Handing out duplicate IP addresses

Ben Wiechman ben at wisper-wireless.com
Thu Jan 15 18:21:35 CET 2009

Can't you do the select and update as part of one transaction? 


For example with MySQL:


SELECT @A:=SUM(salary) FROM table1 WHERE type=1;

UPDATE table2 SET summary=@A WHERE type=1;



The transaction may need to be changed to serializable as well. I don't know
how the DHCP RFC handles preallocations.


You could probably use Repeatable Read level if it is acceptable to mark an
address as taken when a DHCPOFFER is sent while waiting for a DHCPREQUEST
from the client, so long as the address if confirmed to be free before the
DHCPACK is sent, or a DHCPNAK in the case that the address was offered to
multiple clients. 

Ben Wiechman



From: freeradius-users-bounces+ben=wisper-wireless.com at lists.freeradius.org
[mailto:freeradius-users-bounces+ben=wisper-wireless.com at lists.freeradius.or
g] On Behalf Of Padam J Singh
Sent: Thursday, January 15, 2009 9:39 AM
To: FreeRadius users mailing list
Subject: Re: Handing out duplicate IP addresses


Hello Ivan,

Would adding a mutex around the select-update code in the sqlippool module
solve this issue? 


tnt at kalik.net wrote: 

The requests all came in at the same time, to the second (among others),
its like FR took 3 requests and looked at the database at the exact same
time, saw it was an available IP and all those 3 requests assigned it.

That can't be avoided. SELECT (allocate-find) will always work much
faster than UPDATE (allocate-update).

My NAS rejects two of the 3 because the IP is assigned,

I think that you make a good point here. If the allocate-update query was
made to fail in the case that the IP address was already issued to
another thread between allocate-find and allocate-update (by expanding
it with AND expiry_time IS NULL in WHERE), point of failure will be in
sqlippool module and not on the NAS. Logic can then perhaps try to issue
a new IP address (best just once more in order not to create a loop).
That way issuing same IP address to multiple threads can be handled by
the sqlippool module.
Ivan Kalik
Kalik Informatika ISP
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