[Tutor] telnetlib - character hex00 missing

Alan Gauld alan.gauld at btinternet.com
Mon Nov 22 10:07:06 CET 2010

"Joachim Roop" <no.dr at gmx.de> wrote 

> Even though my non-python telnet-server on the other side is 
> sending #00-bytes, they are not recognized by python's telnetlib 
> (characters #01-#FF seem to work fine though).
> My C++ implementation has no problems with this.  
> I have to use Python 3.1 on Windows.
> I'm guessing this a known bug. What is the workaround? :(

Telnetlib has been around for at least 10 years so I'd expect a 
bug that serious to have been fixed long ago! I suspect something 
else is at fault.

> # Receiver
> tn = telnetlib.Telnet()
> tn.open(ip, port)
> while 1:

Python idiom nowadays prefers "while True:"  to "while 1:"

>  response = (tn.read_until(b"\r",20))[1:-1]

It might be worth breaking this line down and printing the results

data = tn.read_until(b"\r",20)    # did telnet even read the 00?
response = data[1:-1]   # did we strip it correctly?

>  if response.find(bytes.fromhex("00")) > -1:
>    print ("There are hex00 characters")
>  else:
>    print ("No hex00 characters found")
> tn.close

Others may have more info, but I'll be surprised if telnetlib can't 
read a '00'.

Alan Gauld
Author of the Learn to Program web site

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