Frustrated DHCP failover not working.. :(

Rob Morin rmorin at
Wed Feb 10 17:48:22 UTC 2016

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...



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
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