451 Unavailable For Legal Reasons (RFC 7725) means the server is denying access to the resource because of a legal demand, for example a court order, a government censorship request or a copyright takedown. The number is a reference to Ray Bradbury's novel Fahrenheit 451.
The response should explain the details in its body: who made the demand, under which law or regulation, and to whom and what it applies. Servers should also send a Link header with rel="blocked-by" that identifies the entity implementing the block (for example an ISP or the hosting provider), not the authority that demanded it. A 451 does not say whether the resource exists.
Some websites use 451 to block visitors from regions whose laws they do not want to comply with, for example European visitors after the GDPR came into force. A 451 is cacheable by default.
Example response
HTTP/1.1 451 Unavailable For Legal Reasons
Date: Fri, 06 Nov 2026 10:15:32 GMT
Link: <https://isp.example.net/legal>; rel="blocked-by"
Content-Type: text/html; charset=utf-8
Content-Length: 175
<!doctype html>
<title>Unavailable For Legal Reasons</title>
<h1>Unavailable For Legal Reasons</h1>
<p>This page is not available in your country because of a court order.</p>SEO and redirect checking
Search engines treat 451 like other 4xx errors and do not index the URL when their crawlers receive it. Blocks are often regional, so a crawler in another country may still see the page. TrueURL checks run in Cloudflare's network, so the result may differ from what visitors in your own country get.
Common causes and use cases
- A court order or government request requires blocking specific content.
- A website blocks visitors from certain countries for regulatory reasons, for example to avoid data protection obligations.
- An ISP or network operator blocks a site and shows a 451 page.
- A copyright or other legal takedown applies to the content.