override the client broadcast bit?

David W. Hankins dhankins at isc.org
Thu Sep 23 19:37:55 UTC 2010

On Tue, Sep 07, 2010 at 12:05:03PM -0500, Marc Perea wrote:
> Dhcpd clearly has no trouble honoring either a unicast or broadcast response, so my question is: can we force the server to only respond with a unicast, even if the client requested a broadcast response? Getting new firmware on the modem may or may not pan out, but we're also going down that road, FYI. Being able to force the server into unicast mode would still be a useful tool though.

Not currently.  One concern is that if the client advertises the
broadcast bit, it is advertising that it is not capable of receiving a
directed unicast.  So it would be unusual (but possible) for the
parameter to actually help or work; although the packet may be
delivered to the client, it may not be received.

> Has anyone else had any experience with this - any tales to tell?

New versions of Windows have suddenly started setting the broadcast
bit to true.  Older versions were quite capable of receiving
directed unicasts when unconfigured, so this is a surprising
de-evolution of their software, regressing to a state where it
advertises a lesser capability.  It's been reported that these Windows
versions receive directed unicasts just fine even tho they have set
the broadcast bit true.

So there may already be a case for an 'always-unicast' configuration
flag to oppose the 'always-broadcast' configuration flag.

David W. Hankins	BIND 10 needs more DHCP voices.
Software Engineer		There just aren't enough in our heads.
Internet Systems Consortium, Inc.		http://bind10.isc.org/
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