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.




 
RFC 1945                        HTTP/1.0                        May 1996


9.5  Server Error 5xx

   Response status codes beginning with the digit "5" indicate cases in
   which the server is aware that it has erred or is incapable of
   performing the request. If the client has not completed the request
   when a 5xx code is received, it should immediately cease sending data
   to the server. Except when responding to a HEAD request, the server
   should include an entity containing an explanation of the error
   situation, and whether it is a temporary or permanent condition.
   These response codes are applicable to any request method and there
   are no required header fields.

   500 Internal Server Error

   The server encountered an unexpected condition which prevented it
   from fulfilling the request.

   501 Not Implemented

   The server does not support the functionality required to fulfill
   the request. This is the appropriate response when the server does
   not recognize the request method and is not capable of supporting
   it for any resource.

   502 Bad Gateway

   The server, while acting as a gateway or proxy, received an invalid
   response from the upstream server it accessed in attempting to
   fulfill the request.

   503 Service Unavailable

   The server is currently unable to handle the request due to a
   temporary overloading or maintenance of the server. The implication
   is that this is a temporary condition which will be alleviated
   after some delay.

       Note: The existence of the 503 status code does not imply
       that a server must use it when becoming overloaded. Some
       servers may wish to simply refuse the connection.

10.  Header Field Definitions

   This section defines the syntax and semantics of all commonly used
   HTTP/1.0 header fields. For general and entity header fields, both
   sender and recipient refer to either the client or the server,
   depending on who sends and who receives the message.





 
RFC 1945                        HTTP/1.0                        May 1996


10.1  Allow

   The Allow entity-header field lists the set of methods supported by
   the resource identified by the Request-URI. The purpose of this field
   is strictly to inform the recipient of valid methods associated with
   the resource. The Allow header field is not permitted in a request
   using the POST method, and thus should be ignored if it is received
   as part of a POST entity.

       Allow          = "Allow" ":" 1#method

    Example of use:

       Allow: GET, HEAD

   This field cannot prevent a client from trying other methods.
   However, the indications given by the Allow header field value should
   be followed. The actual set of allowed methods is defined by the
   origin server at the time of each request.

   A proxy must not modify the Allow header field even if it does not
   understand all the methods specified, since the user agent may have