upstream openstreetmap {
        least_conn;
        server a.tile.openstreetmap.org;
        server b.tile.openstreetmap.org;
        server c.tile.openstreetmap.org;
}

server {

        location /tile/ {
                rewrite ^/tile/([0-9]+)/([0-9]+)/([0-9]+) /$1/$2/$3.png break;
                #proxy_pass http://openstreetmap;
                proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
                proxy_set_header X_FORWARDED_PROTO http;
                proxy_set_header Host $http_host;
                proxy_cache openstreetmap-backend-cache;
                proxy_cache_valid  200 302  365d;
                proxy_cache_valid  404      1m;
                proxy_redirect off;
#                if (!-f $request_filename) {
                        proxy_pass http://openstreetmap;
 #                       break;
#                }
        }