5xx Server Error

HTTP 502 Bad Gateway

The server, while acting as a gateway or proxy, received an invalid response from the upstream server. This often indicates a problem with the backend server, not the gateway itself.

Common Causes

  • Backend server is down or unreachable
  • Upstream server returned an invalid or malformed response
  • Network issues between the proxy/load balancer and backend
  • Backend server crashed or is being restarted

How to Fix

  1. 1Check if the backend server is running and healthy
  2. 2Review proxy/load balancer logs for connection errors
  3. 3Verify network connectivity between proxy and upstream
  4. 4Restart the backend server if it's unresponsive

Example

HTTP 502 Bad Gateway
GET /app HTTP/1.1 → Nginx (proxy)
→ Nginx tries to reach backend at port 3000
→ Backend is down
→ 502 Bad Gateway

Related Server Error Codes

500

Internal Server Error

501

Not Implemented

503

Service Unavailable

504

Gateway Timeout

505

HTTP Version Not Supported

506

Variant Also Negotiates

507

Insufficient Storage

508

Loop Detected

510

Not Extended

511

Network Authentication Required