combining lease files

Bob Harold rharolde at
Wed Aug 29 20:36:48 UTC 2018

On Wed, Aug 29, 2018 at 10:41 AM project722 <project722 at> wrote:

> Thanks. So would this syntax be correct on the secondary?
> failover peer "dhcp-failover" {
>         secondary; # declare this to be the secondary server
>         address <server IP address>;
>         port 647;
>         peer address <A-master>;
>         peer address <B-master>;
>         peer port 647;
>         max-response-delay 30;
>         max-unacked-updates 10;
>         load balance max seconds 5;
> }

I don't configure by hand any more, we bought IPAM software, but looking at
what it produces, I see:
On mine the peer port is different than the other port (647 vs 847), and
reversed on the other end.

But more importantly, you need a whole separate "failover peer" section for
each pairing, with a different name:

failover peer "pairA" {

failover peer "pairB" {

Then in each pool use the correct failover peer name:

subnet netmask
    option subnet-mask;
    option broadcast-address;
    ddns-rev-domainname "";
    ddns-updates false;
    option netbios-node-type 8;
    option netbios-name-servers,;
    option routers;
        failover peer "pairB";

Bob Harold

> On Wed, Aug 29, 2018 at 9:35 AM Bob Harold <rharolde at> wrote:
>> On Wed, Aug 29, 2018 at 10:14 AM project722 <project722 at> wrote:
>>> Thanks for the feedback guys!
>>> So, in this scenario I'll have 2 masters temporarily - B will become a
>>> new master and sync to A-failover, and A-master will still be in the
>>> picture as well? If I try this approach, then, I will have to add the
>>> subnets for server B to A-failover, then enable failover between B and
>>> A-failover? Wouldn't A-master have a problem with A-failover having subnets
>>> in its conf file that A-master doesn't?
>>> Also, I'm starting to think aboout ip helper addresses. Seems like the
>>> flow would be:
>>> Add an IP helper address on the router for server B's subnets to also
>>> point to A-failover
>>> Once they are completely in sync then remove the IP helper for server B
>>> altogether and add another for A-master
>>> Does that sound about right?
>> That is correct.  A DHCP server can be master for some subnets with
>> failover 1, and others with failover 2, and still others with no failover.
>> A DHCP failover server can be failover for some subnets with master 3,
>> others with master 4, and other subnets that are not using failover.  The
>> only limitation is that I don't think a server can be master for some
>> subnets and failover for others.
>> p.s. Here at U of Michigan we currently have 8 pairs, and have some
>> experience moving subnets around  :)  The trick is to verify that the dhcp
>> forwarding on the routers is really working and that there is no firewall
>> rule or acl somewhere blocking it.
>> --
>> Bob Harold
>>> On Wed, Aug 29, 2018 at 8:36 AM Bob Harold <rharolde at> wrote:
>>>> On Wed, Aug 29, 2018 at 9:33 AM Sten Carlsen <stenc at>
>>>> wrote:
>>>>> On 29/08/2018 14.28, project722 wrote:
>>>>> Hey All,
>>>>> We are consolidating all subnets on server B and moving to an exisitng
>>>>> failover pair we have, In order to decom server B. I'll need to take the
>>>>> leases file from server B and combine whats there with the leases file on
>>>>> both servers in the failover pair. (doing this to make the failover pair
>>>>> aware of what leases are already out there that were assigned by server B)
>>>>> I am not sure, but I think one other way could be to make server B
>>>>> part of a failover pair with one of the existing failover servers for a
>>>>> period of time. That way the remaining server will have all the information
>>>>> transferred from server B. Later that configuration could be changed to
>>>>> have the other remaining failover server act as the peer, that way the
>>>>> servers would transfer the data, probably with less risk.
>>>>> Others should confirm or reject this idea.
>>>>> I'm thinking just a simple cat on both files should be fine to combine
>>>>> the data. And, all 3 servers are running the same verison of dhcp
>>>>> ( and RHEL so I don't expect and formatting problems.
>>>>> However, since failover pairs have more transitional states with the
>>>>> "binding state" variable, is it possible I will run into any issues doing
>>>>> this? Is there a better, more preferred way of doing this instead of
>>>>> merging the leases file?
>>>> The benefit of a failover pair is that we can upgrade/repair/replace
>>>> one server at a time without any interruption in service.
>>>> Let's call the failover pair A-master and A-failover.
>>>> The recommended method is to configure server B as master and the
>>>> A-failover server as failover for server B's subnets also.  Give it some
>>>> time (watch the failover messages in the logs) to sync the data from B to
>>>> A-failover.  Then move the subnets from B to A-master, and A-master will
>>>> sync the data from A-failover.
>>>> --
>>>> Bob Harold
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