How to make dhcpd send NAKs instead of silently ignoring DHCPREQUESTS?

Simon Hobson dhcp1 at
Tue Apr 3 07:04:34 UTC 2012

Andre Albsmeier wrote:

>I have a client (WinXP Professional)

Well, enough said. We've had people at work with related problems 
where their laptop has failed to work out that it's moved between 
their home and our office networks. I think I had them change the 
subnet on their home network to be different to ours.

>  > There is potentially another way to do things.
>>  The ISC server now supports reserved leases - these are "normal"
>>  leases but even if expired will never be re-allocated to other
>>  clients. If you create reserved leases for your fixed address clients
>>  instead of using host statements then the addresses cannot be given
>>  to anything else and will result in a NACK in the circumstances you
>>  describe. You can either just extend your main range, or add small
>>  ranges to just include the addresses in question.
>But from what I have seen I have to hack the dhcpd.leases file
>for this. (BTW, I found this possibility before but found it
>quite ugly so I decided to ask on the list ;-)).

I believe you can also do it via OMAPI - but I've never used it myself.

>In my personal opinion (I am no DHCP expert, just a user with the
>above observations) I think that if a server is authoritative one
>should be able to NAK unknown client's DHCPREQUESTs for addresses
>not in dynamic ranges.

It's valid for there to be multiple, independent, authoritative DHCP 
servers on a network as long as they have non-overlapping ranges. 
Unless you tell the server that an address is explicitly not 
available, it will remain silent regarding those it's not responsible 
It's just unfortunate that you've found a corner case where it is 
valid to have a fixed-address and a dynamic range which overlap.

Simon Hobson

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