Switch from Single DHCP to Master-Slave DHCP setup

Gregory Sloop gregs at sloop.net
Tue Jul 30 00:14:32 UTC 2013

Thanks Steven amd Steinar...

Steven, you gave me the info about the split in an earlier thread - so
I did know that already - but you should get credit for it. Thanks! :)

The other stuff I know from the docs - but thanks to you both for
re-highlighting that.

One final issue I'll note, as a pointer for anyone else stumbling on
this thread...

Time synchronization is *very* important too. Use NTP to make sure the
master and slave are in very close sync time-wise. [I've heard greater
than 60 seconds time skew between the two will cause huge issues. I'm
not sure if that's the exact right value where problems start, but the
gist is the same: Keep them time sync'd.]

Two follow-ups.

What steps would one take when you need to take one of the DHCP
servers off-line for a while. [Hours to more than days?]

[Looks like use omshell, and...]

server someserver.fqdn
key keyname secretkey
new failover-state
set name downservername
local-state = 4

Does that look right?

[Provided you have the proper keys etc.]

...And if so, how would one query for the current state?
[Use "open" instead of "local-state = 4"?

And related:
What steps for having one of the DHCP servers fail and get replaced
from a "from-scratch" new server. [Assume no backup of old
server/lease-file etc.] 

...From related discussions - it appears you can just (re-)configure a new
peer and bring it up - and they'll sync up fine. [Ignore complications
around DNS/DDNS/Master/Slave etc.] 

Is this right?



>> One thing you can't do is do it step by step, when enabling a failover
>> association between two DHCP servers it has to be done on both systems at
>> the same time (otherwise neither system will issue leases as neither will
>> have been able to reconcile/balance the lease pools).
>> Make sure your network IP helper configuration is pointing at both DHCP
>> servers and for DDNS make sure both DHCP servers are able and allowed to
>> update DNS on the master.

snn> Also, just to have mentioned this (it should be pretty obvious): Make
snn> sure that the servers in a failover pair run the same (or at least
snn> *compatible*) versions of dhcpd. Specifically, 3.0.x is *incompatible*
snn> with anything recent.

snn> Steinar Haug, Nethelp consulting, sthaug at nethelp.no

Gregory Sloop, Principal: Sloop Network & Computer Consulting
Voice: 503.251.0452 x82
EMail: gregs at sloop.net

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