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RFC 3315 says:
The client SHOULD perform duplicate address detection [17] on each of
the addresses in any IAs it receives in the Reply message before
using that address for traffic.  If any of the addresses are found to
be in use on the link, the client sends a Decline message to the
server as described in section 18.1.7.


Attached is patch (dhcp-4.1.1-sendDecline.patch) that makes client send 
Decline
message when dhclient-script returns value indicating that DAD failed.


Next part of this problem is how to make dhclient-script discover that 
DAD failed.
I'm not skilled with other OS besides linux so my patch 
(dhcp-4.1.1-dhclient-script-DAD.patch)
fixes only dhclient-script for linux.
It uses the fact that 'ip addr show' shows 'tentative' flag in address 
info when DAD
is still not complete or failed.

Thanks

Jiri Popelka
Red Hat, inc.