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I have a PPTP daemon V1.3.4 running on a CentOS server.

My client connections come from both Linux and Windows machines. When PPTP connection is active, the clients are unable to download full web pages from many websites (stackoverflow, twitter, ip-lookup for example). For some other websites, the clients are not even able to ping the IP address ( for example).

However they are able to download full web pages of Facebook, Youtube for example.

The CentOS server itself is able to download full web pages from all the web sites. It does not have any iptable rules despite a few masquerade rules in NAT table.

Please help, thanks.

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up vote 2 down vote accepted

This sounds VERY much like an MTU issue (and one I have experienced a few times).

/sbin/iptables -I FORWARD -o eth0 -s IP.OF.PPTP.CLIENT -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

What is probably happening is your external interface on your CentOS box has a "standard" MTU of 1500 bytes, however the mtu is less then that on the PPTP link because it needs to encapsulate the packets (I think it will typically leave 1492 bytes or something like that), and something in the path does not like it.

I believe the inability to ping may be a bit of a red herring - I can't ping it either (which means they are blocking ICMP).

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Thank you very much David. My issue has been resolved. I used a slightly different iptables rule though: iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu – Howard Feb 13 '13 at 8:05

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