I was trying to implement telnet server with telnetlib3 in python. But everything works fine until i start using some UTF-8 encoding character, that moment it’s break.

Here is my simple mock server to test about this problem

import asyncio
import telnetlib3

async def shell(reader, writer):
    writer.encoding = 'utf-8'
    writer.write('ดีคับn')
    writer.write('Type "exit" to close the connection.n')
    
    while True:
        writer.write('> ')
        inp = await reader.read(1024)
        if not inp:
            break

        inp = inp.strip()
        if inp.lower() == 'exit':
            break

        writer.write(f'You said: {inp}n')

    writer.write('Goodbye!n')
    writer.close()

if __name__ == "__main__":
    loop = asyncio.get_event_loop()
    server = telnetlib3.create_server(port=8023, shell=shell, host='0.0.0.0', encoding='utf-8')
    loop.run_until_complete(server)
    loop.run_forever()

I have try to specify encoding in server = telnetlib3.create_server(port=8023, shell=shell, host='0.0.0.0', encoding='utf-8'), of course i try both “utf-8” and “utf8”. But the telnetlib3 still uses ascii encoder to encode the string.

Here is the error (after i explicitly define a encoder utf-8)

Task exception was never retrieved
future: <Task finished name='Task-4' coro=<shell() done, defined at /home/rinz/dev/catnet/c2/whytelnet3issostupid.py:4> exception=UnicodeEncodeError('ascii', 'ดีคับn', 0, 5, 'ordinal not in range(128)')>
Traceback (most recent call last):
  File "/home/rinz/dev/catnet/c2/whytelnet3issostupid.py", line 6, in shell
    writer.write('ดีคับn')
  File "/home/rinz/dev/catnet/.venv/lib/python3.12/site-packages/telnetlib3/stream_writer.py", line 2614, in write
    self._write(self.encode(string, errors))
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/rinz/dev/catnet/.venv/lib/python3.12/site-packages/telnetlib3/stream_writer.py", line 2594, in encode
    return bytes(string, encoding, errors or self.encoding_errors)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-4: ordinal not in range(128)