DHCP pair messed up, second one only running cant get primary up.

Rob Morin rmorin at datavalet.com
Mon Jan 16 19:58:33 UTC 2017

Hey all, so my re-pairing went fine that I did early Saturday morning.


What I did was;


Stop dhcpd on both servers

Deleted the dhcpd.leases file which sits on a 4 gig ramdisk on both servers

Then I rebooted the primary server, I did a reboot rather than start dhcpd, as I had kernel updates to do, so I figure kill 2 birds with one stone!

Before dhcpd starts, in the init.d script, I do a “touch /ramdisk/dhcpd.leases “

Then dhcpd starts up fresh, all worked fine on primary, it started giving leases out after about 4 minutes after it started up.


Then after the MCLT time, I did the same thing to the secondary, and it came up flawlessly and both started to give out leases in a load balancing way just fine.


BTW I do copy /ramdisk/dhcpd.leases file to disk every 5 mins to be safe via cron.


Other than this reboot it ran for almost a year with zero issues.


Thanks for all the comments.

Have a great day!



Rob Morin

Gestionnaire des systèmes | Senior Systems Administrator

Tel: 514 385-4448 #174                         


5275, chemin Queen-Mary, Montréal (Québec) H3W 1Y3 Canada

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From: dhcp-users [mailto:dhcp-users-bounces at lists.isc.org] On Behalf Of Bob Harold
Sent: January 16, 2017 2:49 PM
To: Users of ISC DHCP <dhcp-users at lists.isc.org>
Subject: Re: DHCP pair messed up, second one only running cant get primary up.



On Mon, Jan 16, 2017 at 12:14 PM, Drew Derbyshire <swhobbit at derbyshire.us <mailto:swhobbit at derbyshire.us> > wrote:

As an aside ...

The thought of putting recovery state/log files (i.e. the DHCP leases file and its backup) on a RAM disk to make top look pretty leaves me dismayed. That leaves zero local protection against a system crash (or misguided deliberate reboot).

And yes, the server has been running 346 days -- Doesn't matter. Services can be five nines reliable, but hardware won't be.

An SSD, or if you prefer multiple SSD units configured as a raid, will give you the same order of performance without the premortem fodder.

The machine should be audited for other critical files which are written to volatile storage, and moved to the SSD or other storage as well.




I agree that putting state in ram is a concern, but with a failover pair, there should be a duplicate copy on the other server of all but the very latest changes.  There is a risk when one server is rebooting, so having a copy of the backup lease file on real disk would help.

I think SSD (particularly write speed) is still orders of magnitude slower than ram.


That said, I would not ever want to have both servers rebooted at the same time, or even restart DHCP at the same time.  Restart one, allow it to sync data and get to normal-normal, then restart the other.



Bob Harold



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