451 Unavailable For Legal Reasons means the resource cannot be accessed due to legal demands — censorship, a court order, or government regulation. The number 451 references Ray Bradbury's novel Fahrenheit 451 about information censorship.
Where 403 Forbidden blocks for lack of permission, 451 transparently signals that specific content is blocked for legal reasons. The response may include a rel="blocked-by" Link header identifying who demanded the block.
GET /article/banned-topic HTTP/1.1
Host: news.example.comHTTP/1.1 451 Unavailable For Legal Reasons
Link: <https://spqr.example.org/legal>; rel="blocked-by"
Content-Type: text/html
<h1>451 Unavailable For Legal Reasons</h1>
<p>This content is blocked in your region.</p>app.get('/article/:slug', (req, res) => {
if (isBlockedInRegion(req.ip, req.params.slug)) {
return res.status(451)
.set('Link', '<https://example.org/legal>; rel="blocked-by"')
.send('Unavailable For Legal Reasons');
}
// serve the article...
});