Linux Networking Cookbook

19.4. Finding Duplicate IP Addresses with arping

Problem

You want to know how to test an IP address on your LAN to see whether it is a duplicate.

Solution

Use arping, like this:

	$ arping -D 192.168.1.76
	ARPING 192.168.1.76 from 0.0.0.0 eth0
	Unicast reply from 192.168.1.76 [00:14:2A:54:67:D6] for 192.168.1.76 [00:14:2A:54:67:
	D6] 0.605ms
	Sent 1 probes (1 broadcast(s))
	Received 1 response(s)

Received 1 response(s) means that this address is already in use, and arping even gives you the MAC address. You may also test with a hostname:

	$ arping -D uberpc
	ARPING 192.168.1.76 from 0.0.0.0 eth0
	Unicast reply from 192.168.1.76 [00:14:2A:54:67:D6] for 192.168.1.76 [00:14:2A:54:67:
	D6] 0.590ms
	Sent 1 probes (1 broadcast(s))
	Received 1 response(s)

You should set a time limit or count limit, or arping will keep running when it gets no response. This example sets a time limit of 10 seconds:

	$ arping -w10 -D 192.168.1.100
	ARPING 192.168.1.100 from 0.0.0.0 eth0
	Sent 11 probes (11 broadcast(s))
	Received 0 response(s)

Use -c5 instead of -w10 to tell arping to run for five counts.

Discussion

Of course, you may use any value for -c and -w that you like.

This is a good test to run when you have mobile users with static IP addresses on their laptops that come and go a lot, or to use before assigning a static address to a new host. If you're having intermittent connectivity problems with a particular host, run arping to see if it has a duplicate address.

arping is also useful to see if a host is up when ping fails. ...