[pptp-server] GRE: Bad checksum from pppd

[Lillian Kulhanek]

Hi all,
So close and yet so far.  PPTP on a Linux box, masqueraded behind a
firewall.  We could establish a connection with a client, but it disconnects
almost immediately, looks like from the peer side.  In our debugging
efforts, we tried to isolate the problem by connecting from a Win98SE
computer within the network directly to the vpn (modems/firewall bypassed).
In this manner, we are able to login, with MSCHAP-v2 peer authentication, as
you'll see in the logs, then the peer disconnects.