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got a very strange behaviour with a Gentoo-based PPTP-Gateway here. I configured the VPN and the default routes - from the Gateway itself, the connections out to the VPN work great. But from the Clients, connected to the Router, I just can do pings into this VPN. No HTTP-Traffic, no FTP etc.

iptables is not used and flushed, so I don't think, that's the point of failure.

Does anyone have some hints to look for?

(Edited the title for a better understanding)

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Check the MTU settings. – topdog Aug 17 '10 at 7:36
Yeah, thought of it too - but mtu's of 1492, 1450, 1400 and 1000 are all giving the same results. – braindump Aug 17 '10 at 7:39
Use tcpdump to see what is happening to the traffic flow. – topdog Aug 17 '10 at 7:52

Okay, it's been the MTU. a

iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

did it. Thanks to everyone.

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