nginx-proxy

# map block must be outside the server{} block
map $host $productionurl {
    default       mainsite.com;

    ## uncomment for multisite
    # subsite.dev   subsite.com;
    # subsite2.dev  subsite2.com;
    # subsite3.dev  subsite3.com;
}

server {
	# . . .

	location ~* \.(png|jpg|jpeg|gif|ico|mp3|mov|tif|tiff|swf)$ {
		try_files $uri $uri/ @production;
	}
	location @production {
		resolver 8.8.8.8;
		proxy_pass      http://$productionurl/$uri;
	}

	# . . .
}