connect to local area network through PPTP VPN (Synology DSM)

Credit goes to this forum post by vibrationbaby

In short, assign PPTP managed IPs into the same pool of local private network.

1. login as root through SSH.
2. go to /usr/syno/etc/packages/VPNCenter/pptp
3. edit the two lines for localip and remoteip pptp.conf. (@vibrationbaby provided the setting below. i used .200 and .201-205 while my DHCP managed 100-199. just choose some addresses that is not overlapping with existing DHCP pool.)
   

   localip 192.168.1.210
   remoteip 192.168.1.211-220
   

4. then it should work after restart the VPN Server through DSM web interface.

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__