How to read telnet output with telnetlib3 and python?

Question

Using python 3.8.10 and telnetlib I want to connect to a host and just print out the message the host is sending upon connection. I have been trying the following code:

import telnetlib3
import asyncio
async def run_telnet_session():
    total_output = ""
    # Establish a Telnet connection with timeout
    reader, writer = await telnetlib3.open_connection("192.168.200.10", 9000)
    
    try:
        # Read the Telnet output
        while True:
            output = await reader.read(4096)  
            if output:
                print(output)
                total_output += output
            else:
                break  
    
    finally:
        writer.close()
        await writer.wait_closed()
    return total_output

output = asyncio.run(run_telnet_session())
print(output)

and I get the expected output, but the code is blocked! The function never returns. How to change the code so there is a timeout and the function returns the string?

score 0 · Answer 1 · answered Jun 05 '23 at 10:10

The code you provided is attempting to use the telnetlib3 library, which is not a standard library in Python. It seems that you intended to use the telnetlib library, which is the built-in Telnet client library in Python.

import telnetlib
import time

def run_telnet_session():
    total_output = ""
    # Establish a Telnet connection
    tn = telnetlib.Telnet("192.168.200.10", 9000, timeout=2)
    
    try:
        # Read the Telnet output
        while True:
            output = tn.read_very_eager().decode('utf-8')
            if output:
                total_output += output
                print(output)
                # Reset the timer if new data is received
                start_time = time.time()
            else:
                # Check if no new data received for 2 seconds
                if time.time() - start_time > 2:
                    break
    
    finally:
        tn.close()
    
    return total_output

output = run_telnet_session()
print(output)

In this code, the read_very_eager() method is used to read data from the Telnet connection without blocking. The received data is printed and appended to total_output. The timer (start_time) is reset whenever new data is received, and if no new data is received for 2 seconds, the loop is terminated.

Make sure to replace "192.168.200.10" and 9000 with the correct host IP address and port number you want to connect to.

No, with telnetlib it does not work. Let me rephrase my question — Alex, Jun 05 '23 at 10:34

score 0 · Answer 2 · answered Jun 05 '23 at 11:18

The solution seems to be that you wrap the reading function into a wait_for like follows:

output = await asyncio.wait_for(reader.read(4096), timeout=2)

That construct now seems to wait for 2 seconds if there is no further output from the read method...

So complete code is

async def telnet_welcome_message(host, port, timeout=2):
    output = ""
    # Establish a Telnet connection with timeout
    reader, writer = await telnetlib3.open_connection(host, port)
        
    try:
        # Read the Telnet output
        while True:
            data = await asyncio.wait_for(reader.read(4096), timeout=timeout)
            if data:
                output += data
    except asyncio.exceptions.TimeoutError:
        # Timeout occurred -> Exiting
        pass
    finally:
        reader.close()
        writer.close()
    return output

message = asyncio.run(telnet_welcome_message(HOST, PORT))
print(message)

How to read telnet output with telnetlib3 and python?

2 Answers2