nginx selecting the backend by header

    upstream backend13 {
    # sticky - link backend by cookies
    # must build nginx with  --add-module=/usr/local/src/nginx-goodies-nginx-sticky-module-ng
    sticky;
    server 127.0.0.1:6001;
    server 127.0.0.1:6002;
    server 127.0.0.1:6003;
    }
    upstream backend47 {
    sticky;
    server 127.0.0.1:6004;
    server 127.0.0.1:6005;
    server 127.0.0.1:6006;
    server 127.0.0.1:6007;
    }
    map $http_x_backend $backend {
    default backend13;
    13      backend13;
    47      backend47;
    }

        location / {
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_set_header Host $host;
            proxy_pass http://$backend;
        }

    set_real_ip_from  127.0.0.1/32;
    real_ip_header    X-Forwarded-For;

curl -c curl_cookie -b curl_cookie -sS  --header "X-Backend: 13" http://192.168.0.65/