451 Unavailable For Legal Reasons is an HTTP status code that signals the server denies access to a resource because of a legal demand. It is defined in RFC 7725 and belongs to the 4xx client error class.
At a glance
- Status code
- 451 Unavailable For Legal Reasons
- Class
- 4xx (client error)
- Duration
- not specified by RFC 7725
- In Google
- handled like other 4xx codes except 429; 451 is not listed separately in Google's documentation
- Caching
- cacheable by default
- Specification
- RFC 7725 (February 2016)
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What does 451 mean?
According to RFC 7725, the blocking party does not have to be the origin server. Internet providers, search engines, cache providers and DNS servers can also block. The response should explain in its body who made the legal demand, which law or regulation applies and to which people and resources. A Link header with rel="blocked-by" can identify the party that implements the block. MDN adds that this header names the blocker and not the party that demanded it. A 451 response is cacheable by default. The RFC notes that clients cannot rely on 451 being used, because a legal order may not allow disclosing the demand.
Which legal requirements trigger a block is a matter of the applicable law; the status code itself makes no statement on that. This is not legal advice.
The number refers to Ray Bradbury’s novel Fahrenheit 451.
451 and SEO
Google’s documentation says that content from URLs returning 4xx codes (except 429) is ignored and that indexed URLs are removed from the index. It does not mention 451 specifically, so how Google treats it follows from those general rules (inference).
Compared with similar codes:
- 403 Forbidden denies access without naming a reason.
- 404 and 410 mean the page does not exist (any more).
- 451 means the page exists and is withheld for a legal reason.
Example
HTTP/1.1 451 Unavailable For Legal Reasons
Link: <https://example.com/legal-notice>; rel="blocked-by"
Content-Type: text/html
<h1>Unavailable for legal reasons</h1>
<p>This content is blocked in your region following a court order.</p>
Common mistakes
- Returning 403 or 404 for a legal block, so users and clients get no explanation.
- A 451 response without an explanatory body, although RFC 7725 asks for the demanding party, the legal basis and the affected scope.
Sources
- RFC 7725: An HTTP Status Code to Report Legal Obstacles – IETFrfc-editor.org
- 451 Unavailable For Legal Reasons – MDN Web Docsdeveloper.mozilla.org
- How HTTP status codes, and network and DNS errors affect Google Search – Google Search Centraldevelopers.google.com
- Error 451 – Cloudflare Docsdevelopers.cloudflare.com
- HTTP 451 – Wikipedia (secondary, for history)en.wikipedia.org
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