1 ) activate Telnet for the Syno from within DSM (Control Panel / Terminal)
2 ) Telnet into the Syno, logging in with user : root and your Syno admin password
If you use Windows, you might want to get a TelNet client (e.g. Putty)
3 ) Now we need to modify the options.pptpd file. To do this, type :
vi /usr/syno/etc/synovpn/pptp/options.pptpd
You should see the options.pptpd file in the terminal / telnet windows.
4 ) With the cursor scroll down right after the nobsdcomp line
5 ) Press I (as in Insert) to switch vi to Insert mode
6 ) add the two following lines :
nopcomp
noaccomp
7 ) to save the modifications, do the following :
– Press Escape to exit the Insert mode
– Press :wq
8 ) Now go back to DSM, disable the PPTP VPN, Save, enable PPTP VPN & Save (easy way to restart the services so the modifications of options.pptp are taken into account).
REF: http://forum.synology.com/enu/viewtopic.php?f=173&t=37216