Option 50 in failover mode

[Glenn Satchell]

Hi Bob

I think there are two different ideas of "hands off" failover. It's true
that the failover protocol has been around for a long time, and I recall
running it myself in the early 2000s. But while that protocol
automatically handled failure of one system it was not entirely hands off.
The surviving dhcp server would only allocate new IPs from it's share of
the pool. If the pool didn't have enough spare IPs to handle all the
clients then eventually it would run out of addresses to allocate. Back in
those days it was a design feature that the surviving server had to be
manually put into "partner down" mode. In partner down mode the surviving
server could allocate IP addresses from the entire pool and thus run
without running out of addresses.

In one of the recent versions was a feature for automatic partner down
after a defined period of time. In this case the surviving server switches
to partner down and can run without running out of addresses. This is what
others have termed "hands off".

regards,
-glenn

> sthaug at nethelp.no wrote:
>> > I feel confident saying that most people setting up two dhcpd servers
>> > in a failover configuration will want it to have hands off handling of
>> > failures.
>>
>> I don't feel confident about that at all.
>>
>> Given that ISC DHCP, until a very recent version, only supported
>> "hands on" failover, my guess is that most failover configurations
>> today are "hands on", i.e. no automatic setting of a failed server
>> in "partner-down" state.
>
> I have personally been using "hands off" failover mode since 3.1.1
> from 2008.  It has been three years and it has been working very well
> for me.  The failover protocol was introduced in isc dhcpd version 3.0
> way back in 2000 sometime but since I didn't use it then I don't know
> how stable it was during that period.  I could believe it wasn't
> usable then.  But if you search the net you will find many howto
> documents such as this one http://www.madboa.com/geek/dhcp-failover/
> from 2005 where apparently at least some people were successfully
> using failover six years ago.
>
> I don't consider at least six years of working failover in the isc
> dhcpd very recent at all.  And I have had years of stability from it.
>
> Bob
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