I've set up a VPN with pptpd on Ubuntu 12.04. I can connect to the server from within the network, but I am unable to connect from a remote network.
The server is behind a router, but as an experiment, I opened up all ports - 1 to 6500, thinking that perhaps it was using a port I wasn't expecting. Still have no luck.
I cannot find the file /var/log/messages on this computer, so I don't know what's going on there.
However, in the syslog, I noted that a line reads "no ifupdown configuration found"
And the next line reads GRE: Bad checksum from pppd.

I'm not sure why I should get these errors from a remote network, but not the local network.

Any help would be appreciated.

I've never used pptp and I only installed Ubuntu once for someone else, but I googled this and found where someone said that MTSE and MPPE are disabled by default in Ubuntu's network manager, and they need to be enabled. It was also suggested that you turn on verbose debugging (pptpd --debug).

According to the Wikipedia article on pptp, the protocol has some serious security vulnerabilities. You might be better off implementing OpenVPN or similar.