配置虚拟主机时，在配置文件中加入如下内容
- DocumentRoot /home/img
- ServerName img.host.com
- ErrorDocument 404 /
- RewriteEngine on
- RewriteCond %{HTTP_REFERER} !^$
- RewriteCond %{HTTP_REFERER} !^http://img.host.com/.*$ [NC]
- RewriteCond %{HTTP_REFERER} !^http://www.host.com/.*$ [NC]
- RewriteRule \.(gif|jpg|jpeg)$ http://img.host.com/forbidden.png [R,L]
