How to sync a Linux secondary DHCP server?

[Sandra Schlichting]

> > At https://www.isc.org/wp-content/uploads/2017/08/dhcp43.html#DHCP%20FAILOVER
> > 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,
> > 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.

Maybe they should add that that case is only when reservations are not
used. For reservations it shouldn't be an issue.

> 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.

Very nice. Both my DHCP servers would be right next to each other with
a direct fail over link.

> >> 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.

Can it be configured how many each serve?

E.g. if it were possible to have dhcp1 to serve 100% of the IP's, then
it would a classic master/slave setup. Or are there reason I wouldn't
want to do that?