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18. Server Response Codes

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The first line in an HTTP server response indicates whether the client request was successful or not, and why. The status is given with a three-digit server response code (also known as a status code) and a descriptive message.

Status codes are usually generated by Web servers, but they might also be generated by CGI scripts that bypass the server's precooked headers and supply their own. Status codes are grouped as follows:

Code Range Response Meaning
100-199 Informational
200-299 Client request successful
300-399 Client request redirected, further action necessary
400-499 Client request incomplete
500-599 Server errors

HTTP defines only a few specific codes in each range, although servers may define their own as needed. If a client receives a code that it does not recognize, it should understand its basic meaning from its numerical range. While most Web browsers handle codes in the 100-, 200-, and 300- range silently, some error codes in the 400- and 500- range are commonly reported back to the user (e.g., "404 Not Found").

Informational

A response in the range of 100-199 is informational, indicating that the client's request was received and is being processed.

100 Continue

The initial part of the request has been received and the client may continue with its request.

101 Switching Protocols

The server is complying with a client request to switch protocols to the one specified in the Upgrade header field.

Client Request Successful

A response in the range of 200-299 means that the client's request was successful.

200 OK

The client's request was successful, and the server's response contains the requested data.

201 Created

This status code is used whenever a new URI is created. With this result code, the Location header (described in Chapter 19, HTTP Headers) is given by the server to specify where the new data was placed.

202 Accepted

The request was accepted but not immediately acted upon. More information about the transaction may be given in the entity body of the server's response. There is no guarantee that the server will actually honor the request, even though it may seem like a legitimate request at the time of acceptance.

203 Non-Authoritative Information

The information in the entity header is from a local or third-party copy and not from the original server.

204 No Content

A status code and header is given in the response, but there is no entity body in the reply. Browsers should not update their document view upon receiving this response. This is a useful code for an imagemap handler to return when the user clicks on useless or blank areas of an image.

205 Reset Content

The browser should clear the form used for this transaction for additional input. Appropriate for data-entry CGI applications.

206 Partial Content

The server is returning partial data of the size requested. Used in response to a request specifying a Range header. The server must specify the range included in the response with the Content-Range header.

Redirection

A response code in the 300-399 range indicates that the request was not performed and the client needs to take further action for a successful request.

300 Multiple Choices

The requested URI refers to more than one resource. For example, the URI could refer to a document that has been translated into many languages. The entity body returned by the server could have a list of more specific data about how to choose the correct resource.

301 Moved Permanently

The requested URI is no longer used by the server, and the operation specified in the request was not performed. The new location for the requested document is specified in the Location header. All future requests for the document should use the new URI.

302 Moved Temporarily

The requested URI has moved, but only temporarily. The Location header points to the new location. Immediately after receiving this status code, the client should use the new URI to resolve the request, but the old URI should be used for all future requests.

303 See Other

The requested URI can be found at a different URI (specified in the Location header) and should be retrieved by a GET on that resource.

304 Not Modified

This is the response code to an If-Modified-Since header, where the URI has not been modified since the specified date. The entity body is not sent, and the client should use its own local copy.

305 Use Proxy

The requested URI must be accessed through the proxy in the Location header.

Client Request Incomplete

A response code in the range of 400-499 means that the client's request was incomplete, and may indicate further information is required from the client.

400 Bad Request

This response code indicates that the server detected a syntax error in the client's request.

401 Unauthorized

The result code is given along with the WW-Authenticate header to indicate that the request lacked proper authorization, and the client should supply proper authorization when requesting this URI again.

402 Payment Required

This code is not yet implemented in HTTP.

403 Forbidden

The request was denied for a reason the server does not want to (or has no means to) indicate to the client.

404 Not Found

The document at the specified URI does not exist.

405 Method Not Allowed

This code is given with the Allow header and indicates that the method used by the client is not supported for this URI.

406 Not Acceptable

The URI specified by the client exists, but not in a format preferred by the client. Along with this code, the server provides the Content-Language, Content-Encoding, and Content-Type headers.

407 Proxy Authentication Required

The proxy server needs to authorize the request before forwarding it. Used with the Proxy-Authenticate header.

408 Request Time-out

This response code means the client did not produce a full request within some predetermined time (usually specified in the server's configuration), and the server is disconnecting the network connection.

409 Conflict

This code indicates that the request conflicts with another request or with the server's configuration. Information about the conflict should be returned in the data portion of the reply.

410 Gone

This code indicates that the requested URI no longer exists and has been permanently removed from the server.

411 Length Required

The server will not accept the request without a Content-Length header supplied in the request.

412 Precondition Failed

The condition specified by one or more If... headers in the request evaluated to false.

413 Request Entity Too Large

The server will not process the request because its entity body is too large.

414 Request-URI Too Long

The server will not process the request because its request URI is too large.

415 Unsupported Media Type

The server will not process the request because its entity body is in an unsupported format.

Server Errors

Response codes in the range of 500-599 indicate that the server encountered an error and may be unable to perform the client's request.

500 Internal Server Error

This code indicates that a part of the server (for example, a CGI program) has crashed or encountered a configuration error.

501 Not Implemented

This code indicates that the client requested an action that cannot be performed by the server.

502 Bad Gateway

This code indicates that the server (or proxy) encountered invalid responses from another server (or proxy).

503 Service Unavailable

This code means that the service is temporarily unavailable, but should be restored in the future. If the server knows when it will be available again, a Retry-After header may also be supplied.

504 Gateway Time-out

This response is like 408 (Request Time-out) except that a gateway or proxy has timed out.

505 HTTP Version not supported

The server will not support the HTTP protocol version used in the request.


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