HTTP Status Code Lookup
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Look up what a status code means and, more usefully, what tends to cause it. Every code here carries the practical note rather than the specification sentence — why a 502 is a different problem from a 504, why 401 is misnamed, and what the Cloudflare 520–526 range is telling you about your origin server. Search by number or by words if you only remember the name.
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How to use HTTP Status Codes
- 1
Enter one or more status codes, one per line — or a word like “timeout”.
- 2
Read the name, the class, and the usual cause.
- 3
Use the class to tell whose problem it is: 4xx is the request, 5xx is the server.
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Frequently asked questions
What is the difference between 401 and 403?+
401 means unauthenticated — the server does not know who you are, and it must send a WWW-Authenticate header saying how to prove it. 403 means authenticated but not permitted. The name “Unauthorized” on 401 is a long-standing misnomer that gets the two swapped in a great deal of production code.
When should I use 301 versus 308?+
Both are permanent. 301 historically allows a client to change a POST into a GET when following it, and most do; 308 guarantees the method and body are preserved. For redirecting pages, 301 is conventional and universally understood. For an API endpoint that receives POSTs, 308 is the one that behaves correctly.
What causes a 502 Bad Gateway?+
A proxy — nginx, a load balancer, a CDN — got an invalid or empty response from the application behind it. Usually the app crashed, is not listening, or died mid-response. It is distinct from 504, where the app is running but did not answer in time; that difference points at completely different fixes.
What are the 520–526 codes?+
Cloudflare-specific, not part of any standard. They all mean Cloudflare could not get a usable response from your origin: 521 is connection refused, 522 is a TCP timeout, 524 is the origin taking over 100 seconds, and 525/526 are TLS problems between Cloudflare and your server. Every one of them points at the origin, not at Cloudflare.
Which code should a maintenance page return?+
503 Service Unavailable, with a Retry-After header. It tells search engines the outage is temporary and to come back — where a 200 on a maintenance page invites them to index the maintenance notice as your content, and a 404 invites them to drop the page altogether.