I encountered an issue recently with trying to make a PPtP connection from a Linux VM as the client to a remote commercial device or server where the GRE packets were being dropped. The same PPtP credentials worked on another server that is bare metal. This lead me speculate that the issue might be something between the routing devices and the client. After a bit of investigative work with wireshark I discovered the GRE packets were in fact getting to the virtualization host but not to the guest VM. I suspect this issue may be present with other types of virtualization software, but to be clear this particular VM host is running KVM/QEMU.
It has been a while (read: years) since I’ve done much with PPtP beyond just using it. Adding a configuration that was working on another server to this particular system I discovered the connection would not complete much to my dismay. Looking at what ppp logged to the system log revealed it never got a proper GRE reply. Well, there were a lot of things in the log but the one that stood out looked like this:
warn[decaps_hdlc:pptp_gre.c:204]: short read (-1): Input/output error
After a bit of Googling and reading the documentation for pptp-client I decided re-try the setup on the previously mentioned working system and watch the log closely for further clues. Where the second system was failing the original system sailed right past and worked fine. My next attempt was to look at what connections the first system had going which lead to me realize and make a mental connection to the documentation/Googling had revealed about PPtP using protocol 47 (GRE) on TCP port 1723 for the control. Watching another attempt on the second system showed the outgoing request for GRE but nothing coming back. Repeating the last test but watching for incoming GRE on the host showed that it was being received but not being passed on to the guest VM. Looking at my options I discovered that there is a whole set of modules and a kernel configuration option to allow forwarding of PPtP.
The missing pieces to the puzzle include adding a line to your sysctl.conf:
Then loading these kernel modules:
As soon as these were in place PPtP started working as expected in the guest VM. What started out as a mystery turned out to be a fairly simple solution. While there are probably not a lot of people still using PPtP these days, it is a better alternative to using a proprietary VPN client.