Linux – how to manually setup pptp vpn in linux

You did not mention your rules file. Have you added appropriate entries to the rules file to allow TCP port 1723 and GRE (IP protocol 47) traffic to your pptp server? See this page for details.

Response to Update

Try replacing vpn all ACCEPT with the equivalent set of policies

vpn fw  ACCEPT
vpn wan ACCEPT
vpn lan ACCEPT

and test whether everything still works. If so, next try removing each of those policies one by one, testing whether everything still works each time, to get down to the minimal set of policies required. Is it possible that the only policy you really need to add is vpn wan ACCEPT ?

In options.pptp, place the following lines anywhere:

mtu 1490
mru 1490
require-mschap
require-mschap-v2
require-mppe-128

Try the connection again. You may need to restart pppd

EDIT 1

There appears to be a mismatch in settings between the server and client. First, let's add the option to enable debug logging into options.pptp. Somewhere in that file, add the line: debug dump.

Next, before starting the connection again, tail the error log: sudo tail -f /var/log/syslog or /var/log/messages

Finally, start the connection again and see what the errors are in fuller detail.

EDIT 2

Looks like we've got an options dice game going on. Since the debug logging didn't shed any additional light on the problem, here's what should probably be done:

man pppd

Look at all the options available and start analyzing them. Change options using options.pptp and see if any change your situation. The error that's being reported strongly indicates that there is some kind of options mismatch between server and client, but it doesn't give any further information. Sadly, in cases like this, the next step is often to play option bingo.