DHCP failover doesn't receive DHCP requests in secondary server

[greg.rabil at bt.com]

Please note that it depends on the type of DHCPREQUEST packets.  DHCPREQUESTs for clients in the RENEWING state will be unicast to the server that issued the lease, i.e. the primary DHCP server.  DHCPREQUESTs for clients in the REBINDING state will be broadcast, and therefore should be received by both primary and failover DHCP servers.  This is true for clients on the same LAN or separated by a DHCP relay.  In the case of a DHCP relay, it will see the broadcast packet and (should be configured to) forward to both primary and failover.  Unicast packets, of course, will not be relayed, but simply routed to the destination IP.  DHCPDISCOVER packets are always broadcast.

Regards,
Greg Rabil

-----Original Message-----
From: dhcp-users [mailto:dhcp-users-bounces at lists.isc.org] On Behalf Of Chris Buxton
Sent: Wednesday, August 15, 2018 12:00 PM
To: Users of ISC DHCP <dhcp-users at lists.isc.org>
Subject: Re: DHCP failover doesn't receive DHCP requests in secondary server

Both are active. Roughly 50% of clients will be answered by each, under normal circumstances.

Chris Buxton

> On Aug 15, 2018, at 8:58 AM, Roberto Carna <robertocarna36 at gmail.com> wrote:
> 
> Dear, thanks for your response...So I will review the DHCP agent as you said.
> 
> But now I have a question:
> 
> If both DHCP servers always receive DHCPREQUEST packets, does it mean 
> that in normal condition (primary and secondary server are running UP) 
> both DHCP servers responds these requests ??? Or only the primary 
> server responds the requests in despite of the secondary server is UP 
> too ???
> 
> Thanks a lot again, regards !!!
> 
> 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!!!
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>>> dhcp-users at lists.isc.org
>>> https://lists.isc.org/mailman/listinfo/dhcp-users
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