[pptp-server] pptp, gre, adsl firewllas and nat

Christopher Tresco ctresco at economics.mit.edu
Wed May 23 09:24:48 CDT 2001

Are you NATing protocol 47?? That is the gre protocol.

> -----Original Message-----
> From: pptp-server-admin at lists.schulte.org
> [mailto:pptp-server-admin at lists.schulte.org]On Behalf Of John Leach
> Sent: Monday, May 21, 2001 7:37 PM
> To: pptp-server mailing list
> Subject: [pptp-server] pptp, gre, adsl firewllas and nat
> Hello, I've been playing with pptpd with much success, but am having
> troubles getting it to work in one particular scenario.
> I have 2 linux boxes, connect over the internet via a bridged cable
> internet connection on one end, and a natted adsl connection complements
> of the friendly British Telecom.
> I'm using the linux client, and can connect to both boxes via their
> local LANs fine, all works great.
> When I try to connect over the internet things go terribly wrong.  The
> port connects and the gre gets set up, ppp starts and seems to get
> going, but then I get LCP: ConfigRequest timeout errors (in my syslog).
> I've found a bit of info on this message re: generic ppp problems. and
> have tried a few suggested solutions, for example playing with my mru
> and mtu settings, but to no avail.
> I'm worried this is caused by the NATing of the adsl router.  I've
> tcpdumped traffic at both ends and observed seemingly normal gre and tcp
> traffic, so I'm pretty sure the router understands gre.  Could gre be
> being affected by the natting, like ipsec does?  (even tho ipsec is only
> affected because the changes in the packets by the NAT breaks the
> security measures, and gre has no such security measure I know of).
> Has anyone else had similar problems?  I'm going to try this connection
> to a 3rd box on a real leased line, using the current boxes as a client
> one at a time to see if I can rule either of them out.
> I'll submit more details when I get them.
> John Leach.
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