DHCP Failover and dhcpd.leases

John Wobus jw354 at cornell.edu
Fri Apr 13 20:07:02 UTC 2012

> I’m currently using a single DHCP server from which I periodically  
> sync the dhcpd.leases file to a second backup DHCP where the deamon  
> is not started. That’s my cold standby failover.
> Now I want to use the ISC-DHCPs build-in failover system which I  
> already successfully configured and tested in a testing environment.
> My question is, what happens to the leases database when the server  
> is in failover mode?
> Are all leases written to the file on both server peers or only  
> those which were handed out by the failover peer? Clearly, do both  
> servers have the same leases file?
> I built some software around the leases database an so I rely on  
> having all active leases in it.
> Another question is, if I change my running config on the productive  
> server to do ISC failover, how does this affect already given out  
> leases? Are they simply synced to the secondary peer?

Each server sends info about the leases it grants
to the peer server so the peer server can add it to
its lease file.  This communication is done
asynchronously, i.e., though servers wait
for their own lease file update to finish
before sending the ACK, they do NOT make
the ACK wait until the info is on the peer
server's disk.

We periodically scan the leases file to tell
us about pool usage, basically to warn us
of full pools and to give us data to help
make adjustments. We scan only one lease
file, and just keep in mind that our
results aren't watertight, just very

John Wobus
Cornell IT

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