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WebFinger Translate `user@domain` mentions to actor profile URIs.
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What is WebFinger, and why is it used?

On Mastodon, user profiles can be hosted either locally on the same website as yours, or remotely on a completely different website. The same username may be used on a different domain. Therefore, a Mastodon user's full mention consists of both the username and the domain, in the form @username@domain. In practical terms, @user@example.com is not the same as @user@example.org. If the domain is not included, Mastodon will try to find a local user named @username. However, in order to deliver to someone over ActivityPub, the @username@domain mention is not enough -- mentions must be translated to an HTTPS URI first, so that the remote actor's inbox and outbox can be found.

Enter WebFinger. WebFinger as described in RFC 7033 is a spec that defines a method for resolving links to a resource, given only a URI on a particular server. This allows anyone to look up where a resource is located without having to know its exact location beforehand; for example, by email or phone number. This lookup is directed at the endpoint /.well-known/webfinger, and a resource query parameter is passed along with the lookup. The resource URI used with Mastodon is the acct: URI as described in RFC 7565, with the username of a profile that is hosted on a particular domain.

{{< hint style="danger" >}} Because Mastodon heavily relies on mentions for addressing other profiles, WebFinger is required for fully interoperating with Mastodon. Users can generally load profiles by searching for the direct HTTPS URI if they know it, or for the username@domain address, but Mastodon's internal logic depends almost completely on acct: URIs or username@domain representations. Searching for any objects or profiles from an ActivityPub implementation without WebFinger will fail because the author cannot be converted to a user in the local database. {{< /hint >}}

Sample WebFinger flow

Suppose we want to lookup the user @Gargron hosted on the mastodon.social website.

Just make a request to that domain's /.well-known/webfinger endpoint, with the resource query parameter set to an acct: URI.

{{< code title="https://mastodon.social/.well-known/webfinger?resource=acct:gargron@mastodon.social" >}}

{
  "subject": "acct:Gargron@mastodon.social",
  "aliases": [
    "https://mastodon.social/@Gargron",
    "https://mastodon.social/users/Gargron"
  ],
  "links": [
    {
      "rel": "http://webfinger.net/rel/profile-page",
      "type": "text/html",
      "href": "https://mastodon.social/@Gargron"
    },
    {
      "rel": "self",
      "type": "application/activity+json",
      "href": "https://mastodon.social/users/Gargron"
    },
    {
      "rel": "http://ostatus.org/schema/1.0/subscribe",
      "template": "https://mastodon.social/authorize_interaction?uri={uri}"
    }
  ]
}

{{< /code >}}

You can parse this JSON response to find a link with your desired type. For ActivityPub id, we are interested in finding application/activity+json specifically.

This way, we have translated @Gargron@mastodon.social to https://mastodon.social/users/Gargron and we can now interact over ActivityPub by referring to this URI as id where appropriate.

{{< code title="Sample activity" >}}

{
"id": "https://social.example/activities/1",
"type": "Create",
"actor": "https://social.example/actors/1",
"object": {
    "id": "https://social.example/objects/1",
    "type": "Note",
    "content": "Hello, Gargron!"
},
"to": "https://mastodon.social/users/Gargron"
}

{{< /code >}}

Note in the above example that social.example does not use the same URI structure as Mastodon. Thus, we cannot guess the actor id given only the username and domain. However, if social.example supports WebFinger, then we can get this id by requesting https://social.example/.well-known/webfinger?resource=acct:username@social.exampleand parsing the response for a link with the application/ld+json; profile="https://www.w3.org/ns/activitystreams" or application/activity+json type. This link should also have the link relation rel="self".

Mastodon's requirements for WebFinger

When given an account in the form username@domain or @username@domain, Mastodon will do the following:

  • Construct an acct: URI using that username and domain
  • Make a WebFinger request for that resource

Using that WebFinger response, Mastodon will check the following:

  • The subject is present
  • The links array contains a link with rel of self and type of either application/ld+json; profile="https://www.w3.org/ns/activitystreams" or application/activity+json
    • The href for this link resolves to an ActivityPub actor

Using that ActivityPub actor representation (which may be provided directly, without the initial WebFinger request), Mastodon will do the following:

  • Take preferredUsername and the hostname of the actor's server
  • Construct an acct: URI using that username and domain
  • Make a Webfinger request for that resource

If the subject matches the resource, then the process stops here. Otherwise, if the subject contains a different canonical account URI, then Mastodon will perform an additional Webfinger request for that canonical account URI in order to ensure that this new resource links to the same ActivityPub actor with the same criteria being checked.

In other words, the following cases are valid:

  • Asking example.com for the resource acct:alice@example.com yields a link to an actor on the domain example.com with a preferredUsername of alice, and the subject matches the requested resource acct:alice@example.com
  • Asking example.com for the resource acct:alice@example.com yields a link to an actor on the domain ap.example.com with a preferredUsername of alice
    • ...then, asking ap.example.com for the resource acct:alice@ap.example.com yields a subject of acct:alice@example.com and a link to the same actor

See also

{{< caption-link url="https://github.com/mastodon/mastodon/blob/main/app/services/activitypub/fetch_remote_actor_service.rb" caption="app/services/activitypub/fetch_remote_actor_service.rb" >}}

{{< caption-link url="https://github.com/mastodon/mastodon/blob/main/app/services/resolve_account_service.rb" caption="app/services/resolve_account_service.rb" >}}

{{< caption-link url="https://github.com/mastodon/mastodon/blob/main/app/lib/webfinger.rb" caption="app/lib/webfinger.rb" >}}