ISC DHCPD giving out reserved IPs for non-reserved hosts

John Wobus jw354 at
Mon Jul 9 19:29:19 UTC 2007

I'm assuming that you were indeed attempting to use the ISC server's
host declaration as the equivalent of the Microsoft servers 'reserved' 

DHCP standards don't apply to this issue.  The subject at hand is how to
configure a specific DHCP server to do something.  Microsoft and ISC 
have their own implementation details, including configuration methods.
Even their own jargon.  Once you've configured them, each in their
own way, one hopes they both carry out the DHCP operations in
RFC-compliant manner.

All I say is that the 'host' declaration in the ISC server is not the
exact equivalent of the 'reserved' feature in the Microsoft server.
To mimic Microsoft's 'reserved' configuration feature, in the ISC
server you would also need to declare a dynamic pool or pools 
specifying which addresses are to be dynamically given out.
Microsoft wins for configuration convenience in this specific case,
though many would not directly mess with the ISC configuration, but
would use software to construct the file, which could take care of
such complications under the covers, and perhaps offer other
configuration convenience features that Microsoft doesn't.

(I actually have zero experience with the Microsoft server, so
I could easily go off track and say something wrong about it.)

John Wobus

On Jul 9, 2007, at 9:11 AM, Scott Lacy wrote:

> So Microsoft DHCP's behavior of reserving addresses specifically for
> certain MACs is not standard?  That's the first DHCP product I've used
> (I know, I know), so I was assuming that ISC dhcpd would behave the 
> same
> way.
> Scott
> John Wobus wrote:
>> It's possible you are seeing normal behavior.  If you are using host
>> declarations
>> to attempt to reserve addresses, you are under a misimpression.  Host
>> declarations
>> do not reserve addresses; they merely tell the server to give a 
>> certain
>> answer when they receive a certain MAC address.
>> You reserve addresses by leaving them out of the configured ranges
>> that are dynamically allocated.
>> John Wobus
>> On Jul 6, 2007, at 3:31 PM, Scott Lacy wrote:
>>> I have a DHCP server running ISC DHCPD v3.0.3 on Fedora Core.  I am
>>> having an issue where dhcpd is giving out leases for reserved IP
>>> addresses to hosts other than that which the addresses are reserved
>>> for.  There are no indication in the logs of any conflicts.  The logs
>>> look fairly vanilla and there was no indication of any trouble until 
>>> I
>>> got a call about an IP conflict with the machine the reservation is
>>> for.  The only quick way to resolve the problem was to power down the
>>> offending device.  Even removing the newly-issued lease from DHCP and
>>> having the other device pull an address did not seem to help.
>>> Is there anything that jumps out to any of you as far as things I
>>> should
>>> check my configuration for?  I've run this installation for almost a
>>> year and I've had this come up twice in the last month, both on
>>> networks
>>> that were set up in DHCP long ago.
>>> Thanks,
>>> Scott Lacy
>>> Systems Manager
>>> Mercer University

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