ISC DHCP server offers fixed IP addresses to ANY device!

Foggi, Nicola NFOGGI at
Sun Dec 28 20:31:40 UTC 2008


We use the "reserved" functionality of dhcpd to handle primarily our network printers (and special pc's on net that need "static" ip's for one reason or another.

For printers, our field guys deploy a printer, set it to dhcp, print the test page, and submit that ip/mac address which then we turn around and "reserve" it for that printer and then a print queue is setup for it etc... we do something similar for pc's that require a "static" ip for some reason or another.

It's legacy from our Cisco Network Registrar that currently runs most of the campus, but we're slowly migrating to DHCPD to replace it.  It has proven to be quite handy.  I initially started doing fixed-addresses for them all, but i would have to shrink my available pool of dynamic ip's or chop it up, and that was much more and disruptive.  Once we got the info about setting the flag via omapi that simplified the "reserved" flag!  (now if there was a way to "unset" the reserved flag!

I think at least in our case the reserved flag is the way to go from a management perspective, so i hope to see it continue to evolve!  For instance, i've seen some cases where the server will allow the client to request a different ip and the server ACK's it, whereas i would prefer the NAK it and then offer it the reserved ip when the client re-discovers.

Nicola Foggi
Networks and Telecom
DePaul University

-----Original Message-----
From: dhcp-users-bounces at on behalf of David W. Hankins
Sent: Mon 12/22/2008 12:43 PM
To: Users of ISC DHCP
Subject: Re: ISC DHCP server offers fixed IP addresses to ANY device!
On Sat, Dec 20, 2008 at 02:37:35PM +0530, Jatinder Goswami wrote:
> Can you please let us know how to configure dhcpd for reserved IP
> functionality.

'fixed-address' declarations are distinct from the idea of reserved
dynamic allocations.  Because fixed-address setups do not carry a
lease through the resulting states, there is no hard drive access,
and consequently, no fsync-rate limit to response performance.

But this doesn't really answer your question either.

When I added reserved IP support, it was new, and I wasn't really sure
how it would be used (most people seem to prefer fixed-address
statements for aforementioned simplicity reasons, 'reserved' flagged
leases seemed at the time to be largely only useful to MS->ISC
conversions, and as such, probably done once at lease-database-
migration time).  So the only ways to reserve an IP address out of
a dynamic pool right now are to edit dhcpd.leases inbetween a restart,
to use OMAPI, or to try the "infinite-is-reserved" feature, which was
a kind of shot in the dark on my part in the area of not-migrating
reserved lease support.

None of those are really good options.  In retrospect, now that I've
seen how folks plan to use it, I think we could have supported a
situation where the server is directed to reserve leases for specific
host records (just add a 'reserve' boolean config value) or classes,
and this could at least be used for hosts that do not roam around the

This would probably take betwen 5-10 lines of code, if anyone feels
like contributing it.

In other news, we're looking at better database backends (and a mode
of operation that is not memory based), and better CLI tools for
runtime configuration management.

Ash bugud-gul durbatuluk agh burzum-ishi krimpatul.
Why settle for the lesser evil?
David W. Hankins	"If you don't do it right the first time,
Software Engineer		     you'll just have to do it again."
Internet Systems Consortium, Inc.		-- Jack T. Hankins

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