It is stated in VirtualBox manual (chapter 6) that VM network interfaces in "NAT" mode do not support GRE protocol. I'm currently trying to setup a pair of VMs as
pptp server and client which would communicate via VM's network interfaces in "INTERNAL NETWORK" mode. I set up modern Linux OSes with
pptp software on both machines and made some basic
pptp configuration. However, I fail to connect the client to the server - server reports a failure when writing to GRE socket.
I now need a quick proof of principal ability to use the GRE protocol between VirtualBox VMs over a network in "INTERNAL" mode, such as a several-line
.c program that would try to open/send/receive/close a GRE protocol connection, or like a specific lightweight tool to diagnose the GRE availability.