dhcp-users Digest, Vol 88, Issue 31

David Li dlipubkey at gmail.com
Sat Feb 27 23:17:21 UTC 2016

I am also very interested in this topic. My first question about
"reserved lease" is how it takes care of the client that is gone. Will
the DHCP server be able to notify DNS and remove its record?

It's confusing from the man pages saying that:

"infinite-is-reserved flag;

ISC DHCP now supports 'reserved' leases. See the section on RESERVED
LEASES below. If this flag is on, the server will automatically
reserve leases allocated to clients which requested an infinite
(0xffffffff) lease-time."

Does this mean this is actually a "forever" lease?


> Date: Fri, 26 Feb 2016 11:10:09 +1100
> From: "Glenn Satchell" <glenn.satchell at uniq.com.au>
> To: "Users of ISC DHCP" <dhcp-users at lists.isc.org>
> Subject: Re: Static IP and IP management
> Message-ID:
>         <6a1896dfd6a9b395ae36125816bb3335.squirrel at mail.uniq.com.au>
> Content-Type: text/plain;charset=iso-8859-1
> Hi Bernard,
> ISC dhcpd ships with a very comprehensive set of man pages, so I would
> suggest perusing these:
> dhcpd.conf, dhcpd, dhcp-options, dhcp-eval
> In this case the reserved leases are mentioned in the dhcpd.conf man page.
> I've included the section below in the hope that google will find it for
> future searches.
> I would also suggest that you can approximate a reserved lease by setting
> the lease time to be very long, say 1 year (31536000 seconds).
> regards,
> -glenn
> man dhcpd.conf
> ...
>      It's often useful to allocate a single address to  a  single
>      client,  in  approximate  perpetuity.   Host statements with
>      fixed-address clauses exist to a  certain  extent  to  serve
>      this  purpose,  but  because host statements are intended to
>      approximate 'static configuration',  they  suffer  from  not
>      being referenced in a littany of other Server Services, such
>      as dynamic DNS, failover, 'on events' and so forth.
>      If a standard dynamic lease, as from any range statement, is
>      marked  'reserved',  then the server will only allocate this
>      lease to the client it is identified by (be that  by  client
>      identifier or hardware address).
>      In practice, this means that the lease  follows  the  normal
>      state  engine,  enters ACTIVE state when the client is bound
>      to it, expires, or is released, and any events  or  services
>      that would normally be supplied during these events are pro-
>      cessed normally, as with any other dynamic lease.  The  only
>      difference is that failover servers treat reserved leases as
>      special when they enter the FREE or  BACKUP  states  -  each
>      server applies the lease into the state it may allocate from
>      - and the leases are not placed on the queue for  allocation
>      to  other  clients.   Instead  they  may  only be 'found' by
>      client identity.  The result  is  that  the  lease  is  only
>      offered to the returning client.
>      Care should probably be taken to ensure that the client only
>      has  one  lease  within a given subnet that it is identified
>      by.
>      Leases may  be  set  'reserved'  either  through  OMAPI,  or
>      through  the 'infinite-is-reserved' configuration option (if
>      this is  applicable  to  your  environment  and  mixture  of
>      clients).
>      It should also be noted that leases  marked  'reserved'  are
>      effectively treated the same as leases marked 'bootp'.
> On Fri, February 26, 2016 2:26 am, Bernard Fay wrote:
>> I have to find out about this "reserved" thing. I don't understand it.
>> Google, please, help me!
>> On Wed, Feb 24, 2016 at 1:44 PM, Chuck Anderson <cra at wpi.edu> wrote:
>>> On Wed, Feb 24, 2016 at 01:17:13PM -0500, Bernard Fay wrote:
>>> > 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.
>>> You can do that, but you have to use "reserved" leases rather than
>>> fixed-address statements.  ISC dhcpd doesn't track the expiry of
>>> fixed-address leases.
>>> _______________________________________________
>>> dhcp-users mailing list
>>> dhcp-users at lists.isc.org
>>> https://lists.isc.org/mailman/listinfo/dhcp-users
>> _______________________________________________
>> dhcp-users mailing list
>> dhcp-users at lists.isc.org
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