DHCP Failover and dhcpd.leases

[Glenn Satchell]

> 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
copy.

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

I hope this helps.

regards,
-glenn

> Thanks in advance!
>
> Regards
> Malte
>