Nginx多个域名指到同一个网站

我的域名是 example.com
我想让 www.example.com 可以跳转到 example.com
我的nginx配置是

server {
       listen 80;
       server_name www.example.com example.com;
       root /var/www/web;

       if ($http_host == "www.example.com") {
                 rewrite ^ http://example.com$request_uri permanent;
       }

不行。。。

---

if ($host = 'www.example.com' ) {
    rewrite ^/(.*)$ http://example.com/$1 permanent;
}

---

if ($http_host = www.example.com) {
    rewrite  (.*)  http://example.com$1 permanent;
}

---

server {
  listen 80;
  server_name www.example.com;
  return 301 http://example.com/$request_uri;
}

server {
  listen 80;
  server_name example.com;
  root /var/www/web;
}