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

[marcus hall]

On Fri, Jan 08, 2010 at 03:36:23PM -0700, Jim Ockers wrote:
> 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.


The "asyncmap" option only maps control characters (0x00 - 0x1f), so
setting some of the bits in that would cause ppp to escape some of the
characters.  But, there is also an "escape" option that can escape
any characters except for 0x20-0x3f and 0x5e.

Now, this looks like it is trying to make ppp avoid sending the escaped
characters because they cause trouble if sent over the line.  It does
not state that ppp will ignore the reception of an escaped character,
although it would be reasonable for it to do so..

marcus hall
marcus at tuells.org