Re: [lwip-users] Help with PPP connection - modem disconnects for no obvious reason...

[Patrick Klos]

On 3/20/2019 1:48 PM, Sylvain Rochet wrote:
> Hi Patrick,

Hi Sylvain,

> On Wed, Mar 20, 2019 at 01:20:02PM -0400, Patrick Klos wrote:
> > For whatever reason, you're not responding to the CHAP challenges.
> >
> > Are you dialing into an ISP (which would require authentication) or into
> > another system configured to be a "server"?  Either way, the peer is
> > expecting you to have a username and password of some kind, but you're not
> > responding to their challenges.
> Well, that's just PPP to a cellular modem, PPP link is only between the
> device and the cellular modem to de/serialize IP packets on a serial
> link. That's why authentication is never required, cellular modems
> usually add some kind of fake authentication to support PPP clients
> where authentication can't be disabled.

OK, but that just begs the question "why is the modem asking for CHAP 
authentication"?  Maybe that's how GSM modems work?  Anyway, let's 
explore the CHAP process further.

> > Also, what version of LwIP are you using?  I can't find some of these debug
> > strings (like "CHAP Challenge") in my (2.1.2) sources. And are you sure all
> > your modules were compiled with PPP_DEBUG defined as non-zero?
> "CHAP Challenge" is from "CHAP" from chap_protent and "Challenge" from
> chap_code_names[].

Oops... sorry, I didn't notice that.

> Debugging seem pretty well enabled, for once in a while I have no clue
> what is wrong here, usually it's straightforward from the trace but here
> it really isn't.

If all the debugging is enabled, I'm curious why we don't see a CHAP 
Response being sent out?  I would suggest adding a few PPPDEBUG 
statements to chap_input() (especially the few places where is silently 
returns) and chap_respond() and see what that reveals?

Patrick