webfinger

WebFinger is a protocol for discovering information about people and other entities on the internet. It provides a standard mechanism to retrieve metadata about a resource identified by a URI such as an email address or acct: URI, using a lightweight web service.

How it works: A client resolves information by issuing a request to the host domain's Well-Known WebFinger endpoint, typically https://example.org/.well-known/webfinger?resource=acct:[e-mail]. The server responds with a Resource Descriptor in JSON (JRD) or XML (XRD) describing the subject and related links, including endpoints for profiles, public keys, or other services. The response includes a subject field and a links array; each link has rel, href, type, etc.

Uses and adoption: WebFinger is used to discover endpoints for federated services, notably ActivityPub and other decentralized identities. It enables actor discovery, profile retrieval, and key material discovery for secure communications. It has been used by email-style identifiers and by some social platforms to locate user profiles and services across domains.

Standards and history: The WebFinger protocol was formalized in RFC 7033 (2013). It builds on earlier web-based resource descriptions and supports multiple serialization formats via JSON Resource Descriptor (JRD) or XRD. While not universally implemented, it remains a foundational tool for interoperable discovery in federated ecosystems.

Privacy and limitations: Because requests reveal a resource identifier and host, WebFinger raises privacy considerations. It relies on TLS and rate-limiting; endpoints may require authentication or restrict information. It is not a universal discovery mechanism and depends on the target domain implementing an endpoint.