502 Bad Gateway

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

When a server should send it. By a gateway or proxy (nginx, a load balancer, a CDN) when the server behind it returned an invalid response or closed the connection. Applications themselves almost never send it.

Common causes. The application server (PHP-FPM, Node, Gunicorn) crashed, restarted or refused the connection.

What to do. Retry in a minute. Site owners should check that the backend service is running and look at the proxy's error log.

Browsers and HTTP clients. Browsers show the proxy's error page. Retrying after a moment often works, because the usual cause is a backend restarting or crashing.

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/1.1 502 Bad Gateway
Content-Type: text/plain; charset=utf-8

Bad Gateway

location /example {
    return 502;
}

RewriteEngine On
RewriteRule ^example$ - [R=502,L]

http_response_code(502);
echo 'Bad Gateway';
exit;

res.writeHead(502, { 'Content-Type': 'text/plain' });
res.end('Bad Gateway');

# in a view function
return 'Bad Gateway', 502