Relay agents, NAT, and offers to giaddr

David W. Hankins David_Hankins at
Fri Sep 15 22:12:21 UTC 2006

On Fri, Sep 15, 2006 at 10:58:53PM +0100, Simon Hobson wrote:
> Clients renewing do not use a relay agent - they unicast directly to 
> the server.

Today.  Tomorrow?

Note that the above document ratifies a fairly longstanding DHCP 'hack'
of setting the server-identifier option to the contents as received
in giaddr (or to some fixed value that is not the DHCP server).  This
causes the client to transmit to the relay which treats the packet the
same as any broadcast and passes it on.

This has been used by folks who set firewall rules on the fly or have
comms problems with their DHCP clients, such as the OP of this thread
will once he assigns an address.

There are also some black magic design considerations to take into
account if you go this route, since giaddr/link-selection/subnet-selection
also govern the server's interface correction logic (you may get NAKs
when the client is on the right network, or not get NAKs when the client
has moved).

ISC Training!  October 16-20, 2006, in the San Francisco Bay Area,
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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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