[clue-tech] How to make pppd ignore certain binary strings?

[Jim Ockers]

Hi everyone,

Does anyone know/remember how to make pppd ignore certain strings on 
receive?  IIRC the asyncmap function was to make pppd escape out certain 
sequences of data which could be interpreted by a modem to hang up.  
E.g. +++ATH0

NO CARRIER


Just kidding.  Anyway I have a piece of communications gear that 
sometimes inserts a string of bits into the bitstream that comes in  on 
a serial port to pppd.  This string of bits is always these 8 bytes: "e9 
c5 00 ca 00 02 00 40"  They really screw up the PPP frames and of course 
cause a frame checksum error and a TCP retransmission from the other end 
for whatever data was corrupted.  Here is the beginning of a normal 
PPP+IP frame:

0000  21 45 08 00 34 be 46 40  00 40 06 68 73 ... and so on and so forth

Here is the same PPP+IP frame but containing the unwanted sequence of 
bits at the beginning of the frame.   It's not always at the beginning, 
but regardless of where in the frame it appears, it is messed up.

0000  e9 c5 00 ca 00 02 00 40  21 45 08 00 34 be 46 40   .......@ !E.. at ..@

I captured the packet traces using pppd's record option.  I'm wondering 
if any of you know or can suggest a way to make pppd ignore these byte 
sequences which are inserted by a piece of serial communications gear?  
Since it's always the same 8 bytes it should be pretty easy to recognize.

Thanks,
Jim

-- 
Jim Ockers, P.Eng. (ockers at ockers.net)
Contact info: http://www.ockers.ca/pason.html


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