What causes a 502 Bad Gateway error?

A 502 error occurs when a reverse proxy or edge server (like Nginx, Cloudflare, or AWS ALB) receives an invalid or empty response from the upstream application server (e.g. Node.js or Python process crashed).

Updated: August 2026 • Verified v1.2

HTTP Status Codes Directory & Searchable Reference

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HTTP Status Code Categorization: 1xx Informational, 2xx Success, 3xx Redirection, 4xx Client Error, 5xx Server Error

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Understanding Your Results

Provides instant search across standard HTTP status codes (e.g. 200, 201, 301, 304, 400, 401, 403, 404, 422, 429, 500, 502, 503, 504) with practical troubleshooting causes.

Practical Guidance & Next Steps

4xx codes indicate client-side problems (bad auth, invalid payload, wrong URL); 5xx codes indicate server-side infrastructure crashes or gateway timeouts.

What is this?

A developer networking reference for identifying, debugging, and understanding HTTP response codes.

How does it work?

1. Search by status code (e.g. 404, 502) or error description. 2. Filter by category (2xx Success, 4xx Client Error, 5xx Server Error). 3. Copy code definitions and root cause explanations in 1 click.

Why use it?

Quickly identifies whether an API failure originated from a client-side request or a backend server outage.

Technical Scope & Calculation Limitations

Covers IANA RFC 9110 HTTP Semantics status codes.

Frequently Asked Questions

HTTP 401 means the client is unauthenticated (missing or invalid login credentials). HTTP 403 means the client is authenticated, but does not have permission to access the requested resource.

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