All HTTP status codes

HTTP 502 Bad Gateway

Server Error

A gateway/reverse proxy got an invalid response from the upstream server — the proxy is fine, the thing behind it isn't answering properly.

Defined in RFC 9110

Common causes

  • Upstream app crashed, still booting, or listening on the wrong port
  • Upstream response malformed or connection reset
  • Wrong upstream host/port in proxy config (nginx upstream, load balancer target group)

How to fix it

  1. Check whether the upstream process is running and healthy

  2. Verify proxy upstream configuration and security-group/firewall paths

  3. Inspect upstream logs — the proxy log only says 'bad response'

  4. Add health checks so the LB stops routing to dead instances

Related status codes

503 Service Unavailable

The server is temporarily unable to handle the request — overloaded, in maintenance, or deliberately shedding load. Retry-After may say when to come back.

504 Gateway Timeout

A gateway/proxy didn't get a response from the upstream in time. The upstream may still be processing — the proxy just gave up waiting.

500 Internal Server Error

A generic 'something broke on the server' — an unhandled exception or misconfiguration with no more specific status to report.

Want to explore status codes interactively? Try the free HTTP Status Code Reference — search, categories and example responses, right in your browser.