ISC DHCP server offers fixed IP addresses to ANY device!

David W. Hankins David_Hankins at
Mon Dec 29 18:44:14 UTC 2008

On Sun, Dec 28, 2008 at 02:31:40PM -0600, Foggi, Nicola wrote:
> 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 it should unset if you set the lease state to 'reset'.  The
reset state is supposed to completely wipe a lease, like a blank

We normally only hear from people when things are broken; it's good to
hear that something's working (fairly) for a change!

> 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.

Ah, the DISCOVER/OFFER should go through allright, but yeah if a
client is requesting another lease that is available, we'll give it.

The trouble is that REQUEST processing is nicely simple; it names a
single IPv4 address.  We can do a hash table lookup to find that
single address, do a quick "permission" check, and then pass/fail.  No
loops (well, except the permission check ACL's), just a pesky disk
access/fsync tick.

We don't know if the client has other leases, much less just reserved
ones, unless we search the by-chaddr or by-client-id hash tables,
which potentially need some loop iterations.  I think we can look at
doing that again now, at least gated by a config parameter.  My memory
is the leases weren't sorted in their respective buckets when we were
filling in the edges around reserved leases; so a search was
potentially very expensive, and we need to do two of them.  I think we
sorted those buckets now, if I recall correctly, so we can look at
doing something like this again.

In terms of reserved lease evolution for your use case, I think the
CLI work we've been thinking about would be the most helpful.  You
really should be able to use some tool to get and set the status of a
lease by IPv4 address ("rndc-alike status/set state/etc"),
and although OMAPI can fill in that gap if you're willing to read
enough text, there needs to be a simpler tool.

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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