How to sync a Linux secondary DHCP server?

Thomas Markwalder tmark at
Tue Jul 31 13:14:05 UTC 2018

Hello all:

Firstly Simon, thank you for providing such a detailed response. It has 
been my experience that you are a prompt, able, and patient presence on 
our users list, and, all of us here at ISC appreciate your contributions 
to the community.

One point of clarification,  Host reservations are not synchronized 
between servers via fail over.  Any host reservations you have in one 
configuration file,  you must include in the peer server's configuration 
file.  For dynamic reservations (i.e. those that do not include a fixed 
address), the leases allocated for each host will be synchronized 
(assuming they come from within a FO pool), but not the actual 
reservation specifications.


Thomas Markwalder
ISC Software Engineering

PS (There are quite a few regular responders on this list, and ALL of 
you are appreciated immensely. I'll be making an effort to make sure 
each of you are recognized accordingly)

On 07/31/2018 05:30 AM, Simon Hobson wrote:
> Sandra Schlichting <littlesandra88 at> wrote:
>> At
>> they mention
>> "
>> It is possible to get into a dangerous situation: if you put one
>> server into the PARTNER-DOWN state, and then *that* server goes down,ss
>> and the other server comes back up, the other server will not know
>> that the first server was in the PARTNER-DOWN state, and may issue
>> addresses previously issued by the other server to different clients,
>> resulting in IP address conflicts. Before putting a server into
>> PARTNER-DOWN state, therefore, make sure that the other server will
>> not restart automatically.
>> "
> There's really no way around that corner case.
>> Are there other cases that an admin should be aware of?
> It is possible to have two servers that lose connectivity between themselves but can both still communicate with some or all of the clients. This could be down to a routing issue, or because a link that carries the failover traffic has failed but that link isn't used for client traffic. So the default is not to automatically go into partner down state upon loss of communications - but there is an option to do that if you are confident that this problem doesn't apply (eg both servers are on the same network segment).
> If a server does fail, you do have some time to deal with it before problems start, so many just rely on system monitoring to alert them and put the surviving partner into partner-down mode.
>>> By default, both servers are active, so it's not really master & slave, and they will automatically rebalance things so that both have roughly half the free addresses in each pool.
>> Is it possible/likely that one server gets broken/corrupt in a way
>> that it doesn't trigger a fail over? This is the worst case I can
>> think of =)
> I suppose anything is possible !
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