NGINX proxy caching - cache buster variable in querystring - possible to ignore?

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http://serverfault.com – We have the following url we would like to proxy cache: file.php?parameter=one&paramater2=two&r=EPOCHTIMESTAMP Query string parameter "parameter" varies between requests. So does "paramater2". Query stringing parameter r is a timestamp we use to make sure the client doesn't serve cached (on the client side) content. Aka "cache buster". Yes we also use all the appropriate don't cache h headers. Now, we would like to proxy cache via nginx some of these requests. Is it possible to instruct nginx to ignore the r querystring param but use all the others when setting a cache key for the (HowTos)