502 Bad Gateway

An HTTP 502 Bad Gateway error means that a server acting as a gateway or proxy received an invalid or incomplete response from an upstream server it contacted to fulfill the request. This commonly occurs in architectures where a reverse proxy like Nginx or a load balancer sits in front of application servers. End users see a failed page load, while developers need to investigate the communication between the proxy and the backend.

Common causes

• The upstream application server has crashed, is unresponsive, or is not running
• Firewall or security group rules blocking traffic between the proxy and the backend server
• The upstream server is returning malformed HTTP responses or headers
• DNS resolution failures preventing the gateway from locating the upstream server
• Network timeouts due to the upstream server taking too long to start sending a response

How to fix it

• Verify that the upstream application server process is running and listening on the expected port
• Check proxy configuration to ensure upstream server addresses, ports, and protocols are correct
• Increase proxy timeout settings (e.g., proxy_read_timeout in Nginx) if the backend needs more time to respond
• Inspect network connectivity and firewall rules between the proxy and backend servers
• Review upstream server logs for crashes, out-of-memory errors, or other failures that could produce invalid responses

Related HTTP errors