[pptp-server] Problem connecting over Dialup connection

[Jason M. Felice]

On Tue, Dec 21, 1999 at 12:45:24PM -0800, Jeff Rosenberg wrote:
> Have installed PPTPD.  The problem I am experiencing is that from cable 
> modems on the internet, there is no problem using Win98_SE to create the 
> PPTP tunnel into the company.  The VPN comes up great from any and all 
> employees who have a cable modem.
> 
> When trying users who have regular Dial-up connections we all get an error 
> from the VPN connection #645.
> 
> Since the Cable modems work I am assuming my configuration (PPPD, 
> chap-secrets, etc. etc. is properly done).  I have played with the MTU and 
> MRU settings in the Options file but no values produce a connection with the 
> #645 error.
> 
> Here is the PPPD Options file and the debug info. from the PPTPD and Kernel:
[snip]

This makes me very curious as to how PoPToP handles gre packets which get
sliced up (due to device MTU or what not).

Changing the MTU to be lower on the server won't change anything, BTW, 
neither will changing the MTU on the client (can you do that under Windoze).
Changing the MTU on an intermediary router is the only thing you can do to
cause packets to get sliced.  Even then, I'm not sure if route MTU discovery
(via some ICMP magic) is involved.  If it is, this is sometimes incorrect as
some firewall vendors filter these ICMP requests and others (for some reason)
always reply 1500, despite the actual MTU.

Really, the only thing to do is do a tcpdump on interfaces on both ends, then
generate some large traffic and see how the packets get sliced up.

-Jay 'Eraserhead' Felice