DHCP failover doesn't receive DHCP requests in secondary server

Gregory Sloop gregs at sloop.net
Wed Aug 15 16:15:45 UTC 2018

Top posting:

I think the term "fail over" causes some confusion.
DHCPd in "fail-over" isn't what I'd typically think of, when I think "fail over."

When I think fail-over, I think of a server that essentially does no active "work" while the primary is up and functioning - other than *getting ready* to handle the load of the primary when it goes down.

Yet DHCPd fail-over is really *load-balancing* [at least in "normal" setups] with a bunch of communication between the two to handle the failure/unavailability of *either* server.

Thus, in regular practice, both servers are seeing requests and answering them, and [again in "normal" setups] will have approximately half of the leases distributed to each server.


RC> Dear, thanks for your response...So I will review the DHCP agent as you said.

RC> But now I have a question:

RC> If both DHCP servers always receive DHCPREQUEST packets, does it mean
RC> that in normal condition (primary and secondary server are running UP)
RC> both DHCP servers responds these requests ??? Or only the primary
RC> server responds the requests in despite of the secondary server is UP
RC> too ???

RC> Thanks a lot again, regards !!!

RC> 2018-08-15 12:38 GMT-03:00 perl-list <perl-list at network1.net>:
>> The problem isn't with the secondary server.  The DHCP packets must always go to both servers.  If the packets are relayed from a relay agent, then the relay agent must be configured to send to both of the DHCP servers (On a Cisco router, that would involve having ip helper-address listed twice, for example).

>> If no relay, then the DHCP servers must both be on the same LAN with the clients.

>> When both of the primary and secondary servers are running, you should see logs of the DHCP packets in both servers logs.

>> ----- Original Message -----
>>> From: "Roberto Carna" <robertocarna36 at gmail.com>
>>> To: "Users of ISC DHCP" <dhcp-users at lists.isc.org>
>>> Sent: Wednesday, August 15, 2018 11:06:08 AM
>>> Subject: DHCP failover doesn't receive DHCP requests in secondary server

>>> Dear people,

>>> I´ve implemented a failover scheme with ISC-DHCP-SERVER in two
>>> different servers, with the same version of the package and OS.

>>> Everything seems to be OK, but when a execute "service isc-dhcp-server
>>> stop" in the primary server, in the secondary server I can see:

>>> Aug 15 11:52:01 dhcp2 dhcpd[5839]: peer dhcp-failover: disconnected
>>> Aug 15 11:52:01 dhcp2 dhcpd[5839]: failover peer dhcp-failover: I move
>>> from normal to communications-interrupted

>>> what is correct, but after that the secondary server doesnt' receive
>>> any DHCPREQUEST packet from clients.

>>> So, my faoilover scheme doesn't work, because if primary server goes
>>> down, the secondary server doesn't receive any IP request from
>>> clients. When I start the isc-dhcp-server on primary server, at this
>>> moment I can see the DHCPREQUEST from clients to primary server.

>>> What can be the problem with the secondary server please ???

>>> Thanks a lot!!!
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.isc.org/pipermail/dhcp-users/attachments/20180815/3822b4d2/attachment.html>

More information about the dhcp-users mailing list