problem with dhcp setup

Glenn Satchell Glenn.Satchell at
Tue Jun 19 13:24:53 UTC 2007

Hi Jason

A DHCP packet can only be of a certain maximum size. If you tried to
provide every option then you would overflow the packet size. How can
the server decide which options a client wants to see? The RFC is very
specific - the client must ask for the options it wants.

Unfortunately other dhcp servers have different defaults in this
regard, and some vendors fall back to "it works with X dhcp server" even
when their own client is clearly not following the rules

To add to Simon's list, lease time is also always sent.

If this is not the case, which options do you think the dhcp server
should provide without asking?


>I do agree with you, however, I am sure you would agree that DHCP servers
>have been accounting for client shortcomings for quite some time.  As much
>as I hate to admit it this I suspect this will continue as we the adoption
>of IPv6 advances.  You seem to be alluding to this below, in fact, you cite
>some great examples.
>On 6/19/07 2:48 AM, "Simon Hobson" <dhcp1 at> wrote:
>> At 12:20 -0400 18/6/07, John Jason Brzozowski (CISSP, RHCT) wrote:
>>> So if a client requests no options, the ADVERTISE/REPLY will contain no
>>> options at all?
>>> Just because a client does not request options does not absolutely indicate
>>> that the client does not know how to handle the same.
>> If a client doesn't request an option it needs then it's broken and
>> there should be no expectation of it working ! It should not be down
>> to one piece of software to second guess what someone else broke and
>> fix it for them. While most clients seem reasonably 'sane', we have
>> come across a few which are broken in very ingenious ways - how about
>> one that doesn't ask for a default gateway but as well as this will
>> not accept a lease that is less than 2 years long ?
>> There are however a handful of options (address, subnet mask, gateway
>> I think) that are automatically sent even if the client doesn't
>> request them.
>John Jason Brzozowski (CISSP, RHCT)
>jjmb at
>(p) 484-994-6787
>(f) 610-616-4535

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