我有一个配置文件，如下所示：

server {

        listen       80;
        server_name  localhost;

        #charset utf-8;
        root   html/laravel/public;
        index  index.html index.php;

        #browse folders if no index file
        autoindex on;

        # enforce NO www
        if ($host ~* ^www\.(.*))
        {
            set $host_without_www $1;
            rewrite ^/(.*)$ $scheme://$host_without_www/$1 permanent;
        }

        # serve static files directly
        location ~* \.(jpg|jpeg|gif|css|png|js|ico|html)$ {
            access_log off;
            #expires max;
        }

        # removes trailing slashes (prevents SEO duplicate content issues)
        if (!-d $request_filename)
        {
            rewrite ^/(.+)/$ /$1 permanent;
        }

        # canonicalize codeigniter url end points
        # if your default controller is something other than "welcome" you should change the following
        # if ($request_uri ~* ^(/lobby(/index)?|/index(.php)?)/?$)
        # {
        #     rewrite ^(.*)$ / permanent;
        # }

        # removes trailing "index" from all controllers
        if ($request_uri ~* index/?$)
        {
            rewrite ^/(.*)/index/?$ /$1 permanent;
        }

        # unless the request is for a valid file (image, js, css, etc.), send to bootstrap
        if (!-e $request_filename)
        {
            rewrite ^/(.*)$ /index.php?/$1 last;
            break;
        }

        # redirect server error pages to the static page /50x.html
        #
        error_page   500 502 503 504  /50x.html;
        location = /50x.html {
            root   html;
        }

        location /backend/ {
            root /html/frontend;
        }

        location ~ \.php$ {
            include fastcgi.conf;
            fastcgi_pass 127.0.0.1:9000;
            fastcgi_index index.php;
            include fastcgi_params;
        }

        location ~ /\.ht {
            deny  all;
        }

        # catch all
        # error_page 404 /index.php;

        # location ~ \.php$ {
        # try_files $uri =404;
        #         fastcgi_pass  unix:/tmp/php.socket;
        #         fastcgi_index index.php;
        #         #include fastcgi_params;
        #         include /home/tamer/code/nginx/fastcgi_params;
        # }
        # access_log /home/tamer/code/laravel/storage/logs.access.log;
        # error_log  /home/tamer/code/laravel/storage/logs.error.log;
    }

我必须改变根文件夹为$host/backend/的任何网址与$host/backend/。所有规则的加载页面必须是相同的，只有根文件夹必须改变。
我该怎么做呢？