I have just created Mastofeeder.com, a proxy from any RSS feed to Mastodon (or ActivityPub) that requires no extra logins or accounts.

Mastofeeder allows you to search your favorite websites with an RSS feed directly from Mastodon. Let's say you're a fan of xkcd.com. All you have to do is search for the handle @xkcd.com@mastofeeder.com. If an RSS feed is found using the search logic, Mastofeeder will create a virtual RSS feed user on Mastofeeder server, which starts posting all items from the RSS feed within the next hour. It's that simple!

Mastofeeder was developed using TypeScript and is open-source, so feel free to check out the project to learn more about it or even contribute to its development.

Building Mastofeeder

The protocol that joins Mastodon servers to each other is called ActivityPub. It's a simple protocol that defines a few common actions that actors (users) can perform and how servers send them to each other. For this blog post I'll concentrate mainly on the server to server commnunication part.

Finding a user

Servers have both active and passive parts; you can use a standard HTTP server to answer when another server looks up a username on your server:

GET /.well-known/webfinger?resource=acct:@jesse@jessesownserver.com

HTTP 200 OK

{
  "subject": "acct:@jesse@@jessesownserver.com",
  "links": [
    {
      "rel": "self",
      "type": "application/activity+json",
      "href": "https://jessesownserver.com/users/jesse"
    }
  ]
}

This is called WebFinger and it's used to find out where the user's profile is located. This kind of request is sent every time you search for a user at Mastodon.

But this is not enugh; Mastodon also needs to know where the user's inbox is. This is done by looking up the user's profile:

GET /users/jesse

HTTP 200 OK

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "id": "https://jessesownserver.com/users/jesse",
  "type": "Person",
  "inbox": "https://jessesownserver.com/users/jesse/inbox"
}

And this information is enough for Mastodon to show the user's profile in the search results; it can send messages like "follow" and to the user's inbox.

Following a user

When you follow a user, a two-way communication needs to be established between the two servers holding the users. This is done by sending a "follow" message to the user's inbox:

POST /users/jesse/inbox

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "id": "https://johnsserver.com/users/john-the-follower/<>",
  "type": "Follow",
  "actor": "https://johnsserver.com/users/john-the-follower",
  "object": "https://jessesownserver.com/users/jesse",
}

The user's server will then send a "Accept" message back to the follower's inbox:

POST /users/john-the-follower/inbox

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "id": "https://jessesownserver.com/users/jesse/<>",
  "type": "Accept",
  "actor": "https://jessesownserver.com/users/jesse",
  "object": {
    "@context": "https://www.w3.org/ns/activitystreams",
    "id": "https://johnsserver.com/users/john-the-follower/<>",
    "type": "Follow",
    "actor": "https://johnsserver.com/users/john-the-follower",
    "object": "https://jessesownserver.com/users/jesse",
  }
}

Before this message is sent, the user interface shows a "pending" status for the follow request.

Creating a new post

Now that we have a follower relationship established, we can send a new post to the follower's inbox:

POST /users/john-the-follower/inbox

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "id": "https://jessesownserver.com/users/jesse/<>",
  "type": "Create",
  "actor": "https://jessesownserver.com/users/jesse",
  "object": {
    "@context": "https://www.w3.org/ns/activitystreams",
    "id": "https://jessesownserver.com/users/jesse/<>",
    "type": "Note",
    "attributedTo": "https://jessesownserver.com/users/jesse",
    "content": "Hello world!"
  }
}

And this message will pop up in the follower's timeline.

Wrapping up

Understanding the concepts in this blog post should be enough to be able to create a simple ActivityPub proxy server for RSS, Twitter or any other service that has an API.

The real world is a bit more complex, as you need to handle signing your own messages, but there are plenty of libraries that can help you with that (or you can chceck my implementation).