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aioax25: AX.25 and APRS library in asyncio

Test Status Coverage Status

The aim of this project is to implement a simple-to-understand asynchronous AX.25 library built on asyncio and pyserial, implementing a AX.25 and APRS stack in pure Python.

Python 3.5+ and above is required as of 2021-11-12

I did try to support 3.4, but this proved to be infeasible for the following reasons:

  • Python 3.8+ makes asyncio.coroutine deprecated (apparently will be removed in 3.10). This meant I needed coroutine and async def versions of some API functions, and the necessary logic to "hide" the latter from Python 3.4.
  • Trying to coax generator-based coroutines to run "in the background" for unit test purposes proved to be a pain in the arse.

Python 3.5 support is planned to continue until it too, becomes infeasible (e.g. if type annotations become required).

What works

  • We can put a Kantronics KPC-3 TNC into KISS mode automatically
  • Multi-port KISS TNCs (tested with Direwolf and the NWDR UDRC-II)
  • We can receive AX.25 UI frames
  • We can send AX.25 UI frames

What doesn't work

  • Connecting to AX.25 nodes
  • Accepting connections from AX.25 nodes

What isn't tested

  • Platforms other than GNU/Linux

Current plans

Right now, I intend to get enough going for APRS operation, as that is my immediate need now. Hence the focus on UI frames.

I intend to write a core class that will take care of some core AX.25 message handling work and provide the basis of what's needed to implement APRS.

After that, some things I'd like to tackle in no particular order:

  • Connected mode operation
  • NET/ROM support

Supported platforms will be GNU/Linux, and possibly BSD variants. I don't have access to recent Apple hardware (my 2008-era MacBook will not run contemporary MacOS X) so I'm unable to test this software there, but it should work nonetheless.

It might work on Windows -- most probably using Cygwin or Subsystem for Linux. While I do have a Windows 7 machine handy, life's too short to muck around with an OS that can't decide if it's pretending to be Linux, VMS or CP/M. There's an abundance of AX.25 stacks and tools for that platform, I'll accept patches here on the proviso they don't break things or make the code unmaintainable.


This is a rough guide regarding how to use aioax25 in your programs.

Create a KISS device interface and ports

Right now we only support serial KISS interfaces (patches for TCP-based interfaces are welcome). Import make_device from aioax25.kiss, then create an instance as shown:

    kissdev = make_device(
        type='serial', device='/dev/ttyS4', baudrate=9600,

Or for a TCP-connected KISS interface:

    kissdev = make_device(
        type='tcp', host='', port=12345,

(Note: if is going over the Internet, I suggest either routing via a VPN or supplying a ssl.SSLContext via the ssl parameter so that your client is authenticated with the server.)

Or for a subprocess:

    kissdev = make_device(
        type='subproc', command=['/path/to/your/command', 'arg1', 'arg2'],

Some optional parameters:

  • reset_on_close: When asked to close the device, try to issue a c0 ff c0 reset sequence to the TNC to put it back into CMD mode.
  • send_block_size, send_block_delay: If a KISS frame is larger than this size, break the transmissions out the serial port into chunks of the given size, and wait send_block_delay seconds between each chunk. (If your TNC has a small buffer, this may help.)

This represents the KISS TNC itself, with its ports accessible using the usual __getitem__ syntax:

    kissport0 = kissdev[0]
    kissport1 = kissdev[1]

These KISS port interfaces just spit out the content of raw AX.25 frames via their received signals and accept raw AX.25 frames via the send method. Any object passed to send is wrapped in a bytes call -- this will implicitly call the __bytes__ method on the object you pass in.

Setting up an AX.25 Interface

The AX.25 interface is a logical routing and queueing layer which decodes the data received from a KISS port and routes it according to the destination call-sign.

AX25Interface is found in the aioax25.interface package. Import that, then do the following to set up your interface:

   ax25int = AX25Interface(
       kissport=kissdev[0],     # or whatever port number you need

Some optional parameters:

  • cts_delay, cts_rand: The number of seconds to wait after making a transmission/receiving a transmission, before we send another transmission. The delay time is cts_delay + (random.random() * cts_rand), the idea being to avoid doubling when two stations attempt transmission.

The AX25Interface is a subclass of Router (see aioax25.router), which exposes the following methods and properties:

  • received_msg: This is a Signal object which is fired for every AX.25 frame received. Slots are expected to take two keyword arguments: interface (the interface that received the frame) and frame (the AX.25 frame itself).

  • bind(callback, callsign, ssid=0, regex=False): This method allows you to bind a call-back function to receive AX.25 frames whose destination field is addressed to the call-sign and SSID specified. The call-sign may be a regular expression if regex=True. This will be compiled and matched against all incoming traffic. Regardless of the value of regex, the callsign parameter must be a string.

  • unbind(callback, callsign, ssid=0, regex=False): This method un-binds a previously bound call-back method from receiving the nominated traffic.

Additionally, for transmitting frames, AX25Interface adds the following:

  • transmit(frame, callback=None): This method allows you to transmit arbitrary AX.25 frames. They are assumed to be instances of AX25Frame (from aioax25.frame). The callback, if given, will be called once the frame is sent with the following keyword arguments: interface (the AX25Interface that sent the frame), frame (the frame that was sent).

  • cancel_transmit(frame): This cancels a pending transmission of a frame. If the frame has been sent, this has no effect.

APRS Traffic handling

The AX25Interface just deals in AX.25 traffic, and does not provide any special handling of APRS UI frames. For this, one may look at APRSInterface.

Import this from aioax25.aprs. It too, is a subclass of Router, and so bind, unbind and received_msg are there -- the messages received will be instances of APRSFrame (see aioax25.aprs.frame), otherwise the behaviour is identical.

   aprsint = APRSInterface(
       ax25int=ax25int,         # Your AX25Interface object
       mycall='VK4MSL-9',       # Your call-sign and SSID

Other optional parameters:

  • retransmit_count, retransmit_timeout_base, retransmit_timeout_rand, retransmit_timeout_scale: These control the timing of retransmissions when sending confirmable APRS messages. Before transmission, a time-out is computed as timeout = retransmit_timeout_base + (random.random() * retransmit_timeout_rand), and a retry counter is initialised to retransmit_count. On each re-transmission, the retry counter is decremented and the timeout is multiplied by retransmit_timeout_scale.
  • aprs_destination: This sets the destination call-sign used for APRS traffic. Right now, we use the experimental call of APZAIO for all traffic except direct messages (which instead are sent directly to the station addressed).
  • aprs_path specifies the digipeater path to use when sending APRS traffic.
  • listen_destinations is a list of AX.25 destinations. Behind the scenes, these are values passed to Router.bind, and thus are given as dicts of the form: {callsign: "CALL", regex: True/False, ssid: None/int}. Setting this may break reception of MICe packets!
  • listen_altnets is an additional list of AX.25 destinations, given using the same scheme as listen_destinations. Setting this may break reception of MICe packets!
  • msgid_modulo sets the modulo value used when generating a message ID. The default value (1000) results in a message ID that starts at 1 and wraps around at 999.
  • deduplication_expiry sets the number of seconds we store message hashes for de-duplication purposes. The default is 28 seconds.

To send APRS messages, there is send_message and send_response:

  • send_message(addressee, path=None, oneshot=False, replyack=False): This sends an APRS message to the addressed station. If path is None, then the aprs_path is used. If oneshot=True, then the message is sent without a message ID, no ACK/REJ is expected and no retransmissions will be made, the method returns None. Otherwise, a APRSMessageHandler (from aioax25.aprs.message) is returned.
    • If replyack is set to True, then the message will advertise reply-ack capability to the recipient. Not all APRS implementations support this.
    • If replyack references an incoming message which itself has replyack set (either to True or to a previous message ID), then the outgoing message will have a reply-ack suffix appended to "ack" the given message.
    • The default of replyack=False disables all reply-ack capability (an incoming reply-ack message will still be treated as an ACK however).
  • send_response(message, ack=True): This is used when you have received a message from another station -- passing that message to this function will send a ACK or REJ message to that station.

The APRSMessageHandler class

The APRSMessageHandler class implements the APRS message retransmission logic. The objects have a done signal which is emitted upon any of the following events:

  • Message time-out (no ACK/REJ received) (state=HandlerState.TIMEOUT)
  • Message was cancelled (via the cancel() method) (state=HandlerState.CANCEL)
  • An ACK or REJ frame was received (state=HandlerState.SUCCESS or state=HandlerState.REJECT)

The signal will call call-back functions with the following keyword arguments:

  • handler: The APRSMessageHandler object emitting the signal
  • state: The state of the APRSMessageHandler object.

TAPR TNC2 packet format

Sometimes, you need the incoming packet in TAPR TNC2 format, notably for APRS-IS interaction. This is somewhat experimental in aioax25 as no one seems to have a definition of what "TNC2 format" is.

All AX25Frame instances implement tnc2 property, which returns the frame in a hopefully TNC2-compatible format. For UI frames, which may be encoded in a number of different formats, there is also a get_tnc2 method, which accepts arguments that are passed to bytes.decode(); the default is to decode the payload as ISO-8859-1 since this preserves the byte values losslessly.

APRS Digipeating

aioax25 includes a module that implements basic digipeating for APRS including handling of the WIDEn-N SSIDs. The implementation treats WIDE like TRACE: inserting the station's own call-sign in the path (which I believe is more compliant with the Amateur License Conditions Determination in that it ensures each digipeater "identifies" itself).

The aioax25.aprs.uidigi module can be configured to digipeat for other aliases such as the legacy WIDE and RELAY, or any alias of your choosing.

It is capable of handling multiple interfaces, but will repeat incoming messages on the interface they were received from ONLY. (i.e. if you connect a 2m interface and a HF interface, it will NOT digipeat from HF to 2m).

Set-up is pretty simple:

from aioax25.aprs.uidigi import APRSDigipeater

# Given an APRSInterface class (aprsint)
# Create a digipeater instance
digipeater = APRSDigipeater()

# Connect your interface

# Optionally add any aliases you want handled
digipeater.addaliases('WIDE', 'GATE')

You're now digipeating. The digipeater will automatically handle WIDEn-N and TRACEn-N, and in the above example, will also digipeat for WIDE, GATE.

Preventing message loops on busy networks

If you have a lot of digipeaters in close proximity (say about 6) and there's a lot of traffic, you can get the situation where a message queued up to be digipeated sits in the transmit queue longer than the 28 seconds needed for other digipeaters to "forget" the message.

This leads to a network with the memory of an elephant, it almost never forgets a message because the digipeats come more than 30 seconds after the original.

The APRSDigipeater class constructor can take a single parameter, digipeater_timeout, which sets an expiry (default of 5 seconds) on queued digipeat messages. If a message is not sent by the time this timeout expires, the message is quietly dropped, preventing the memory effect.