How to sync a Linux secondary DHCP server?

perl-list perl-list at
Wed Aug 1 14:42:49 UTC 2018

When you bring the failover peer back into the loop after a partner down, it will take awhile before the returning peer starts to respond to DHCP packets but that doesn't stop the existing peer that never left from still answering. 

Basically, there wouldn't be an outage but it would be some period of time (MCLT) until the returning peer begins to answer. 

Don't set MCLT low, however, set it to at least 3600 as if the primary goes away, the secondary will only issue new leases and renew existing ones for the length of MCLT. I usually set MCLT to the length of the lease i have set (usually I do 8 hours). 

> From: "Sandra Schlichting" <littlesandra88 at>
> To: "Users of ISC DHCP" <dhcp-users at>
> Sent: Wednesday, August 1, 2018 10:33:47 AM
> Subject: Re: How to sync a Linux secondary DHCP server?

> > > So if I made sure that each dhcp server served dynamic leases in
> > > different ranges, so no overlap, would it then be a good setup?

> > Yes you could do that. But I can think of a few problems:

> > If one server was faster it could have more of the clients getting
> > addresses from it and fewer from the second server. There is no load
> > balancing or sharing.

> That would be acceptable in my setup.

> > If one server is shutdown or fails, then the clients would eventually
> > expire their leases, and would get a different address frm the other
> > server. So persistent connections, such as ssh, web page logns, etc, would
> > disconenct or need to re-authenticate.

> Hmm. Ok, that is not optimal.

> > While failover might look difficult, it is actually quite simple to set up
> > and works very reliably. The failover code has been around for a really
> > long time, early 2000s, so all the kinks have been well and truly worked
> > out.

> Based on Paul's reply, I got the impression that the failover were
> risky, when he wrote:

> "
> DHCP failover is a very fragile protocol under failure. It works
> amazingly well under normal circumstances but if you ever get to the
> point where you're thinking about doing partner-down, be prepared to
> have resync take hours or lose leases.
> "

> But that is not your experience, I can guess?
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> dhcp-users at
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