subenet-mask option in DHCPACK for DHCPINFORM

David W. Hankins David_Hankins at
Mon Jul 23 21:48:37 UTC 2007

On Mon, Jul 23, 2007 at 01:37:22PM -0700, Jane Du (jadu) wrote:
> Actually, the dhcp client we are using doesn't like that the subnet-mask
> option is included in the DHCPACK when it doesn't request it. The vendor
> of the client told us that we need to change the code by not sending the
> option because it doesn't follow the spec. In RFC 2131, the option 1 is
> not a "must" added option in the ACK, right? 

It is neither a MUST nor is it a MUST NOT.  In fact, servers MAY
provide additional options.

In the context of DHCPINFORM, I think this behaviour is not right,
and we'll fix this in a future release.

In the case of DHCPREQUEST, this behaviour is unfortunately mandatory
to be compatible (and to actually work on the modern Internet).

I'm a little bit disappointed to see a client that throws an error if
it sees an option it doesn't understand.  The best thing to do is to
just ignore the option.  These sorts of things do happen!

> Do you see any issue if I just change code not to send option 1 when it
> is not requested?

If you don't have any clients with the aforementioned flaw, nothing
will go wrong (for example, if you were working fine with 3.0.x).

> In which version, does ISC server start sending the subnet-mask for the
> ack to request?  I don't see such behavior in 3.0.4.

3.1.0.  It was incorporated as a new feature since it does change

Ash bugud-gul durbatuluk agh burzum-ishi krimpatul.
Why settle for the lesser evil?
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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