DHCPACK question

David W. Hankins David_Hankins at isc.org
Thu Feb 12 17:32:35 UTC 2009

On Thu, Feb 12, 2009 at 08:01:45AM +0000, Simon Hobson wrote:
>> Feb 12 08:35:18 net1 dhcpd: DHCPINFORM from via 
>> Feb 12 08:35:18 net1 dhcpd: DHCPACK to (00:0d:5e:82:75:d1) 
>> via eth0
> * In the case of INFORM, I think the client just broadcasts the packet 
> (it's interested in any answer, not specifically an answer from the server 
> that gave it it's lease). It does however set the flag to say it can handle 
> a unicast reply and so the server replies directly, not via the relay 
> agent.

It's not the unicast flag that dictates this, the only language in
RFC 2131 that talks about replying to DHCPINFORMs says you SHOULD
unicast the reply to the contents of 'ciaddr'.

In this case, the client appears to be properly filling the ciaddr
field, but doesn't know the servers' unicast IP address(es).  It
broadcasts the DHCPINFORM, which hits relay agents, which forwards
to servers, which make their replies unicast to ciaddr.

This is all correct, and described in RFC 2131.

I think the OP is noticing that this behaviour is unlike 'DORA'
exchanges, which return the reply to 'giaddr' always if it is set
(the relay always handles the reply).

I don't know precisely why the DHCPINFORM exchange is so very
different from other message exchanges, but I have a theory;

DHCP leveraged existing BOOTP relays, so even today DHCP relay
behaviour is simply BOOTP relay behaviour with some DHCP-extension.

BOOTP relays will forward unicast replies (server->relay->client) to
the 'yiaddr' field, which is required to be zeroed in DHCPINFORM

So a unicast reply with DHCPINFORM via a relay agent is not a
possibility without extending BOOTP relays, or clouding the meaning
of the yiaddr field contents, or making all such replies broadcast
in nature.

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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