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Message Rewrite Facility

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. Pleroma 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.

Quarantine Instances

You have the ability to prevent from private / followers-only messages from federating with specific instances. Which means they will only get the public or unlisted messages from your instance.

If, for example, you're using MIX_ENV=prod aka using production mode, you would open your configuration file located in config/prod.secret.exs and edit or add the option under your :instance config object. Then you would specify the instance within quotes.

config :pleroma, :instance,
  [...]
  quarantined_instances: ["instance.example", "other.example"]

Using `SimplePolicy`

SimplePolicy is capable of handling most common admin tasks.

To use SimplePolicy, you must enable it. Do so by adding the following to your :instance config object, so that it looks like this:

config :pleroma, :instance,
  [...]
  rewrite_policy: Pleroma.Web.ActivityPub.MRF.SimplePolicy

Once SimplePolicy is enabled, you can configure various groups in the :mrf_simple config object. These groups are:

media_removal: Servers in this group will have media stripped from incoming messages.
media_nsfw: Servers in this group will have the #nsfw tag and sensitive setting injected into incoming messages which contain media.
reject: Servers in this group will have their messages rejected.
federated_timeline_removal: Servers in this group will have their messages unlisted from the public timelines by flipping the to and cc fields.
report_removal: Servers in this group will have their reports (flags) rejected.

Servers should be configured as lists.

Example

This example will enable SimplePolicy, block media from illegalporn.biz, mark media as NSFW from porn.biz and porn.business, reject messages from spam.com, remove messages from spam.university from the federated timeline and block reports (flags) from whiny.whiner:

config :pleroma, :instance,
  rewrite_policy: [Pleroma.Web.ActivityPub.MRF.SimplePolicy]

config :pleroma, :mrf_simple,
  media_removal: ["illegalporn.biz"],
  media_nsfw: ["porn.biz", "porn.business"],
  reject: ["spam.com"],
  federated_timeline_removal: ["spam.university"],
  report_removal: ["whiny.whiner"]

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":

defmodule Pleroma.Web.ActivityPub.MRF.RewritePolicy do
  @moduledoc "MRF policy which rewrites all Notes to have 'new message content'."
  @behaviour Pleroma.Web.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
  # message itself as `message`.
  @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"]

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

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

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

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

  @impl true
  def describe do
    mrf_sample = Pleroma.Config.get(:mrf_sample)

    {:ok, %{mrf_sample: mrf_sample}}
  end
end

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

config :pleroma, :instance,
  rewrite_policy: [
    Pleroma.Web.ActivityPub.MRF.SimplePolicy,
    Pleroma.Web.ActivityPub.MRF.RewritePolicy
  ]

Please note that the Pleroma developers consider custom MRF policy modules to fall under the purview of the AGPL. As such, you are obligated to release the sources to your custom MRF policy modules upon request.

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