Funkwhale includes a feature that mimics [Pleroma’s Message Rewrite Facility (MRF)](https://docs-develop.pleroma.social/backend/configuration/mrf/). The MRF enables instance admins to create custom moderation rules. You can use these rules to complement Funkwhale's [built-in moderation tools](../../moderator_documentation/index.md).
## 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](https://dev.funkwhale.audio/funkwhale/funkwhale/blob/stable/api/funkwhale_api/moderation/mrf_policies.py)).
- Domain and user blocking, when checking incoming messages ([code](https://dev.funkwhale.audio/funkwhale/funkwhale/blob/stable/api/funkwhale_api/federation/mrf_policies.py))
```{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:
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:
```{code} bash
# 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 {file}`.env` file by adding its name to the {attr}`FUNKWHALE_PLUGINS` list. Add this variable if it's not there.
## Test your MRF policy
To make the job of writing and debugging MRF policies easier, we provide a management command.
