DHCP Failover and dhcpd.leases

Glenn Satchell glenn.satchell at uniq.com.au
Mon Apr 16 14:23:16 UTC 2012

On 04/16/12 16:29, "Küppers, Malte" wrote:
> Hi there and thanks a lot for your replies!
> There are just some questions left:
> -I guess thats ok then, so I can safely enable failover mode without
> losing leases?!


> -How long does it take until the leases files are synchronized and equal
> on both servers? Until next update is pushed ?

As quickly as the leases can be copied over the network. FOr a small 
lease file it could be a few 10s of seconds, a large (few MB) might take 
a couple of minutes.

> -Will I have to start the second server with a copy of the leases file?
> Or does a blank file work fine? Because while testing, I ran into the
> problem that if I wiped the leases files on both sides the
> both server got stuck in some “waiting for update” state an neither
> primary nor secondary was handing out leases anymore. How can I wipe out
> the files safely if needed?

No. Start existing server with existing lease file. Start new server 
with empty lease file. New server will request a lease update from the 
other server.

This is covered in the section titled FAILOVER STARTUP in the dhcpd.conf 
man page.

If you wipe the lease files on both sides then both servers go into a 
state where they will wait before issuing new leases. Generally this is 
a bad thing to do, so you shouldn't wipe both lease files in production. 
Even though the servers have no concept of any leases after wiping the 
lease files, all the clients *do* have leases, but the servers are now 
out of synch with the clients, so the servers wait for the current 
leases to expire before handing out new leases. If the two systems get 
out of synch with each other (network error where they can't communicate 
for example) you can pick one system with the "best" lease file, and 
wipe out the other one, but really it's best to avoid that and let the 
systems heal themselves.

It's best to arrange for syslog from your dhcp servers to go to the same 
host so you can see messages from both together.

The other thing you'll see which may seem odd is initially a new client 
will get a lease that is half of the MCLT setting. Next time it renews 
it will get the full lease time.


> Thanks in advance
> Regards
> Malte
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> dhcp-users at lists.isc.org
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Glenn Satchell                            |  Miss 9: What do you
Uniq Advances Pty Ltd, Sydney Australia   |  do at work Dad?
mailto:glenn.satchell at uniq.com.au         |  Miss 6: He just
http://www.uniq.com.au tel:0409-458-580   |  types random stuff.

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