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]