Maybe you can use the map module(http://wiki.nginx.org/HttpMapModule) in
front of the limit request module.

2011/3/23 j0nes2k <nginx-forum@nginx.us>

> Hello,
>
> I am trying to use the HTTPLimitRequests Module to prevent excessive
> usage of resources on our server. However we also have some spikes which
> seem to originate from search bots or other allowed usage. We want to
> keep these requests from known IP adresses untouched and limit all other
> requests.
>
> Is there a way to whitelist IPs using this module?
>
> Thank you for your answer!
>
> Best regards,
> Jonas Kaufmann
>
> Posted at Nginx Forum:
> http://forum.nginx.org/read.php?2,185127,185127#msg-185127
>
>
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