Error sending a ping message

[decadenza]

Issue

The command:

aiocoap-client -m EMPTY coap://10.0.2.0:5683

produces the following error:

Traceback (most recent call last):
  File "pc/aiocoap-venv/bin/aiocoap-client", line 8, in <module>
    sys.exit(sync_main())
             ~~~~~~~~~^^
  File "pc/aiocoap-venv/lib/python3.13/site-packages/aiocoap/cli/client.py", line 684, in sync_main
    asyncio.run(main(args=args))
    ~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/asyncio/runners.py", line 195, in run
    return runner.run(main)
           ~~~~~~~~~~^^^^^^
  File "/usr/lib/python3.13/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/usr/lib/python3.13/asyncio/base_events.py", line 725, in run_until_complete
    return future.result()
           ~~~~~~~~~~~~~^^
  File "pc/aiocoap-venv/lib/python3.13/site-packages/aiocoap/cli/client.py", line 662, in main
    await single_request_with_context(args)
  File "pc/aiocoap-venv/lib/python3.13/site-packages/aiocoap/cli/client.py", line 578, in single_request_with_context
    await single_request(args, context)
  File "pc/aiocoap-venv/lib/python3.13/site-packages/aiocoap/cli/client.py", line 505, in single_request
    response_data = await requester.response
                    ^^^^^^^^^^^^^^^^^^^^^^^^
  File "pc/aiocoap-venv/lib/python3.13/site-packages/aiocoap/protocol.py", line 768, in _run_outer
    await cls._run(app_request, response, weak_observation, protocol, log)
  File "pc/aiocoap-venv/lib/python3.13/site-packages/aiocoap/protocol.py", line 797, in _run
    await protocol.find_remote_and_interface(app_request)
  File "pc/aiocoap-venv/lib/python3.13/site-packages/aiocoap/protocol.py", line 466, in find_remote_and_interface
    if await ri.fill_or_recognize_remote(message):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "pc/aiocoap-venv/lib/python3.13/site-packages/aiocoap/tokenmanager.py", line 214, in fill_or_recognize_remote
    return await self.token_interface.fill_or_recognize_remote(message)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "pc/aiocoap-venv/lib/python3.13/site-packages/aiocoap/transports/tcp.py", line 461, in fill_or_recognize_remote
    if message.requested_scheme == self._scheme:
       ^^^^^^^^^^^^^^^^^^^^^^^^
  File "pc/aiocoap-venv/lib/python3.13/site-packages/aiocoap/message.py", line 776, in requested_scheme
    return self.request.requested_scheme
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'requested_scheme'

Expected behaviour

The client sends an ping message (EMPTY CON).

Other information

$ python3 -m aiocoap.cli.defaults
Python version: 3.13.5 (main, May  5 2026, 21:05:52) [GCC 14.2.0]
aiocoap version: 0.4.17
Modules missing for subsystems:
    dtls: missing DTLSSocket
    oscore: missing cbor2, cryptography, filelock, ge25519, lakers-python
    linkheader: everything there
    prettyprint: missing cbor2, pygments, cbor-diag
    ws: missing websockets
Python platform: linux
Default server transports:  tcpserver:tcpclient:tlsserver:tlsclient:udp6
Selected server transports: tcpserver:tcpclient:tlsserver:tlsclient:udp6
Default client transports:  tcpclient:tlsclient:udp6
Selected client transports: tcpclient:tlsclient:udp6
SO_REUSEPORT available (default, selected): True, True

[chrysn]

A CoAP ping is sufficiently different from a CoAP request that just requesting with an empty code through the client does not just work.

Good idea, though: thanks! I'll probably make it a flag (--ping) rather than setting the empty code (maybe with a nice hint), either way, taking it up as a feature request. (Currently active in other projects, so don't hold your breath until it's done; also, I'll probably prioritize making this not err as weirdly as it does over implementing the feature).

[decadenza]

I was testing the interoperability between aiocoap and proseccoap (https://github.com/decadenza/ProsecCoAP).
To be honest with you, also with libcoap it is quite difficult to send a simple ping message, and I had to resort to something like:

printf "\x40\x00\x00\x00" | nc -u -w 1 192.168.0.101 5683

to verify that I was handling pings correctly.

I won't hold my breath. But thanks.

[chrysn]

To be fair, CoAP ping is really used rarely: A GET to / can have just as few bytes, and sure, it may send more data back, but more often than not it's just a 4.04, and while that's an error on the face of it, it works just as well as confirmation that that peer is alive.

I think that to some extent, this was something that sounded like a good idea back at RFC7252, and it just didn't take on as a widespread concept because it adds barely any practical value.