Message Rewrite Facility

WARNING: Due to how this app currently handles its configuration, MRF is only usable if you're building your own docker image.

The Message Rewrite Facility (MRF) is a subsystem that is implemented as a series of hooks that allows the administrator to rewrite or discard messages.

Possible uses include:

• marking incoming messages with media from a given account or instance as sensitive
• rejecting messages from a specific instance
• rejecting reports (flags) from a specific instance
• removing/unlisting messages from the public timelines
• removing media from messages
• sending only public messages to a specific instance

The MRF provides user-configurable policies. The default policy is NoOpPolicy, which disables the MRF functionality. Bonfire also includes an easy to use policy called SimplePolicy which maps messages matching certain pre-defined criterion to actions built into the policy module. It is possible to use multiple, active MRF policies at the same time.

See the docs of ActivityPub.MRF.SimplePolicy for details about how to use it.

Use with Care

The effects of MRF policies can be very drastic. It is important to use this functionality carefully. Always try to talk to an admin before writing an MRF policy concerning their instance.

Writing your own MRF Policy

As discussed above, the MRF system is a modular system that supports pluggable policies. This means that an admin may write a custom MRF policy in Elixir or any other language that runs on the Erlang VM, by specifying the module name in the rewrite_policy config setting.

For example, here is a sample policy module which rewrites all messages to "new message content":

# This is a sample MRF policy which rewrites all Notes to have "new message
# content."
defmodule Site.RewritePolicy do
  @behavior ActivityPub.MRF

  # Catch messages which contain Note objects with actual data to filter.
  # Capture the object as `object`, the message content as `content` and the
  # entire activity itself as `activity`.
  @impl true
  def filter(%{"type" => "Create", "object" => %{"type" => "Note", "content" => content} = object} = message)
      when is_binary(content) do
    # Subject / CW is stored as summary instead of `name` like other AS2 objects
    # because of Mastodon doing it that way.
    summary = object["summary"]

    # edits go here.
    content = "new message content"

    # Assemble the mutated object.
    object =
      object
      |> Map.put("content", content)
      |> Map.put("summary", summary)

    # Assemble the mutated activity.
    {:ok, Map.put(activity, "object", object)}
  end

  # Let all other messages through without modifying them.
  @impl true
  def filter(message), do: {:ok, message}
end

If you save this file as lib/site/mrf/rewrite_policy.ex, it will be included when you next rebuild Bonfire. You can enable it in the configuration like so:

config :activity_pub, :instance,
  rewrite_policy: [
    ActivityPub.MRF.SimplePolicy,
    Site.RewritePolicy
  ]