RFC 7725 / START HERE
HTTP 451, before the details
HTTP means Hypertext Transfer Protocol. Status 451 is named Unavailable For Legal Reasons; it reports a legal access obstacle.
A 451 response says that a server is denying access because of a legal demand. The server returning it can be an intermediary, not just the publisher of the resource.
The idea in one picture
Request a resource
Apply the legal access block
Explain the 451 response
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
A general refusal can hide why access failed. RFC 7725 provides a more explicit status, recommends an explanation and a link identifying the entity that implements the block, and states the limits of that signal.
A familiar example
A fictional filter refuses a request for a report and returns 451. Its blocked-by link identifies the filter; the body explains the fictional demand. The status does not prove the report exists or that every legal restriction will be disclosed.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- Report a legal access blockA server reports that a legal demand caused it to deny access; the server returning 451 need not be the origin.→
- Identify who implements the blockA blocked-by link identifies the entity implementing a block, while the body can explain who demanded it.→
- Apply cache controls to the responseA 451 response is cacheable by default, subject to method rules and explicit cache controls; this does not force a cache to store it.→
- Read the limits of the signalA 451 neither proves the resource exists nor guarantees that every legal restriction will be reported transparently.→
This introduction explains RFC 7725 (2016). Read the original RFC ↗