问一个 nginx 反向代理的问题?

[jingwentian]

location / {

proxy_pass http://www.aaa.com/;
proxy_set_header Host $proxy_host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
}

被反代的域名: http://www.aaa.com
代理的域名: http://www.bbb.com

但是在访问www.bbb.com的时候 $_SERVER['SERVER_NAME'] 的值还是www.aaa.com,
请问怎么配置能实现$_SERVER['SERVER_NAME']的值是当前访问的域名呢?
谢谢