Q. I run quite large network with different subnets. I’d like to know how can I find out duplicate IP address under Linux / UNIX?

A. You can use arpping command. The arping utility performs an action similar to ping command, but at the Ethernet layer. You can send ARP REQUEST to a neighbor host / computers.

Task: Send ARP request

Let us find out reachability of an IP on the local Ethernet with arping i.e send ARP request 192.168.1.1:
$ sudo arping -I eth0 -c 3 192.168.1.1
Output:

ARPING 192.168.1.1 from 192.168.1.106 ra0
Unicast reply from 192.168.1.1 [00:18:39:6A:C6:8B]  2.232ms
Unicast reply from 192.168.1.1 [00:18:39:6A:C6:8B]  1.952ms
Sent 3 probes (1 broadcast(s))
Received 3 response(s)

Where,

• -I eth0 : Specify network interface i.e. name of network device where to send ARP REQUEST packets. This option is required.
• -c 3 : Stop after sending 3 ARP REQUEST packets

Task: Find duplicate IP

The -D option specifies duplicate address detection mode (DAD). It returns exit status 0, if DAD succeeded i.e. no replies are received.
$ sudo arping -D -I eth0 -c 2 192.168.1.1
If 192.168.1.1 duplicated you should see zero exit status:
$ echo $?
Always use following syntax for duplicate address detection with arping:
$ sudo arping -D -I <interface-name> -c 2 <IP-ADDRESS-TO-TEST>
$ echo $?

References:

• Ethernet Layer Tools – arping
• arping man page