Dhcprelay Unicastiong its reply

David W. Hankins David_Hankins at isc.org
Thu Jan 22 22:39:23 UTC 2009

On Wed, Jan 21, 2009 at 10:25:27AM -0700, commo dore wrote:
> What would cause dhcrelay to unicast the reply back to the client?

The broadcast flag was not set.

This is a DHCP protocol feature (stemming from BOOTP relay semantics
as well here).

A DHCP relay directs server replies to the 'yiaddr', using the
'chaddr' contents for MAC address.

If your DHCP relay is trying to ARP, it is broken.

If you've encountered a client that does not set the broadcast flag,
but also cannot receive unicasts when unconfigured, then it is broken.

Either way, you should take it up with the relevant manufacturer.  In
the meantime, also either way, you might be able to work around the
issue by setting the 'always-broadcast' configuration option in the
server, if it is ISC DHCP.

David W. Hankins	"If you don't do it right the first time,
Software Engineer		     you'll just have to do it again."
Internet Systems Consortium, Inc.		-- Jack T. Hankins
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