Debian Apache2 启动报错 ServerName

Debian安装完Apache2后重启Apache2报错：
apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.0.1 for ServerName

几乎所有Ubuntu/Debian Apache2的文章之中都说是要修改httpd.conf，加入ServerName，而实际上在Debian中httpd.conf是空的，ServerName需要加在ports.conf之中才会起作用。

echo "ServerName ubuntu.mydomain.com" >> /etc/apache2/ports.conf