HTTP 502

The 502 Bad Gateway error is an HTTP status code that occurs when a server acting as a gateway or proxy receives an invalid or faulty response from another server in the communication chain. ^[1]

Specification

The 502 Bad Gateway error is defined in the RFC 7231 ^[2] specification, in Section 6.6.3. ^[3] The 502 Bad Gateway error is part of the Hypertext Transfer Protocol (HTTP) specification. It is defined in the HTTP/1.1 specification by the World Wide Web Consortium (W3C). According to the specification, the 502 status code indicates that the server, while acting as a gateway or proxy, received an invalid response from an upstream server.

Occurrence

The 502 Bad Gateway error can occur due to several reasons. Some of the common causes include: ^[4]

• Server overload: If a proxy server or gateway is overwhelmed with a large number of requests and is unable to process all of them in a timely manner, it can result in a 502 error. Overload can be caused by high user activity, poor configuration, or inadequate resources on the server.
• Misconfiguration of server infrastructure: A misconfiguration of the server infrastructure, including the proxy server or gateway, can lead to communication problems and trigger the 502 error. For example, invalid DNS settings, firewall restrictions, or inadequate routing rules can result in connection errors between the servers.
• Upstream server failure: If the server that the proxy or gateway is accessing is not functioning properly or experiencing an outage, it can lead to the 502 Bad Gateway error. Failure can occur due to maintenance, hardware issues, software problems, or other technical issues.
• Network issues: Unstable network connections, packet loss, or other network problems can cause communication disruptions between the proxy server and the upstream server. This can result in a 502 error as the proxy server fails to receive a valid response from the upstream server.

Example

Consider the following example to illustrate the occurrence of a 502 Bad Gateway error:

A user attempts to access a website by entering the URL in their browser. A proxy server first receives the user's request. Acting as an intermediary, the proxy server forwards the request to an upstream server. The upstream server processes the request but encounters an internal error. Instead of sending a valid response, the upstream server returns an invalid or faulty response to the proxy server. The proxy server, unable to handle the invalid response, returns a 502 Bad Gateway error to the user's browser. The user sees the error page indicating the 502 Bad Gateway status. ^[5]

References

1. developer.mozilla.org 502 Bad Gateway
2. RFC7231.txt
3. Fielding; et al. (June 1999). 6.6.3 502 Bad Gateway. IETF. p. 63. sec. 6.6.3. doi: 10.17487/RFC7231 . RFC 7231.
4. How do I solve the problem with the 502 Bad Gateway status code?
5. "502 Bad Gateway". http.dev. Retrieved 31 July 2024.

Sources

• HTTP status code 502 Bad Gateway at http-statuscode.com
• HTTP 502 Bad Gateway Error Explained at KeyCDN
• HTTP/1.1: Status Code Definitions - World Wide Web Consortium (W3C)
• Troubleshooting 502 Errors - Microsoft Azure
• Common Causes of a 502 Bad Gateway Error and How to Fix It - NGINX