Subject: [dhcwg] Minimum size of a dhcp message

From: [email-masked]
Date: Mon, 13 Oct 2003 19:09:21 +0200

I recently experienced some problems with our dhcp implementation
in combination with some clients which didn't accept our OFFERs.
In this special case it was a relatively old COM21 cablemodem with
VXworks OS.

As it turned out, the dhcp client of the modem insisted on a minimum
dhcp message size of 300 bytes, which is in fact the size of the 'old'
bootp-packet (with it's 64 bytes of vendor specific informations)

It looks like ISC-DHCPD is respecting this minimum size (padding the
remaining bytes after the end option with zeroes). I fixed our server
to ensure this minimum size in responses, unfortunately I couldn't
find where this minmum mis specified in the DHCP RFCs (it is spefified
in the bootp rfc 1497)

Is this client behaving correctly ?

	-Patrick
--
Patrick Guélat, ImproWare AG Network Services, CH-4133 Pratteln
Mail: [email-masked] - Phone: +41 61 826 93 00 (ext: 13)

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From: Bud Millwood <[email-masked]>
Date: Tue, 14 Oct 2003 09:59:00 +0200

> Is this client behaving correctly ?

To my knowledge the DHCP packet can be smaller than 300 bytes, and in the 
deployment of our DHCP server the only client I was aware of that had 
problems working with our server was vxWorks.

That said, we changed it to be a minimum to be 300 bytes quite a while back, 
and have not had any problems. RFC 2131, section 2, par. 7 (after Table 2) 
says:

   "A DHCP client must be prepared to receive DHCP messages
   with an 'options' field of at least length 312 octets."

We haven't had any problems that I am aware of after extending the minimum 
message size to 300 bytes.

Bud Millwood
Weird Solutions, Inc.
http://www.weird-solutions.com
tel: +46 8 758 3700
fax: +46 8 758 3687
mailto:[email-masked]

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From: "Rob Stevens" <[email-masked]>
Date: Tue, 14 Oct 2003 19:37:19 -0700

Barr Hibbs and I are working on an update to RFC2131 and many of the
issues will be described in the draft
"draft-ietf-dhc-implementation-01" (forthcoming).

Our recommendation there is that the minimum size of a DHCP packet
MUST be 300 octets.
This is based solely on the stated design goal that DHCP be largely
compatible with BOOTP, and this requirement was imposed on BOOTP
(RFC951).

Why the original BOOTP stipulated this requirement is hard to say, but
in the past certain routers were known to drop BOOTP packets with a
smaller size. I have no idea whether this is true today, but I think
we should stick with the historical requirement.

Rob Stevens

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From: Richard Johnson <[email-masked]>
Date: Wed, 15 Oct 2003 22:36:51 -0700

Interesting!  I just finished taking care of an incompatibility between 
our DHCP client and one particular ISP and (after trying various 
changes to get it to work) found the change which finally fixed the 
problem was to *remove* the additional padding of the packets to a 
minimum of 312 bytes.  If I padded the DISCOVER packets, I never got 
any responses.  They were showing me packet traces of a Windows system 
which did *not* do this padding and which got responses just fine.  
Removing our padding got a response as well.  I guess you can take this 
as proof that there exists at least one ISP with a configuration which 
will NOT work if the packets are padded.

/raj