FAQ

Frequently Asked Questions

:maxdepth: 3
:caption: Frequently Asked Questions

Where can I find project policies and conventions?

How do I get this running?

To configure your environment for developing with the multicast library, follow the steps in the Install Guide. Key steps include:

Ensuring you have a supported version of Python installed.
Using pip to install (e.g., pip install multicast)
Verifying the installation works:

in `Python`	in `bash`
`import multicast`	`python3 -m multicast --help`

How do I use this `multicast` to receive some UDP multicast?

With multicast installed and set up, this guide will assume you want to listen to multicast group 224.0.0.1 on the UDP port 59595.

Prototype in `bash`

This command will allow you to quickly prototype and test receiving multicast messages without needing to write a full Python script. Once you confirm that the command works as expected, you can then proceed to implement a more advanced solution in Python as necessary.

python3 -m multicast --daemon --use-std HEAR --port 59595 --group 224.0.0.1

Explanation of the Command-Line Options

Option	Description
`--daemon`	This option runs the multicast listener as a background daemon.
`--use-std`	This specifies the action to take when any output is produced. In this case, it uses the standard output to print received messages instead of the default to just log messages.
`HEAR`	This specifies the action to take. In this case, it will receive multicast.
`--port 59595`	This sets the UDP port number to `59595`, which is used to identify/filter the multicast messages that will be accepted.
`--group 224.0.0.1`	This specifies the multicast group address to join. You can replace `224.0.0.1` with your desired multicast group address.

Steps to Run
1. Open your terminal.
2. Ensure you have the multicast module installed and accessible.
3. Run the command above, adjusting the port and group as needed for your specific use case.

Most users will want to stick to using HEAR when receiving multicast from the CLI. Alternatively, users can use the ephemeral RECV (by omitting the --daemon flag) to receive individual UDP messages, no more than one message at a time.

Tip

Caveat: RECV is much more useful if actually used in a loop, for example:

while true ; do  # until user Ctrl+C interrupts
    python3 -m multicast --use-std RECV --port 59595 --group 224.0.0.1 --groups 224.0.0.1
done

Develop in `Python`

While the command line interface is useful for prototyping, the Python API is better for the rest of the development process. So, once you are ready to proceed to implement a more advanced solution in Python, you can import the multicast library module with the usual import multicast logic.

How do I use this to send UDP Multicast?

With multicast installed and set up, this guide will assume you want to send a message to multicast group 224.0.0.1 on the UDP port 59595.

from the python module

Sending is similar to listening, you create a sender and pass it the options, except that unlike listeners and receivers, senders are always synchronous (e.g., allowing only one message per call).

from multicast import send

# Create a multicast sender
sender = send.McastSAY()

# Send a message
sender(group='224.0.0.1', port=59595, ttl=1, data='Hello, Multicast!')

from the CLI

python3 -m multicast SAY --group 224.0.0.1 --port 59595 --message "Hello World!"

Explanation of the Command-Line Options

Option	Description
`SAY`	This specifies the action to take. In this case, it will transmit multicast.
`--group 224.0.0.1`	This specifies the multicast group address to transmit messages to. You can replace `224.0.0.1` with your desired multicast group address.
`--port 59595`	This sets the UDP port number to `59595`, which is used by other members of the multicast group to help identify/filter the multicast messages to accept.
`--message`	This specifies the rest of the input is to be the message to transmit.
`"Hello World!"`	This specifies the multicast message content to transmit. In this case, it is the greeting "Hello World!"

Steps to Run
1. Open your terminal.
2. Ensure you have the multicast module installed and accessible.
3. Run the command above, adjusting the message, port and group as needed for your specific use case.

This command will allow you to quickly prototype sending multicast messages without needing to write a full Python script. You can test the sending functionality alongside the receiving functionality to ensure that messages are being transmitted and received correctly.

What is the basic `multicast` Python API?

Warning

Caveat: this example is still prototype focused, and should be considered a minimal implementation (e.g., lacks useful input validation, lacks error handling, etc.)

# Optional setup console logging
import logging
multicast_logging_sink = logging.getLogger()
multicast_logging_sink.setLevel(logging.INFO)  # increase default logging from multicast module
handler = logging.StreamHandler()  # example trivial log handler
multicast_logging_sink.addHandler(handler)

# imports
import multicast
from multiprocessing import Process
import random  # for random port

# Multicast group address and port
MCAST_GRP = "224.0.0.1"  # Replace with your multicast group address (use IPv4 dotted notation)
MCAST_PORT = int(random.SystemRandom().randint(49152, 65535))  # Replace with your multicast port

# Options for multicast listener
listener_options = {
    "is_daemon": True,  # bool: enable daemon mode
    "port": MCAST_PORT,  # int: UDP port for multicast
    "group": MCAST_GRP  # str: multicast group address (use IPv4 dotted notation)
}

# Create a multicast listener
listener = multicast.hear.McastHEAR()

# create a separate process for the listener
p = Process(
    target=listener,
    name="HEAR", kwargs=listener_options
)
p.daemon = listener_options["is_daemon"]
p.start()

# ... use CTL+C (or signal 2) to shutdown the server 'p'

Low-level with example handler:

# imports
import multicast
import random  # for random port
import functools  # for decorator func metadata

# Multicast group address and port
MCAST_GRP = "224.0.0.1"  # Replace with your multicast group address (use IPv4 dotted notation)
MCAST_PORT = int(random.SystemRandom().randint(49152, 65535))  # Replace with your multicast port

# Other important settings (Non-Multicast)
# Multicast does not care about the host IP, but the UDP protocol layer of the Python socket does
# There are 3 logical choices for the vast majority of users:
# 1. '0.0.0.0' for Promiscuous mode (Usually needs privileges to use on most Operating Systems)
# 2. The actual interface IPv4 dot notation address for unprivileged mode
# 3. None, Linux and macOS implementations can let the system choose by passing the
#    MCAST_GRP to the Python socket.bind operation (handled by multicast.skt when missing Host IP)
#    Windows users must use option 1 or 2 for now.
# This address is per socket (e.g., can be chosen per socket even if on a single interface)

# Options for multicast listener
listener_options = {
    "is_daemon": False,  # bool: enable/disable daemon mode
    "groups": [MCAST_GRP],  # list[str]: multicast group addresses (use IPv4 dotted notation list)
    "port": MCAST_PORT,  # int: UDP port for multicast
    "iface": None,  # str: System specific interface name, or None to let system choose
    "group": MCAST_GRP  # str: primary multicast group address (use IPv4 dotted notation)
}

# Example setup for Low-Level use-case
# Define a decorator to loop until able to print a string result of enough length


def printLoopStub(func):
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        while True:
            # cache function result
            cache = func(*args, **kwargs)
            if cache and len(cache) > 1:
                print( str( cache ) )
                break
            elif not cache:
                continue
            else:
                print("Result is too short.")
    return wrapper


# Example low-level handler
# Define a decorated handler to only return successfully received messages


@printLoopStub
def inputHandler():
    # Create an ephemeral multicast receiver
    receiver = multicast.recv.McastRECV()
    # create an empty default string
    out_string = str()
    # try to receive some multicast messages
    (didWork, buffer_string) = receiver(
        **listener_options
    )
    # check the result and "handle" if successful
    if didWork:
        out_string += buffer_string
    del receiver  # optionally cleanup receiver beforehand
    return out_string


# create the actual handler instance
inputHandler()

and elsewhere (e.g., in another module or even on another system); an example for the sender API:

# imports
import multicast
from multiprocessing import Process

# Multicast group address and port
MCAST_GRP = "224.0.0.1"  # Replace with your multicast group address (use IPv4 dotted notation)
MCAST_PORT = int(59595)  # Replace with your multicast port (use the same port as the listeners)

# Other important settings (Non-Multicast)
# Multicast does not care about the host IP, but the UDP protocol layer of the Python socket does
# The sender will default to letting the system choose
# HOST_BIND_IP = "0.0.0.0"

# Options for multicast sender
sender_options = {
    "port": MCAST_PORT,  # int: UDP port for multicast
    "group": MCAST_GRP,  # str: multicast group address (use IPv4 dotted notation)
    "data": "Default listener only knows: STOP"  # str: message content to try to _transmit_
}

# Create an ephemeral multicast sender
sender = multicast.send.McastSAY()

# create a separate process for the sender
p = Process(
    target=sender,
    name="SAY", kwargs=sender_options
)
p.daemon = False  # sender should not be a daemon

try:
    p.start()
except Exception as baton:
    p.join()  # good practice to handle clean up
    raise RuntimeError("multicast seems to have failed.") from baton  # re-raise
finally:
    # clean up some stuff
    if p:
        p.join() # if not already handled don't forget to join the process and other overhead
    # hint: if you use a loop and need to know the exit code
    didWork = (p is not None and p.exitcode <= 0)  # e.g. check for success

Warning

Caveat: the above examples assume the reader is knowledgeable about general interprocess communication theory and the standard python multiprocessing module and its use. Together these examples demonstrate a trivial message passing IPC using multicast python sockets.

Why does multicast module logging require explicit handler setup instead of just basicConfig?

The multicast module uses a null log handler by default as part of its logging architecture. This means that simply calling logging.basicConfig(level=logging.INFO) is insufficient to display the multicast module's log records, as there's no active handler to capture and output them.

To see multicast module logging output, you must explicitly add a log handler:

%%{ init: { 'theme': 'base', 'themeVariables': { 'darkMode': true, 'primaryColor': '#3e3e3e', 'background': 'transparent', 'primaryBorderColor': 'Orange', 'lineColor': 'darkOrange', 'secondaryColor': 'transparent', 'tertiaryColor': '#030303' }} }%%
sequenceDiagram
    participant Python as Your Python Code
    participant multicast as Multicast Module
    participant logging as `logging` module

    Python-->>logging: import logging
    opt setup custom log handler
      create participant Handler as Your Explicit Log Handler
      Python-->>Handler: create `logger` object
      Python-->>logging: add custom log handler
    end
    Python-->>multicast: import multicast
    multicast-->>logging: add default logging.NullHandler log handler
    multicast-->>logging: logs any multicast module output
    opt has added custom log handler
      logging->>Handler: handle logging
      Handler->>logging: the handler logic
    end
    multicast-->>Python: initialized
    loop main stuff
      alt doing whatever
        Python-->>Python: Your Non-multicast logic
      else using multicast
        Python-->>multicast: make API call to multicast
        opt module has log output
          multicast-->>logging: logs any multicast module output
          opt has handler
            logging->>Handler: handle logging
            Handler->>logging: the handler logic
          end
        end
        multicast-->>Python: reply to API call
      end
    end
    Python-xPython: done
    opt has custom log handler
      destroy Handler
      Python--xHandler: cleanup
    end

Required approach - explicit handler setup

Important

logging must be set up before importing multicast otherwise the NullHandler set up by multicast will be first and still be-able to intercept the module's logging.LogRecords just as it normally would.

import logging
multicast_logging_sink = logging.getLogger()
multicast_logging_sink.setLevel(logging.INFO)  # optional, otherwise the module is very quiet
handler = logging.StreamHandler()  # replace with your explicit handler
multicast_logging_sink.addHandler(handler)

This explicit setup ensures that:

The log level is set appropriately (e.g., logging.INFO)
An active handler (e.g., StreamHandler) is added to capture module log records
The multicast module's log output becomes visible to users
While logging.basicConfig() might seem simpler, it won't work with the multicast module's logging architecture and will result in no visible log output from the library.

How do I run and interpret the test suite for this project?

To run the test suite, follow the instructions in the Testing Guide.

The guide explains test organization, and how to run tests outside CI/CD.

How is continuous integration (CI) set up for this repository?

CI is configured as described in the CI Guide. Key points:

Automated builds and tests are run via GitHub Actions. Each pull request is tested for compatibility and code quality. The guide details the CI workflow files, environment variables, and what to expect when contributing changes.

What are the defaults?

Default Multicast Group

Important

The default multicast group address is 224.0.0.1.

From the API documentation:

The Value of "224.0.0.1" is chosen as a default multicast group as per RFC-5771 on the rational that this group address will be treated as a local-net multicast (caveat: one should use link-local for ipv6).

Default Multicast Bind Address

Note

The default multicast bind address is the default group. This is effectively 224.0.0.1.

Default TTL

Important

The default multicast Time-to-Live (TTL) is 1.

From RFC-1112 §6.1

... If the upper-layer protocol chooses not to specify a time-to-live, it should default to 1 for all multicast IP datagrams, so that an explicit choice is required to multicast beyond a single network.

From the API documentation:

A Value of 1 (one TTL) is chosen as per RFC-1112 §6.1 on the rational that an explicit value that could traverse beyond the local connected network should be chosen by the caller rather than the default value. This is in line with the principle of none, one or many.

Default Port

Important

The default UDP port used by multicast is 59595.

From the API documentation:

Arbitrary port to use by default, though any dynamic and free port would work.

While developers and network administrators must consider other factors in real-world deployments, it is fair to say any free port in the dynamic or "ephemeral" port range of 49152-65535 should work as far as the multicast module is concerned.

For SAY the port refers to the destination port.
for RECV and HEAR the port refers to the port to listen on.

Caution

It is best to specify the port in use at this time, as the default will not be properly assigned to multicast ( see related #62 ) by any central authority.

CLI exit code meanings

0 success

1 non-success - and is often accompanied by warnings or errors

2-225 failure of specific reason

Any exit value outside the range of 0-255 inclusive should be decoded with the formula: | input % 256 | which will yield the correct exit code.

Note

These are inline with CEP-8's POSIX-based guidelines.

How do I build the documentation?

Typically, the documentation will be automatically built by CI/CD and posted to the official documentation site.
However, if one still wishes to build the documentation manually, there is a make target specifically for building from source:
```
make build-docs
```

Building Documentation with a Custom Git Reference

By default, the documentation links to the stable branch on GitHub. To override this and link to the specific commit you're working on, set the DOCS_BUILD_REF environment variable:

# git clone ...
# git checkout <specific commit>
# shellcheck disable=SC2155
export DOCS_BUILD_REF=$("${GIT:-git}" rev-parse --verify HEAD)
make build-docs  # or your own documentation build command

This command dynamically sets DOCS_BUILD_REF to the current Git commit hash, ensuring that documentation links point to the exact version of your code.

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FAQ

Frequently Asked Questions

Where can I find project policies and conventions?

How do I get this running?

How do I use this `multicast` to receive some UDP multicast?

Prototype in `bash`

Develop in `Python`

How do I use this to send UDP Multicast?

from the python module

from the CLI

What is the basic `multicast` Python API?

Why does multicast module logging require explicit handler setup instead of just basicConfig?

Required approach - explicit handler setup

How do I run and interpret the test suite for this project?

How is continuous integration (CI) set up for this repository?

What are the defaults?

Default Multicast Group

Default Multicast Bind Address

Default TTL

Default Port

CLI exit code meanings

How do I build the documentation?

Building Documentation with a Custom Git Reference

Copyright (c) 2021-2026, Mr. Walls

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FAQ

Frequently Asked Questions

Where can I find project policies and conventions?

How do I get this running?

How do I use this multicast to receive some UDP multicast?

Prototype in bash

Develop in Python

How do I use this to send UDP Multicast?

from the python module

from the CLI

What is the basic multicast Python API?

Why does multicast module logging require explicit handler setup instead of just basicConfig?

Required approach - explicit handler setup

How do I run and interpret the test suite for this project?

How is continuous integration (CI) set up for this repository?

What are the defaults?

Default Multicast Group

Default Multicast Bind Address

Default TTL

Default Port

CLI exit code meanings

How do I build the documentation?

Building Documentation with a Custom Git Reference

Copyright (c) 2021-2026, Mr. Walls

How do I use this `multicast` to receive some UDP multicast?

Prototype in `bash`

Develop in `Python`

What is the basic `multicast` Python API?