451 Unavailable For Legal Reasons

Client error: the problem lies with the request. Repeating it unchanged will normally fail again.

When a server should send it. When access is refused because of a legal demand, such as a court order or a government block. The response should say who made the demand, and may name them in a Link header with rel="blocked-by".

Common causes. Content withheld in some countries, or a legal takedown.

What to do. Nothing a visitor can fix. The response should say who demanded the block.

Browsers and HTTP clients. Browsers show the body. Crawlers treat it like any other 4xx.

Google Search. Like every 4xx except 429, it tells Google the content does not exist: an indexed URL is removed from the index, and a new one is not processed. Crawl frequency for the URL gradually drops. Source: Google Search Central, How HTTP status codes, and network and DNS errors affect Google Search.

Caching. Not cacheable by default (RFC 9110 section 15.1 does not list it). A cache stores it only when the response says so with Cache-Control or Expires.

HTTP/1.1 451 Unavailable For Legal Reasons
Link: <https://authority.example/order>; rel="blocked-by"

Unavailable For Legal Reasons

location /example {
    return 451;
}

RewriteEngine On
RewriteRule ^example$ - [R=451,L]

http_response_code(451);
echo 'Unavailable For Legal Reasons';
exit;

res.writeHead(451, { 'Content-Type': 'text/plain' });
res.end('Unavailable For Legal Reasons');

# in a view function
return 'Unavailable For Legal Reasons', 451