Connecting to a Windows PPTP VPN from Linux (a problem solved)
I'd been struggling to connect to one of our client's Windows VPNs from my Linux Mint (Cinnamon) OS for a little while but not had any urgent need for it so put off solving it. Today, the urgency arose and so I went about fixing the issue which was thankfully pretty painless.
The VPN was configured using the built in network manager UI in my version of Linux Mint:
Network connections -> Add -> Type: PPTP
I entered the gateway address and credentials as supplied and left all other options as default. Attempting to connect to the VPN would fail, without any message explaining the failure.
Getting an error message
The first job was to find out what was going wrong. After a little Duck-Duck-Go-ing, I discovered that the vital logs were being written to
/var/log/syslog. I tailed this while trying to connect and discovered this message that was causing an authentication failure:
pppd: EAP: unknown authentication type 26; Naking
Fixing the problem
A little searching later gave me the solution. Some Windows VPNs will not accept a request that will attempt multiple Authentication schemes and will use MPPE for the connection. I fixed my problem by enabling MPPE and disabling all authentication methods with the exception of MSCHAPv2. The settings dialog for me could be found at:
Network connections -> Edit (my connection) -> Advanced...
Hopefully that helps someone else out (or indeed me, when I forget).