Static IP and IP management

[Bernard Fay]

I like this idea.

But thinking about it....

When the dhcpd server assign a static IP to a device, it also instruct bind
to add an entry in the DNS zone file.  One thing I realized is that if a
device didn't renew his lease, the entry in the DNS zone file is not
removed.  I would have thought to use the zone files to know if a device is
in use or not.  I had in mind that the lease time would have help to know
if a device therefore an IP is use or not.  In other words, a device
requires an IP and the dhcpd server assigned it a statically defined IP
address. The dhcpd server also instruct bind to add an entry in the
appropriate zone file.  Eventually the device is turned off, the lease time
reach its limit then I would have expected the dhcpd server to instruct
bind to remove the entry regarding this device but it is not the case.
Then I could have take a look at the zone files to know what is in used and
I would know what is not in use.

Either I made something in my configuration or I was expecting too much
from dhcpd and bind.

Thanks,


On Wed, Feb 24, 2016 at 12:11 PM, Chuck Anderson <cra at wpi.edu> wrote:

> On Wed, Feb 24, 2016 at 05:04:10PM +0000, Simon Hobson wrote:
> > Patrick Trapp <ptrapp at nex-tech.com> wrote:
> >
> > > If you are using host entries to dictate what address a device gets
> (and not allowing devices to grab random addresses - effectively making
> them static without having to configure it on the device), then when you
> delete that host entry from the dhcpd.conf, you would know that address is
> free.
> >
> > Yes, but I think the primary issue is knowing that the assignment is no
> longer needed - as in, that device hasn't been here for a while. Jim has
> given an example of how I suspect most systems manage it - literally keep
> track of what IPs and MACs are in use, and see if any of them go stale.
> >
> > An alternative approach could be to use reserved leases. That way, each
> usage of the assignment goes through the normal DHCP lifecycle - including
> DNS updates. By tracking lease usage etc you can then see if a lease is no
> longer being used.
> >
> > Basically it's the old problem - when something is needed for something
> else to work then it gets noticed, when that something is no longer needed
> then it just gets forgotten about.
>
> One other possibility if you can force everyone to use DHCP is just
> keep the DHCP logs and look at them from the last time a device
> DHCP'd.  That way you can keep using fixed-address assignments, but
> managed via DHCP.  It helps if you have switches that support DHCP
> Snooping, ARP Inspection, IP Source Guard so you can really enforce
> the use of DHCP.
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