511 Network Authentication Required

Server error: the request may be fine, but the server failed to carry it out.

When a server should send it. By a captive portal (hotel, airport or train Wi-Fi) that intercepts traffic until the user signs in. The body should link to the login page. Real websites never send it.

Common causes. Hotel, airport or café Wi-Fi with a login page (a captive portal).

What to do. Open any http page in the browser to get the login page, then sign in.

Browsers and HTTP clients. Operating systems detect captive portals by fetching a known test URL and open the login page themselves; browsers show the body.

Google Search. Googlebot treats 5xx responses as a sign of trouble and temporarily slows its crawl rate. Already indexed URLs are kept at first, but are dropped if the errors continue. 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/1.1 511 Network Authentication Required
Content-Type: text/html; charset=utf-8

Network Authentication Required

location /example {
    return 511;
}

RewriteEngine On
RewriteRule ^example$ - [R=511,L]

http_response_code(511);
echo 'Network Authentication Required';
exit;

res.writeHead(511, { 'Content-Type': 'text/plain' });
res.end('Network Authentication Required');

# in a view function
return 'Network Authentication Required', 511