After setup pptp vpn tunnel with pptpd under linux, you can access most of sites with no issue. But if you find you can’t open some sites like: www.alexa.com, then this is more like a tcp mss size issue, simply ssh into your pptpd server, and run the command below:
iptables -I FORWARD -p tcp --syn -i ppp+ -j TCPMSS --set-mss 1356
Most likely, it will fix your problem.
Thanks the post here point me to the right direction:
For what this TCPMSS is: