I'm rearranging our network and putting some DHCP failover since we have a lot of machines, and even servers, getting IP addresses with DHCP.

Today we have a nice DNS topology with one master server and two slaves. The DHCP gives the slaves address to the clients, and recursion is only permitted on the slave servers. Considering this I want to do something like with DHCP.

The main ideia is have one primary DHCP server and two secondaries. But here's come the question: I don't know, and I haven't find it on the documentation if its a supported feature and if there's a way to put the massive DHCP load only on the secondaries.

Here is the documentation about ISC DHCPd failover: https://kb.isc.org/article/AA-00502/0/A-Basic-Guide-to-Configuring-DHCP-Failover.html; but it doesn't cover my usage scenario.

Thanks in advance,

  • 1
    What do you mean with massive DHCP load? How many million queries do you handle per second? – ott-- Aug 17 '13 at 20:19
  • I mean all my queries. I just don't want the primary server handling this. As we've done with the BIND9 servers. – Vinícius Ferrão Aug 17 '13 at 21:17
  • What is "a lot"? Help us with some specifics. – ewwhite Aug 17 '13 at 22:28
  • It's not much. I just want failover with DHCP servers and isolation with the primary one... Theres no huge DHCP traffic. I think I've used bad words to describe the problem. But speaking about numbers we have 1000 hosts in the dhcpd.conf and the max lease time is 30 minutes. – Vinícius Ferrão Aug 17 '13 at 22:39

With DNS, there's a concept of recursive and authoritative queries, so what you're doing there - commonly called hidden master - makes sense. You have a the hidden master holding the data, the slaves copying it, and only enable recursion at the slaves.

With DHCP, there's no such concept; a DHCP server either does not reply, grants or refuses a lease. Whether it does so is based entirely on it's config and lease database, not on whether a DHCP request is "recursive" or not.

Specifically with ISC dhcpd, there's no concept of a "master". You can run two DHCP servers with the same config and failover, but you can't run a "master" server and two "slave" servers.

So - the feature you want doesn't exist. There's no such thing, so you can't do it.

If you don't want DHCP load on your hidden master, just don't run a DHCP server there.

If you want to run two servers and have one ready to take over from the other, you should run failover, but be aware it has caveats.

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  • Thank you. I'll keep the hidden master but will install the DHCP servers on the slave DNS servers. I think it will be ok. – Vinícius Ferrão Feb 24 '14 at 14:00
  • There is a MASTER. It's called primary and it's described in failover section in ISC DCHP server – ChewbaccaKL Dec 13 '19 at 13:27

Not sure if you still care, but I believe you can achieve what you want by adding a "dhcp relay" on the slaves and configure it to relay requests to the master.

Its a different solution to the "fail-over" you had in mind, but possible more analogous to that way you already have your DNS.

I've done this before in a large corporate - we put a small box on every LAN that did DHCP Relay + DNS Resolver and they sent all DHCP requests to one central server.

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  • Is your DHCP server capable of being failover itself? E.g. if the master they relay to went down can they still serve a DHCP response? – Adambean Jul 15 '19 at 14:30

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