Nginx+Lua+Redis で動的upstream

nginx.conf

worker_processes  1;
error_log  /dev/stderr debug;

events {
    worker_connections  256;
}

http {

    server {
        listen       8888;
        server_name  localhost;

        location / {
            set $upstream "";
            rewrite_by_lua '
               local res = ngx.location.capture("/redis")
               if res.status == ngx.HTTP_OK then
                  ngx.var.upstream       = res.body
               else
                  ngx.exit(ngx.HTTP_FORBIDDEN)
               end
            ';
            proxy_pass http://$upstream;
        }

        location /redis {
             internal;
             set            $redis_key $host;
             redis_pass     127.0.0.1:6379;
             default_type   text/html;
        }
   }
}