The 4xx class of status code is intended for cases in which the
client seems to have erred. These status codes are applicable to any request method.
400 Bad Request
The request could not be understood by the server due to malformed
syntax. The client should not repeat the request without
modifications. 1
401 Unauthorized
The request requires user authentication. The response must include
a WWW-Authenticate header field containing a
challenge applicable to the requested resource. The client may
repeat the request with a suitable Authorization header field
. If the request already included Authorization
credentials, then the 401 response indicates that authorization has
been refused for those credentials. If the 401 response contains
the same challenge as the prior response, and the user agent has
already attempted authentication at least once, then the user
should be presented the entity that was given in the response,
since that entity may include relevant diagnostic information.
403 Forbidden
The server understood the request, but is refusing to fulfill it.
Authorization will not help and the request should not be repeated.
If the request method was not HEAD and the server wishes to make
public why the request has not been fulfilled, it should describe
the reason for the refusal in the entity body. This status code is
commonly used when the server does not wish to reveal exactly why
the request has been refused, or when no other response is
applicable.
404 Not Found
The server has not found anything matching the Request-URI. No
indication is given of whether the condition is temporary or
permanent. If the server does not wish to make this information
available to the client, the status code 403 (forbidden) can be
used instead.
407 Proxy Authentication Required **
This code is similar to 401 (Unauthorized), but indicates that the
client MUST first authenticate itself with the proxy.
409 Conflict **
The request could not be completed due to a conflict with the current state of the resource. Ideally, the response entity would include enough information for the user or user agent to fix the problem; however, that may not be possible and is not required.