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This has bothered me for quite a while but I never found out why or how to change the behavior.

ip duplicates could be caused by typos or dhcp errors etc., but they do occur from time to time.

in rpm-based systems such as centos, the old server with the duplicate ip wins, and the new server will get an error in bringing up the nic (ip address already used). this is somewhat harmless because we can just fix the system that is coming up.

ubuntu only the other hand happily grabs the used ip for itself and leave the old server/device without a valid ip. this is the more dangerous behavior because it causes outages.

what i want is to change the ubuntu behavior to that of the centos/rhel so would appreciate any help.

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up vote 2 down vote accepted

This functionality in redhat based systems is provided by arping.

They use the command (line 261 in /etc/sysconfig/network-scripts/ifup-eth)

/sbin/arping -q -c 2 -w 3 -D -I <dev> <ip>

The important bit is -D which is used for duplicate detection mode. Basically throw that in your startup scripts to check if the address is available before adding it.

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cool, thanks! that was what i was looking for. i am going to – johnshen64 Nov 21 '11 at 21:54

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