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

[Sylvain Rochet]

Hi Patrick,

On Wed, Mar 20, 2019 at 02:19:48PM -0400, Patrick Klos wrote:
> 
> 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.

This is to support PPP clients that require a secure CHAP authentication 
although we don't care at all for authentication on a cellular modem. 
The cellular modem is going to accept a downgrade to PAP or even no 
authentication at all. PPP is designed that authentication methods are 
asked/(n)acked in order from the best one to the worst one until both 
agrees on a common one, that's why the cellular modem start by CHAP.


> >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?

(Answering so this question is not left unanswered for future readers, 
although there are clues in the thread)

This is because packet allocation for the reponse failed, probably 
because PPP_USE_PBUF_RAM is not set and PBUF_POOL_BUFSIZE is too small 
(< RESP_MAX_PKTLEN).


Sylvain

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