Trying to make asyncio working with telnetlib

Question

I'm having a hard time trying to make asyncio working with telnetlib for interrogate some hardware.

I think I clearly don't understand the way asyncio is working and I'm a completely lost in all of this. It's really unclear.

My basic version (which is synchrone) is working well but interrogating the complete list of equipments takes 6 hours actually and a large part of the equipements are not responding because they are unreachable.

Since Asyncio make us able to parallelize the connections without waiting for each timeout to trigger I would like to transform my code in proper asynchrone code, without success.

Here is what I tried :

import telnetlib
import time
import datetime
import asyncio

from env.fonctions import *
from env.variables import *


first_cmds = ['term length 0', \
              'show run', \
              'exit']

#create lists to iterate through
hosts = ['router-1', 'router-2', 'router-3', 'router-4', 'router-5']

async def main(hosts, user_rw_hw ,password_rw_hw, first_cmds):
    class ContinueI(Exception):
        pass

    continue_i = ContinueI()

    for host in hosts:
        print(f'{host} | Trying to connect...')
        try:
            tn = await async_establish_telnet_connexion(user_rw_hw ,password_rw_hw, host, 23, 0.5, True)
        except:
            continue

        print(f'{host} | Checking if equipment is not Nexus')

        tn.write('show version'.encode('ascii') + b"\n")
        sh_ver = await async_read_telnet_output(tn)
        if 'Nexus' in sh_ver or 'NX-OS' in sh_ver or 'nexus' in sh_ver:
            print(f'{host} | Equipment is Nexus, closing connection...')
            tn.write('exit'.encode('ascii') + b"\n")
            continue

        tn.write(''.encode('ascii') + b"\n")

        try:
            for cmd in first_cmds:
                tn.write(cmd.encode('ascii') + b"\n")
                if not 'exit' in cmd:
                    response = await async_read_telnet_output(tn)
                    if '\r\n% Invalid' in response:
                        print(f'{host} | Commande "{cmd}" pas reconnue')
                        raise continue_i
                else:
                    print(f'{host} | Commands are accepted')
        except ContinueI:
            tn.write(b"exit\n")
            tn.write(b"exit\n")
            print(f'{host} | Logout for command not recognized')
            continue


if __name__ == "__main__":
    try:
        loop = asyncio.get_event_loop()
        loop.set_debug(1)
        loop.run_until_complete(main(hosts, user_rw_hw ,password_rw_hw, first_cmds))

    except Exception as e:
        pass

    finally:
        loop.close()

and functions :

async def async_read_telnet_output(tn, timeout=2, timestep=0.1):
   timer = 0
   data = b''
   while timer <= timeout:
       new_datas = tn.read_very_eager()
       if len(new_datas) != 0:
           timer = 0
           data += new_datas
       await asyncio.wait(timestep)
       timer += timestep
   return data.decode('utf-8')


async def async_establish_telnet_connexion(user_rw_hw, password_rw_hw, host, port=23, timeout=1, debug=False):
   try:
       tn = telnetlib.Telnet(host, port)    # Here I don't know how to make it awaitable, if I put await before the IDE said that this method is not an awaitable, btw even if I put an awaitable like "asyncio.sleep" the behavior is still the same so it's not the only point bad
   except:
       if debug == True:
           print(f"{host} | Telnet not responding.")
       raise Exception

   if debug == True:
       print(f"{host} | Telnet is responding.")

   response = loop.create_task(async_read_telnet_output(tn, 15))

   if not 'Username:' in response and not 'login' in response:
       if debug == True:
           print(f"{host} | Don't see Username asked by equipment.")
       raise Exception
   else:
       tn.write(user_rw_hw.encode('ascii') + b"\n")

   if debug == True:
       print(f"{host} | Username entered.")

   try:
       await tn.read_until(b"Password: ", timeout)
   except:
       if debug == True:
           print(f"{host} | Don't see Password asked by equipment.")
       raise Exception
   finally:
       tn.write(password_rw_hw.encode('ascii') + b"\n")
       response = await async_read_telnet_output(tn, 10)
       if '% Authentication failed' in response or 'Rejected' in response:
           if debug == True:
               print(f"{host} | Connection failed bad credentials.")
           raise Exception
       if debug == True:
           print(f"{host} | Connection succeed waiting for commands.")
       return tn

If some people know where I fail I would be grateful i'm stuck since one week on it... Reading some books and youtube tutos but nothing help me..

Thank you by advance !

While it won't actually solve your problem, you might want to read up on [How does asyncio actually work?](https://stackoverflow.com/questions/49005651/how-does-asyncio-actually-work). Code has to *be* asynchronous, you cannot just ``await`` a synchronous function and expect it to become concurrent. Using threads instead of coroutines might be much simpler to solve your issue. — MisterMiyagi, Nov 12 '20 at 18:14
I understand your point now @MisterMiyagi, I was thinking that since telnet is waiting for answers sometimes it should automatically be "asynchronousable", but now I understand what you mean, I will take a look closer at this point. In fact I think it could be possible, since my function "async_read_telnet_output" is using a asyncio.sleep method which is awaitable if I understand well, then I will see to refactor my code, if it is the telnetlib that is not compatible, I could maybe take a look if the socket lib is "asynchronousable" and create a new telnet client from scratch. Thanks again ! — gotzilla, Nov 13 '20 at 01:36

score 0 · Answer 1 · answered Nov 29 '21 at 21:58

For the ones landing here... I found https://pypi.org/project/asynctelnet/

quick exmple:

client:

import anyio, asynctelnet

async def shell(tcp):
    async with asynctelnet.TelnetClient(tcp, client=True) as stream:
        while True:
            # read stream until '?' mark is found
            outp = await stream.receive(1024)
            if not outp:
                # End of File
                break
            elif '?' in outp:
                # reply all questions with 'y'.
                await stream.send('y')

            # display all server output
            print(outp, flush=True)

    # EOF
    print()

async def main():
    async with await connect_tcp('localhost', 56023) as client:
        await shell(client)
anyio.run(main)

Server:

import anyio, asynctelnet

async def shell(tcp):
    async with asynctelnet.TelnetServer(tcp) as stream:
        # this will fail if no charset has been negotiated
        await stream.send('\r\nWould you like to play a game? ')
        inp = await reader.receive(1)
        if inp:
            await stream.echo(inp)
            await stream.send('\r\nThey say the only way to win '
                              'is to not play at all.\r\n')

async def main():
    listener = await anyio.create_tcp_listener(local_port=56023)
    await listener.serve(shell)
anyio.run(main)

Still this lib has some bugs. so the current state is "be prepared to code some bug workarounds if you want to use it".

Trying to make asyncio working with telnetlib

1 Answers1