Status Codes (Apache: The Definitive Guide, 3rd Edition)

21.2. Status Codes

The HTTP 1.1 standard defines many status codes that can be returned as a response to a request. Most of the functions involved in processing a request return OK, DECLINED, or a status code. DECLINED generally means that the module is not interested in processing the request; OK means it did process it, or that it is happy for the request to proceed, depending on which function was called. Generally, a status code is simply returned to the user agent, together with any headers defined in the request structure's headers_out table. At the time of writing, the status codes predefined in httpd.h were as follows:

#define HTTP_CONTINUE                      100
#define HTTP_SWITCHING_PROTOCOLS           101
#define HTTP_OK                            200
#define HTTP_CREATED                       201
#define HTTP_ACCEPTED                      202
#define HTTP_NON_AUTHORITATIVE             203
#define HTTP_NO_CONTENT                    204
#define HTTP_RESET_CONTENT                 205
#define HTTP_PARTIAL_CONTENT               206
#define HTTP_MULTIPLE_CHOICES              300
#define HTTP_MOVED_PERMANENTLY             301
#define HTTP_MOVED_TEMPORARILY             302
#define HTTP_SEE_OTHER                     303
#define HTTP_NOT_MODIFIED                  304
#define HTTP_USE_PROXY                     305
#define HTTP_BAD_REQUEST                   400
#define HTTP_UNAUTHORIZED                  401
#define HTTP_PAYMENT_REQUIRED              402
#define HTTP_FORBIDDEN                     403
#define HTTP_NOT_FOUND                     404
#define HTTP_METHOD_NOT_ALLOWED            405
#define HTTP_NOT_ACCEPTABLE                406
#define HTTP_PROXY_AUTHENTICATION_REQUIRED 407
#define HTTP_REQUEST_TIME_OUT              408
#define HTTP_CONFLICT                      409
#define HTTP_GONE                          410
#define HTTP_LENGTH_REQUIRED               411
#define HTTP_PRECONDITION_FAILED           412
#define HTTP_REQUEST_ENTITY_TOO_LARGE      413
#define HTTP_REQUEST_URI_TOO_LARGE         414
#define HTTP_UNSUPPORTED_MEDIA_TYPE        415
#define HTTP_INTERNAL_SERVER_ERROR         500
#define HTTP_NOT_IMPLEMENTED               501
#define HTTP_BAD_GATEWAY                   502
#define HTTP_SERVICE_UNAVAILABLE           503
#define HTTP_GATEWAY_TIME_OUT              504
#define HTTP_VERSION_NOT_SUPPORTED         505
#define HTTP_VARIANT_ALSO_VARIES           506

For backward compatibility, these are also defined:

#define DOCUMENT_FOLLOWS    HTTP_OK
#define PARTIAL_CONTENT     HTTP_PARTIAL_CONTENT
#define MULTIPLE_CHOICES    HTTP_MULTIPLE_CHOICES
#define MOVED               HTTP_MOVED_PERMANENTLY
#define REDIRECT            HTTP_MOVED_TEMPORARILY
#define USE_LOCAL_COPY      HTTP_NOT_MODIFIED
#define BAD_REQUEST         HTTP_BAD_REQUEST
#define AUTH_REQUIRED       HTTP_UNAUTHORIZED
#define FORBIDDEN           HTTP_FORBIDDEN
#define NOT_FOUND           HTTP_NOT_FOUND
#define METHOD_NOT_ALLOWED  HTTP_METHOD_NOT_ALLOWED
#define NOT_ACCEPTABLE      HTTP_NOT_ACCEPTABLE
#define LENGTH_REQUIRED     HTTP_LENGTH_REQUIRED
#define PRECONDITION_FAILED HTTP_PRECONDITION_FAILED
#define SERVER_ERROR        HTTP_INTERNAL_SERVER_ERROR
#define NOT_IMPLEMENTED     HTTP_NOT_IMPLEMENTED
#define BAD_GATEWAY         HTTP_BAD_GATEWAY
#define VARIANT_ALSO_VARIES HTTP_VARIANT_ALSO_VARIES

Details of the meaning of these codes are left to the HTTP 1.1 specification, but there are a couple worth mentioning here. HTTP_OK (formerly known as DOCUMENT_FOLLOWS) should not normally be used, because it aborts further processing of the request. HTTP_MOVED_TEMPORARILY (formerly known as REDIRECT) causes the browser to go to the URL specified in the Location header. HTTP_NOT_MODIFIED (formerly known as USE_LOCAL_COPY) is used in response to a header that makes a GET conditional (e.g., If-Modified-Since).