon, this can be performed in the Network Connections control panel)
  • Double click the network connection icon, and select the properties button
  • Add the new IP
  • Close all property windows again
  • Windows will now announce the new IP address to the LAN

This must be performed for each IP. I am not 100% sure why the Repair is necessary between each IP address, as the only ARP behaviour that is reported, is that Windows clears it's own ARP cache during a repair. However, it also seems to clear the state-flag that remembers whether it has sent a gratuitous ARP or not, and it works if done this way.

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