add Dynu.com to DDNS list on Synology NAS

https://www.dynu.com/DynamicDNS/IPUpdateClient/SynologyNAS

(install ipkg to install nano follow the instructions here if necessary)
(Note: on my DSM 5.2, PATH setting in /root/.profile need to be commented out)
add below codes to /etc.defaults/ddns_provider.conf

[Dynu.com] 
        modulepath=DynDNS 
        queryurl=api.dynu.com/nic/update?hostname=__HOSTNAME__&username=__USERNAME__&myip=__MYIP__&password=__PASSWORD__