Due to CloudFlare only support some ports, I need to change Synology NAS admin console to use port 443 instead of default 5001. However I cannot get it done using Synology web admin, because port 80/443 are reserved for PHP website hosting, even I turned them off...

Google and found out that I can hack the config file directly.
Login using SSH, modify /etc/synoinfo.conf file, change port from 5000/5001 to 80/443.

admin_port="80"  
secure_admin_port="443"  

References:
http://forum.synology.com/enu/viewtopic.php?f=19&t=50796