DHCP peer failure and pool exhaustion...

Shawn Routhier sar at isc.org
Mon Sep 9 20:30:19 UTC 2013

As Greg.Rabil mentions you might want to look at the auto-partner
down option in 4.2.  You also should be pretty careful with it.

In 4.2 we also added a "rewind"  feature.  This has the effect of
increasing the length of time a secondary can be out of contact
with the primary before the leases are exhausted.  


On Sep 9, 2013, at 10:50 AM, Gregory Sloop wrote:

> I just had a case like the following and I'm trying to understand it
> and resolve it long-term.
> I think I know what happened, but want to see if it makes sense.
> ---
> Two DHCP servers, peer'd.
> [4.1-R4 under Ubuntu 12.04 x64 if it matters]
> The relevant pool is quite heavily utilized. [Only a few free
> addresses in the whole pool of, say 150 when at full load, which
> happens often.] 
> In this case, one of the peers went down and we didn't realize it.
> [I understand this is a primary problem, but I'm using peer/fail-over
> to help resolve a situation where a DHCP server fails and I can't fix
> the down box, or am unavailable etc.]  
> Monday AM, the peer eventually gets to this point:
> "...peer holds all free leases" and stations can't get an IP address.
> So, I think what's happened is:
> 1) That the peer is "communications interrupted" [not peer down -
> which I think would "fix" it.]
> 2) The peer went down during a "low" use period and many of the
> "normal" client had NO assigned address when the peer went down. 
> 3) Eventually with only half the large [at the time] available pool of
> addresses, and when we hit a high use period Monday AM, [which would
> deplete the *whole* pool to nearly zero free addresses] the remaining
> peer had no free addresses.
> 4) It couldn't re-balance because the peer was down, and it wasn't set
> in partner down state.
> [The "up" peer had, say 60 addresses (half the pool) at low use, when
> the other peer went down. At high use, it exhausted it's 60 addresses
> in the pool, but couldn't re-balance to get the other 60, and we ran
> out of addresses with 60 still free, but tied up in the down peer.]
> Having the peer come back up will fix things, or having the down peer
> set in partner down state would also fix things as the "up" peer could
> grab all the leases.  
> ---
> So, does that sound right?
> If so, is there any way to "automagically" put the peer in partner
> down state if it's not able to be contacted in X amount of time? 
> ...
> Or, perhaps I should simply say - I don't want the DHCP server to end
> up without available addresses to lease, if the peer goes off-line and
> I'm not able to do some manual process to intervene. That's my goal.
> With that goal in mind, what's the best way to accomplish it?
> -Greg
> -- 
> Gregory Sloop, Principal: Sloop Network & Computer Consulting
> 503.251.0452 x121 Voice | 503.251.0452 Fax
> www.sloop.net
> mailto:gregs at sloop.net
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