DHCP Failover and dhcpd.leases

Randall C Grimshaw rgrimsha at syr.edu
Fri Apr 13 13:17:01 UTC 2012

You should also be aware, [I believe this is still true], that leases to machines with static assignments are not kept in the leases file. For this reason you might consider tracking leases and pool states using syslog.

Randall Grimshaw rgrimsha at syr.edu
From: dhcp-users-bounces+rgrimsha=syr.edu at lists.isc.org [dhcp-users-bounces+rgrimsha=syr.edu at lists.isc.org] on behalf of Glenn Satchell [glenn.satchell at uniq.com.au]
Sent: Friday, April 13, 2012 9:01 AM
To: Users of ISC DHCP
Subject: Re: DHCP Failover and dhcpd.leases

> Hello!
> I'm currently using a single DHCP server from which I periodically sync
> the dhcpd.leases file to a second backup DHCP where the deamon is not
> started. That's my cold standby failover.
> Now I want to use the ISC-DHCPs build-in failover system which I already
> successfully configured and tested in a testing environment.
> My question is, what happens to the leases database when the server is in
> failover mode?
> Are all leases written to the file on both server peers or only those
> which were handed out by the failover peer? Clearly, do both servers have
> the same leases file?
> I built some software around the leases database an so I rely on having
> all active leases in it.

The lease files are not identical - new lease records are appended by the
daemon on each system. Leases issues by one server are sent to the other
server, but may not be written in exactly the same order. The net result,
however, is that the same leases should be in both files.

> Another question is, if I change my running config on the productive
> server to do ISC failover, how does this affect already given out leases?
> Are they simply synced to the secondary peer?

When the new secondary starts up, it requests a copy of all the leases on
the other server. so, yes, it will synchronise automatically.

The failover protocol is documented, there is a References file in the doc
subdirectory of the source distribution which points to
draft-ietf-dhc-failover-12.txt. You can google for this file to download a

The dhcpd.conf man page also describes the operational aspects of the
failover protocol in the sections titled DHCP FAILOVER and FAILOVER

I hope this helps.


> Thanks in advance!
> Regards
> Malte

dhcp-users mailing list
dhcp-users at lists.isc.org

More information about the dhcp-users mailing list