DHCP option 82 behavior

David W. Hankins David_Hankins at isc.org
Fri Sep 8 20:13:27 UTC 2006

On Thu, Sep 07, 2006 at 01:59:07PM -0400, Curt Rask wrote:
> In reading the RFC, it suggests that anytime a DHCP server receives 
> option 82 information, it should always be contained in the reply or 
> whatever communication is going back to the client.  Does anyone know of 
> a way to enable the inclusion of option 82 information for a local subnet?

It's not really clear to me what to do in this case.

DHCP packets are not allowed to exceed 576 bytes (unless the client
supplied an option indicating it was capable of more).  We're allowed
to append relay agent options beyond this limit because the relay will
strip them out before replying to the client.

So we want to avoid including relay agent options if we don't know
for certain that there is an intervening device that will remove the
option, such as when giaddr is set.  But possibly we should relax this
when doing so keeps us under the limit still.

To confound this I think the code currently is 'confused' as to
wether the relay agent options in its cache came from the packet
or were fetched from the lease state - the cached option 82
contents on the lease structure when the client was assigned their

You don't want to include option 82 when you respond to the client
renewals either.

At any rate, we should look more closely at this.

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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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