Finding DUPLICATE IP addresses on a level 2 switch

JonMartin

It would be a nice feature if my Fing Box could discover duplicate IP and alerted me.

I had an issue where a user assigned his own static IP in a DHCP range.  The DHCP server did not know it and would assign the same IP to a different MAC.  Both devices with same IP where connected to a level 2 switch (MAC address routing).  When a third device would try to reach/respond to one of the devices with a duplicate IP, an ARP race would result and packets would SOMETIMES get routed by the switch to the wrong device.   Of course I did not realize this and was looking at VLAN configuration, Routing tables, etc to track the issue.  I finally stumbled on the Duplicate IP by packet sniffing multiple ports on the switch.  The switch was not smart enough to know it had duplicate IPs as it was happily routing packets by MAC.

Are there any tools that can be used to discover duplicate IPs besides examining all the ARP tables for mismatched MAC?

Robin_Ex_Fing

Thanks for the suggestion @JonMartin
I will pass this suggestion to our developers for consideration. For other users who would like to get this idea implemented, please upvote the initial post.