Frustrated DHCP failover not working.. :(

Attila Szalay mochrul at
Wed Feb 10 19:37:42 UTC 2016

In the master node, you have this configuration option: split 128;

This means, that half of the ip addresses are linked to the master and the
secondary only own the other half.

Of course time-to-time they rebalance the ip pools, but (if I remember
correctly) it only happens after a timeout. So if the clients only reach
one of the hosts, it is possible, that those host (temporally) run out of

This is because in normal operation (and with split 128, which means
50-50%) they run more likely as a load-balancing scenario than
master-slave. (Also they split the mac address word to half too and the
slave answer to one half of the macs and the master to the other))

On Wed, 10 Feb 2016 at 18:48 Rob Morin <rmorin at> wrote:

> Hey  Niall, thanks fro your prompt reply... :)
> Your suggestion is interesting, however currently our controllers only
> point to dhcp-2 server and not to both, so making dhcp-1 primary before
> we modify the controllers to go to both machines, would not work.
> Pretty much what i was trying to do is to keep dhcp-2 server secondary,
> and sync it up with dhcp-1, even though dhcp-1 is not giving out any
> leases, because no traffic goes to it currently, as all traffic goes to
> dhcp-2 only.
> Downtime must be kept to a min as we normally have 10'000s of request at
> any given time to these dhcp servers. Thats why i wanted to keep djcp-2
> secondary and dhcp-1 primary as no requests go to dhcp-1 now, once
> dhcp-1 looks like its communicating with dhcp-2 ok, then we would direct
> some controllers to send requests to both servers rather than just the
> one, dhcp-2
> I am still thinking that maybe what i did last night actually worked,
> and maybe i just did not wait long enough fro dhcp-1 to come out of
> recovery=wait because the mclt time was 30 mins.
> I was also thinking to simply shut down dhcpd on both servers, delete
> the dhcpd.leases files on both servers, then start up dhcp-2, then
> dhcp-1 and hopefully they would be in sync faster? Or even maybe
> decrease the mclt time from 30 mins to 5 mins? not sure what that would
> do? Our leases times are 20 minutes. and at 3AM est, not many people
> should care if they loose their IP for 20 mins, i hope...
> :)
> Comments/suggestions?
> Rob Morin
> Montreal, Canada
> On 2016-02-10 12:27 PM, Niall O'Reilly wrote:
> > On 10 Feb 2016, at 16:36, Rob Morin wrote:
> >
> >> Currently dhcp-1 daemon is not running, dhcp-2 is running and giving
> >> out leases just fine, but in "stand alone" mode, meaning failover is
> >> not configured.
> >
> >   At least that's good!
> >
> >> What i need to do is re-config dhcp-2 to be the secondary in a
> >> failover mode,
> >
> >   How important is it that dhcp-2 be the secondary, and not the primary?
> >   [Pas besoin de me répondre: c'est un point de réflexion]
> >
> >> which would be just commentating the line in the dhcpd.conf file that
> >> tells it that ir is the secondary, and then add dhcp-1 to the mix.
> >
> >   It might be simpler to introduce dhcp-1 as secondary.
> >
> >   If it's not acceptable to keep the servers in this "reversed"
> > configuration,
> >   I'ld suggest the sequence outlines below.  It's tedious, but avoids
> > the solo/secondary
> >   transition which (a) scares me and (b) seems to be giving you trouble.
> >
> >   I've done this kind of juggling, but long enough ago that I don't
> > remember enough
> >   detail to give you a tested worked example.  Besides, I don't work
> > there any more
> >   and so no longer have access to the boxes.
> >
> >   Initial condition: dhcp-2 in "solo" (less typing than "stand
> > alone"!) mode
> >   Ensure dhcp-1 is running the same version of the server code as dhcp-2
> >   Prepare dhcp-2 to be primary and dhcp-1 to be secondary
> >   Enable failover and wait until stable operating conditions are
> > established
> >   Note: this failover phase is simply a preparation for migrating to
> > "solo" mode on dhcp-1
> >   Disable dhcp-2 and set dhcp-1 in "partner-down" mode
> >   Prepare dhcp-1 for "solo" operation and then restart it
> >
> >   You should at this stage have the same situation as now, but with
> > dhcp-1 in "solo" mode.
> >
> >   Finally, prepare the target failover configuration (with dhcp-1 as
> > primary) on each node
> >   and activate when ready.
> >
> >   I'm not entirely sure that it will, but I hope this helps.
> >
> >   Best regards,
> >   Niall O'Reilly
> > _______________________________________________
> > dhcp-users mailing list
> > dhcp-users at
> >
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