--- title: WebFinger description: Translate `user@domain` mentions to actor profile URIs. menu: docs: weight: 20 parent: spec --- ## What is WebFinger, and why is it used? {#intro} 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](https://tools.ietf.org/html/rfc7033) 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](https://tools.ietf.org/html/rfc7565), 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 {#example} 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" >}} ```json { "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" >}} ```json { "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.example`and 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" >}}