[pptp-server] Tracking the problem down

[Neale Banks]

On Tue, 29 Feb 2000, Michael Barsalou wrote:

> How do we find which router might be blocking the GRE packets?
> 
> I am trying to connect a Windows Workstation to my PoPToP 
> server.  I know connections work because I can do it from my 
> home.  However, from a machine at another company doesn't work. 

Sounds sadly familiar {:-(

> So that means they are probably blocking GRE packets. Or is 
> there some other explanation?

Don't forget that, for reasons best known to MS, pptp first has a chat on
a TCP control channel (IIRC, port 1723) and then establishes the PPP over
GRE.  It is possible that the TCP path is clear but the GRE path is not.

Can anyone definitively state what pptpd log messages would be seen under
these circumstances?

> I can see the machine trying to connect....so I guess that means 
> on the way here GRE packets are fine.  But then the connection 
> times out.  We're both on highspeed links so I don't think it is a 
> problem with network traffic.
> 
> Anyone got any thoughts on this?

As David pointed out, you can't assume symmetry of either routing paths on
the internet nor symmetry of filtering.  The practical consequence of this
is that you really should test GRE in both directions.