I've set up a Debian box as a PPTP VPN router for other computers on our network - the Debian box establishes a PPTP connection to a remote VPN service and then routes all Internet traffic via the VPN.
From the router itself I can access all websites, everything works fine.
But from other client computers on the network using the router as the default gateway, these machines can only access about 50% of websites for some reason. For example, I can access www.bbc.co.uk but not www.speedtest.net - yet both of these sites work fine when testing with a text-based browser directly on the router's command line.
From the other computers we can ping and traceroute to the non-working sites, it's just web access which won't load (the browser times out waiting for a response).
The following are the iptables rules we've used on the router to enable NAT:
iptables --table nat --append POSTROUTING -j MASQUERADE iptables --append FORWARD --in-interface eth0 -j ACCEPT
Also IP forwarding has been enabled with:
echo 1 > /proc/sys/net/ipv4/ip_forward
If I disconnect the VPN on the Debian box and traffic routes through the DSL connection via the Debian router, all websites load properly from client computers - so this problem only occurs when the VPN is connected (though all sites can be accessed directly from the router when connected via the VPN so this rules out a problem with the VPN as such).
Does anyone have any suggestions as to what might be causing this and what I need to do to fix this?