This appendix explains the HTTP result codes, provided by the Web server, that can be send to visitors as replies to requests.
The 4xx class of status code is intended for cases in which the client seems to have erred. 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 status codes are applicable to any request method. User agents should display any included entity to the user.
If the client is sending data, a server implementation using TCP should be careful to ensure that the client acknowledges receipt of the packet(s) containing the response, before the server closes the input connection. If the client continues sending data to the server after the close, the server's TCP stack will send a reset packet to the client, which may erase the client's unacknowledged input buffers before they can be read and interpreted by the HTTP application.
The request could not be understood by the server due to malformed syntax. The client should not repeat the request without modifications.
The request requires user authentication. The response must include a WWW-Authenticate header field (RFC 2616 document, section 14.47) 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 with the entity that was specified in the response, because that entity might include relevant diagnostic information.
Currently, this code is not implemented by most web servers. It is reserved for future use.
The server understood the request, but is refusing to fulfil 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. If the server does not wish to make this information available to the client, the status code 404 (Not Found) can be used instead.
The server has not found anything matching the Request-URI. No indication is given of whether the condition is temporary or permanent. The 410 (Gone) status code should be used if the server knows, through some internally configurable mechanism, that an old resource is permanently unavailable and has no forwarding address. 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.
The method specified in the Request-Line is not allowed for the resource identified by the Request-URI. The response must include an Allow header containing a list of valid methods for the requested resource.
The resource identified by the request is only capable of generating response entities which have content characteristics not acceptable according to the accept headers sent in the request.
Unless it was a HEAD request, the response should include an entity containing a list of available entity characteristics and location(s) from which the user or user agent can choose the one most appropriate. The entity format is specified by the media type given in the Content-Type header field. Depending upon the format and the capabilities of the user agent, selection of the most appropriate choice may be performed automatically. However, this specification does not define any standard for such automatic selection.
HTTP/1.1 servers are allowed to return responses which are not acceptable according to the accept headers sent in the request. In some cases, this may even be preferable to sending a 406 response. User agents are encouraged to inspect the headers of an incoming response to determine if it is acceptable.
This code is similar to 401 (Unauthorized), but indicates that the client must first authenticate itself with the proxy. The proxy must return a Proxy-Authenticate header field containing a challenge applicable to the proxy for the requested resource. The client may repeat the request with a suitable Proxy-Authorization header field.
The client did not produce a request within the time that the server was prepared to wait. The client may repeat the request without modifications at any later time.
The request could not be completed due to a conflict with the current state of the resource. This code is only allowed in situations where it is expected that the user might be able to resolve the conflict and resubmit the request. The response body should include enough information for the user to recognize the source of the conflict. Ideally, the response entity would include enough information for the user or user agent to fix the problem. However, that might not be possible, and is not required.
Conflicts are most likely to occur in response to a PUT request. For example, if versioning was being used and the entity being PUT included changes to a resource which conflict with those made by an earlier (third-party) request, the server might use the 409 response to indicate that it cannot complete the request. In this case, the response entity would likely contain a list of the differences between the two versions in a format defined by the response Content-Type.
The requested resource is no longer available at the server, and no forwarding address is known. This condition is expected to be considered permanent. Clients with link-editing capabilities should delete references to the Request-URI after user approval. If the server does not know, or has no facility to determine, whether or not the condition is permanent, the status code 404 (Not Found) should be used instead. This response is cacheable unless indicated otherwise.
The 410 response is primarily intended to assist the task of web maintenance by notifying the recipient that the resource is intentionally unavailable, and that the server owners desire that remote links to that resource be removed. Such an event is common for limited-time, promotional services and for resources belonging to individuals no longer working at the server's site. It is not necessary to mark all permanently unavailable resources as "gone", or to keep the mark for any length of time. That is left to the discretion of the server owner.
The server refuses to accept the request without a defined Content- Length. The client may repeat the request if it adds a valid Content-Length header field containing the length of the message-body in the request message.
The precondition specified in one or more of the request-header fields evaluated to false when it was tested on the server. This response code allows the client to place preconditions on the current resource meta-information (header field data) and, therefore, prevent the requested method from being applied to a resource other than the one intended.
The server is refusing to process a request because the request entity is larger than the server is willing or able to process. The server may close the connection to prevent the client from continuing the request.
If the condition is temporary, the server should include a Retry- After header field to indicate that it is temporary and after what time the client may try again.
The server is refusing to service the request because the Request-URI is longer than the server is willing to interpret. This rare condition is only likely to occur when a client has improperly converted a POST request to a GET request with long query information, when the client has descended into a URI "black hole" of redirection (that is, a redirected URI prefix that points to a suffix of itself), or when the server is under attack by a client attempting to exploit security holes present in some servers using fixed-length buffers for reading or manipulating the Request-URI.
The server is refusing to service the request because the entity of the request is in a format not supported by the requested resource for the requested method.
A server should return a response with this status code if a request included a Range request-header field (RFC 2616 document, section 14.35), and none of the range-specifier values in this field overlap the current extent of the selected resource, and the request did not include an If-Range request-header field. (For byte-ranges, this means that the first- byte-pos of all of the byte-range-spec values were greater than the current length of the selected resource).
When this status code is returned for a byte-range request, the response should include a Content-Range entity-header field specifying the current length of the selected resource (see RFC 2616 document, section 14.16). This response must not use the multipart/byteranges content- type.
The expectation specified in an Expect request-header field (see RFC 2616 document, section 14.20) could not be met by this server, or, if the server is a proxy, the server has unambiguous evidence that the request could not be met by the next-hop server.
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. 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. User agents should display any included entity to the user. These response codes are applicable to any request method.
The server encountered an unexpected condition which prevented it from fulfilling the request.
The server does not support the functionality required to fulfil 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.
Section 10 of the RFC 2616 document describes this as "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 fulfil the request.
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. If known, the length of the delay may be indicated in a Retry-After header.
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.
Section 10 of the RFC 2616 document describes this as "504 Gateway Timeout". The server, while acting as a gateway or proxy, did not receive a timely response from the upstream server specified by the URI (such as HTTP, FTP, or LDAP) or some other auxiliary server (such as DNS) it needed to access in attempting to complete the request.
Note:Some deployed proxies are known to return 400 or 500 when DNS lookups time out.
The server does not support, or refuses to support, the HTTP protocol version that was used in the request message. The server is indicating that it is unable or unwilling to complete the request using the same major version as the client other than with this error message. The response should contain an entity describing why that version is not supported, and what other protocols are supported by that server.
Number of hits requested by the client to which the server did not respond to at all. This could be caused by a server-error and/or network-error.
Network errors are hits which were not delivered completely from the TCP level view. There are several possible causes:
This status indicates a server-related problem with the connection. Any of the following situations will be reported:
Server resets the connection.
This is an indication of a server application problem. It is not possible to verify that all data was transmitted or received correctly.
Server sends incorrect data.
The data sent from the server is malformed in such a way that it is not possible to extract the high-level HTTP information. This can be caused by a number of factors, such as packet loss, too many out-of sequence packets, and so on.
Client went away.
Sometimes the client might disappear unexpectedly (computer crash, modem crash, ISP down, or some other hardware problem that results in immediate loss of connectivity). This situation manifests itself as a server error, because the server eventually times out, and resets the connection. It is not possible to determine how much of the transmitted data was received by the client.
Impact on visitors
The visitor receives a server-error message, or at least not the requested information. In some cases, the partially received information is shown to the visitor. This is often an indication that there are problems with the server.
Server errors should not occur regularly. If a high number of server-errors is reported, the network and server components should be investigated using Network Protocol Analysis (NPA) tools.
Some indications for analysis on the cause of server errors:
Load: too many connections to the server and/or load balancer can lead to resource problems.
Balancer: is the load distributed correctly over all the servers, or is one server consistently becoming overloaded and generating errors?
URLs: are only specific application URLs generating this type of problems?
A server timeout occurs when a server fails to reply to a client request. In a timeout situation, the server never transmits any data over the line; that is, no response, or part thereof, is ever sent out. (Server aborts are reported under completion status 4).
The exact interpretation of this completion status is:
The client sent a complete HTTP request.
No data at all was sent back by the server.
Note:A timeout means no data was sent. That is, the server's TCP stack might acknowledge that the client's request was received by sending an acknowledgment segment, but the server application itself is unable to send back any data.
Impact on visitor
The client never received any content. The server simply failed to respond. This can only indicate a network or server application problem.
The cause of server-timeouts can be investigated by analyzing the networks where this problem occurs. Server timeouts occur sporadically, and should not be considered problematic unless a high percentage of requests is involved. In cases where all clients experience a high percentage of timeouts, network and server components should be investigated using network analysis tools and application performance testing tools.
The received client or server header packets was truncated. This was caused by a network problem timeout.
One exception which should normally be seen as a network-error. But since the cause of this issue cannot be solved by the customer and is normally seen as standard behavior, we do not add this one in the failed cubes and see the hit as "success".
Client aborted the transfer, possibly because the client closed the browser, or clicked reload, or clicked away, or was redirected, while the page was still loading.