HTTP Pocket Reference

Client Request Incomplete (400 Range)

Sometimes the server just can’t process the request. Either something was wrong with the document or with the request itself. By far, the server status code that web users are most familiar with is 404 (Not Found), the code returned when the requested document does not exist. This isn’t because it’s the most common code that servers return, but because it’s one of the few codes that the client passes to the user rather than intercepting and handling it in its own way.

For example, when the server sends a 401 (Unauthorized) code, the client does not pass the code directly to the user. Instead, it triggers the client to prompt the user for a username and password, and then resend the request with that information supplied. With the 401 status code, the server supplies the WWW-Authenticate header to specify the authentication scheme and realm it needs authorization for, and the client returns the username and password for that scheme and realm in the Authorization header.

When testing clients you have written yourself, watch out for code 400 (Bad Request), indicating a syntax error in your client’s request, and code 405 (Method Not Allowed), which declares that the method the client used for the document is not valid. (Along with the 405 code, the server sends an Allow header, listing the accepted methods for the document.)

The 408 (Request Time-out) code means that the client’s request wasn’t completed, and the server gave up waiting for the client ...