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Message Rewrite Facility (MRF)

Funkwhale includes a feature that mimics Pleroma’s Message Rewrite Facility (MRF). The MRF enables instance admins to create custom moderation rules. You can use these rules to complement Funkwhale’s built-in moderation tools.

Architecture

The MRF is a pluggable system that processes messages and forwards them to a list of registered policies. Each policy can mutate the message, leave it as is, or discard it.

We implement some of Funkwhale’s built-in moderation tools as a MRF policy. For example:

Allow-list, when checking incoming messages (code).
Domain and user blocking, when checking incoming messages (code)

Note

Pleroma MRF policies can also affect outgoing messages. This is not currently supported in Funkwhale.

Disclaimer

Writing custom MRF rules can impact the performance and stability of your pod. It can also affect message delivery. Every time your pod receives a message it calls your policy.

The Funkwhale project consider all custom MRF policies to fall under the purview of the AGPL. This means you’re required to release the source of your custom MRF policy modules publicly.

Write your first MRF policy

MRF policies are written as Python 3 functions that take at least one payload parameter. This payload is the raw ActivityPub message, received via HTTP, following the HTTP signature check.

In the example below we write a policy that discards all Follow requests from listed domains:

import urllib.parse
from funkwhale_api.moderation import mrf

BLOCKED_FOLLOW_DOMAINS = ['domain1.com', 'botdomain.org']

# You need to register the policy to apply it.

# The name can be anything you want. It will appear in the mrf logs
@mrf.inbox.register(name='blocked_follow_domains')
def blocked_follow_domains_policy(payload, **kwargs):
    actor_id = payload.get('actor')
    domain = urllib.parse.urlparse(actor_id).hostname
    if domain not in BLOCKED_FOLLOW_DOMAINS:
        # Raising mrf.Skip isn't necessary but it provides
        # info in the debug logs. Otherwise, you can return:
        raise mrf.Skip("This domain isn't blocked")

    activity_type = payload.get('type')
    object_type = payload.get('object', {}).get('type')

    if object_type == 'Follow' and activity_type == 'Create':
        raise mrf.Discard('Follow from blocked domain')

You need to store this code in a Funkwhale plugin. To create one, execute the following:

# Plugin names can only contain ASCII letters, numbers and underscores.
export PLUGIN_NAME="myplugin"
# This is the default path where Funkwhale will look for plugins.
# If you want to use another path, update this path and ensure
# your PLUGINS_PATH is also included in your .env file.
export PLUGINS_PATH="/srv/funkwhale/plugins/"
mkdir -p $PLUGINS_PATH/$PLUGIN_NAME
cd $PLUGINS_PATH/$PLUGIN_NAME

touch __init__.py  # required to make the plugin a valid Python package
# Create the required apps.py file to register our plugin in Funkwhale.
cat > apps.py <<EOF
from django.apps import AppConfig

class Plugin(AppConfig):
    name = "$PLUGIN_NAME"

EOF

Once you’ve created the plugin, put your code in an mrf_policies.py file. Place this file inside the plugin directory. Next, enable the plugin in your .env file by adding its name to the FUNKWHALE_PLUGINS list. Add this variable if it’s not there.