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.
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**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.
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.
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"`.
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
