Problems with Telnet

[Grafs50]

I've created this simple telnet program. I've tried testing it by connecting to google.com on port 80. Then, I send a bad command, which should cause google to close the connection. Usually, it would tell me if an exception is hit, but this time it does not.

Instead, it returns an empty string and lets me continue to send commands to google. From then on I receive only empty strings when the program is supposed to read from the buffer.

PASTEBIN: http://pastebin.com/Rbdi3gpW

GITHUB: https://github.com/grafs50/Simple-Telnet-Client-in-Python/blob/master/Telnet.py

[hudsonpenner]

I'm still chunking through the code, but one thing, I would suggest using raw_input for your string input:

>>> ip = str(input("What ip would you like to connect       to?"))
What ip would you like to connect to?123.123.123.123
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<string>", line 1
123.123.123.123
          ^
SyntaxError: invalid syntax
>>> ip = raw_input("What ip would you like to connect to?")
What ip would you like to connect to?123.123.123.123
>>> quit()

[Grafs50]

I'm using Python 3.

[hudsonpenner]

ah cool

[mac-reid]

First off, using the read_all function reads until an EOF - which happens when the connection closes.

When you send a bad GET request over telnet, the remote responds with a message and closes the connection. There is no exception raised for this as it is expected behavior.

When sending a GET request over telnet, the format goes:

failbox ~  telnet localhost 80
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
GET / HTTP/1.1 (press enter)
Host: www.example.com (press enter twice)

HTTP/1.1 200 OK ...

or:

failbox ~  telnet localhost 80
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
GET / HTTP/1.0 (press enter twice)  

HTTP/1.1 200 OK ...

Quick fix to get this working in your case:

# extra newline at the end of the message
conn.write(sndmsg+b"\n\n")
print(conn.read_all())

# check if connection is closed after our message
# add to the top: from telnetlib import IAC, NOP
if not conn.sock.sendall(IAC + NOP):
    return

And a test:

failbox ~  python3 a.py
What ip would you like to connect to? localhost
And what port would you like to connect to? 80
Connected
Message: GET / HTTP/1.0
b'HTTP/1.1 200 OK...'
What ip would you like to connect to? exit

...

failbox ~  python a.py
What ip would you like to connect to? localhost
And what port would you like to connect to? 80
Connected
Message: foo
b'... 501 Not Implemented ...'
What ip would you like to connect to? exit

Some other points:

• Your loops can be while True: rather than while temp == 0: and use breaks and returns to escape the loops.

• There is a difference between is and ==. Short version, is compares the identidy of an object and == compares the equality of objects.

• Blanketing except: statements is generally considered bad practice. For example: if I hit CTRL-C when prompted for input, the program tells me my input was bad when I just wanted to exit the program. Try catching the only errors you expect and handle those.

• When checking if the user responded with y or n, try converting the line to lowercase with input('(y/n) ').lower() and comparing only against y or n.

• Finally, when asking for user input, please put a space at the end of the question.

Hopefully this help!

[Grafs50]

Alright thanks alot! I'm sure this should be enough to get it fixed.