invalid DISCOVER packet - update

Duane Cox duanec at
Tue Apr 10 16:17:32 UTC 2007

> > "The discover packet from the router is 28 bytes longer than normal. The
> > [embedded device] checks the length of the discover packet, if it is
> > the packet is discarded. I have asked engineering if the code can be
> > modified to accept the longer DHCP discover packet."
> >
> > David, I wasn't aware there was a set length, or in this case "normal"
> > length, for dhcp packets.  Am I missing something here?
> Well, this is something of a longstanding problem.  I think RFC2131
> references down to another document that defines IP's minimum MTU
> as being 576 bytes...and alerts implementers that this is the smallest
> value that is guaranteed to work (since you can't predict the MTU
> outside of your local interface).
> It seems some implementations (such as your relay) have taken this
> as a fixed upper limit, rather than a fixed lower limit.

Yes, this is their response...

"According to our engineering Cisco is not following RFC2131 - DHCP.
According to the RFC the maximum datagram size is 576bytes. The datagram
coming from this router is 604bytes."

Would you agree to that?  That's not how I read the RFC...

   The 'options' field is now variable length. A DHCP client must be
   prepared to receive DHCP messages with an 'options' field of at least
   length 312 octets.  This requirement implies that a DHCP client must
   be prepared to receive a message of up to 576 octets, the minimum IP

   datagram size an IP host must be prepared to accept [3].  DHCP
   clients may negotiate the use of larger DHCP messages through the
   'maximum DHCP message size' option.  The options field may be further
   extended into the 'file' and 'sname' fields.

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