502 Bad Gateway

Invalid responses from an upstream server cause the gateway or proxy to return 502 Bad Gateway.

Table of Contents

Usage

The 502 Bad Gateway status code means the server is acting as a proxy or gateway for an upstream server. After forwarding the HTTP request, the upstream server returned an invalid response. The time to resolve this problem is unpredictable.

When the origin server returns an HTTP error status code, the same status code and related information is expected to be forwarded to the client for the most accurate description of the problem.

When the proxy receives no response from the upstream server at all, a 504 Gateway Timeout status code is more appropriate.

Note

502 Bad Gateway means the proxy received an invalid response from the backend. The backend responded, but the response was malformed or unexpected. 504 Gateway Timeout means the backend did not respond at all within the timeout period. 502 points to a backend crash or protocol error, while 504 points to a backend that is slow or unreachable. Neither code indicates the client did anything wrong.

SEO impact

Googlebot treats 5xx errors as a signal to temporarily reduce the crawl rate across the site. Previously indexed URLs are preserved initially, but extended downtime causes Google to drop them from the index. Any content returned with a 502 response is ignored. Google treats 500, 502, and 503 identically in this regard. A 5xx error on the robots.txt file is far more severe, even when every other page on the site returns 200. Google halts crawling for up to 12 hours, then falls back to a cached copy for up to 30 days. If the error persists, Google fully deindexes the site. Bingbot also treats 5xx responses as crawl failures. Persistent 5xx errors prevent indexing in Bing.

Example

The client requests a resource and the server responds with a 502 Bad Gateway status code because the upstream server returned an invalid response.

Request

GET /news HTTP/1.1
Host: www.example.re

Response

HTTP/1.1 502 Bad Gateway
Content-Type: text/html; charset=UTF-8
Content-Length: 134

<html>
  <head>
    <title>Bad Gateway</title>
  </head>
  <body>
    <p>The server is unreachable at this time.</p>
  </body>
</html>

How to fix

Check /var/log/nginx/error.log or the equivalent proxy error log first. The log message identifies the upstream failure mode: connection refused, prematurely closed connection, or invalid header.

Common root causes and their fixes:

  • Backend not running. The upstream process (PHP-FPM, Gunicorn, Node.js, Tomcat) crashed or was never started. Verify with systemctl status php-fpm or equivalent, then restart the service. Check the backend's own log for crash details.
  • Wrong upstream address or port. In nginx, the proxy_pass or fastcgi_pass directive must point to the correct socket or host:port. A mismatch between the configured port and the port the backend listens on produces an immediate 502.
  • DNS resolution failure. When proxy_pass references a hostname, DNS failures prevent nginx from reaching the backend. Test with dig or nslookup from the proxy server. Consider using an IP address or a local /etc/hosts entry to eliminate DNS as a variable.
  • Upstream response too large for buffers. If the backend sends headers exceeding the default buffer size, nginx logs "upstream sent too big header." Increase proxy_buffer_size (or fastcgi_buffer_size) and proxy_buffers/fastcgi_buffers in the nginx configuration.
  • Backend timeout. A slow backend triggers a 502 when the connection closes mid-response. Increase proxy_read_timeout in nginx or ProxyTimeout in Apache. Also raise the backend's own timeout (e.g. request_terminate_timeout in PHP-FPM).
  • Firewall blocking. A host-level firewall (iptables, nftables, security groups) on the backend server blocking the proxy's IP causes connection refused. Verify connectivity with curl http://backend:port/ from the proxy.

In Apache reverse proxy setups, check ProxyPass/ProxyPassReverse directives and confirm mod_proxy and mod_proxy_http are enabled. Run apachectl configtest before restarting.

After applying fixes, run nginx -t to validate configuration, then reload.

Code references

.NET

HttpStatusCode.BadGateway

Rust

http::StatusCode::BAD_GATEWAY

Rails

:bad_gateway

Go

http.StatusBadGateway

Symfony

Response::HTTP_BAD_GATEWAY

Python3.5+

http.HTTPStatus.BAD_GATEWAY

Java

java.net.HttpURLConnection.HTTP_BAD_GATEWAY

Apache HttpComponents Core

org.apache.hc.core5.http.HttpStatus.SC_BAD_GATEWAY

Angular

@angular/common/http/HttpStatusCode.BadGateway

See also

Last updated: April 4, 2026