When a request is made to your server for a page on your site (for instance, when a user accesses your page in a browser), your server returns an HTTP status code in response to the request. This status code provides information about the status of the request. 4xx errors are client or browser request type errors while 5xx errors orginated at the web server but both are are error in nature. 4xx errors are typically more cosmetic and have a negative effect on user experience and search enigne classification while 5xx are catastrohpic causing the web page to totally fail.
Common IIS Status Codes and Causes.
| Code |
Description |
| 200 (OK) |
IIS successfully processed the request.
|
| 304 (Not Modified) |
The client browser requests a document that is already in the cache, and the document has not been modified since the document was cached. The client browser uses the cached copy of the document instead of downloading the document from the server.
|
| 400 (Bad Request) |
The Http.sys file blocks IIS from processing the request because of a problem in the request. Typically, this HTTP status code means that the request contains characters or sequences that are not valid or that the request contradicts the security settings in the Http.sys file.
|
| 401 (Access Denied) |
IIS defines several HTTP status codes that indicate a more specific cause of a 401 error. The following specific HTTP status codes are displayed in the client browser but are not displayed in the IIS log:
- 401.1 - Logon failed.
- 401.2 - Logon failed due to server configuration.
- 401.3 - Unauthorized due to ACL on resource.
- 401.4 - Authorization failed by filter.
- 401.5 - Authorization failed by ISAPI/CGI application
|
| 403 (Forbidden) |
IIS defines the following HTTP status codes that indicate a more specific cause of a 403 error:
- 403.1 - Execute access forbidden.
- 403.2 - Read access forbidden.
- 403.3 - Write access forbidden.
- 403.4 - SSL required.
- 403.5 - SSL 128 required.
- 403.6 - IP address rejected.
- 403.7 - Client certificate required.
- 403.8 - Site access denied.
- 403.12 - Mapper denied access.
- 403.13 - Client certificate revoked.
- 403.14 - Directory listing denied.
- 403.16 - Client certificate is untrusted or invalid.
- 403.17 - Client certificate has expired or is not yet valid.
- 403.18 - Cannot execute requested URL in the current application pool.
- 403.19 - Cannot execute CGI applications for the client in this application pool.
|
| 404 (Not Found) |
IIS defines the following HTTP status codes that indicate a more specific cause of a 404 error:
- 404.0 - Not found.
- 404.2 - ISAPI or CGI restriction.
- 404.3 - MIME type restriction.
- 404.4 - No handler configured.
- 404.5 - Denied by request filtering configuration.
- 404.6 - Verb denied.
- 404.7 - File extension denied.
- 404.8 - Hidden namespace.
- 404.9 - File attribute hidden.
- 404.10 - Request header too long.
- 404.11 - Request contains double escape sequence.
- 404.12 - Request contains high-bit characters.
- 404.13 - Content length too large.
- 404.14 - Request URL too long.
- 404.15 - Query string too long.
|
| 405 (Method Not Allowed) |
The request is made by using an HTTP method that is not valid.
|
| 406 (Invalid MIME Type) |
The request is made by using an Accept header that contains a MIME value that is not valid.
|
| 412 (Precondition Failed) |
The request is made by using an If-Match request header that contains a value that is not valid.
|
| 500 (Internal Server Error) |
IIS defines the following HTTP status codes that indicate a more specific cause of a 500 error:
- 500.0 - Module or ISAPI error occurred.
- 500.11 - Application is shutting down on the Web server.
- 500.12 - Application is busy restarting on the Web server.
- 500.13 - Web server is too busy.
- 500.15 - Direct requests for Global.asax are not allowed.
- 500.19 - Configuration data is invalid.
- 500.21 - Module not recognized.
- 500.100 - Internal ASP error.
|
| 501 (Not Implemented) |
Header values specify a configuration that is not implemented.
|
| 502 (Bad Gateway) |
IIS defines the following HTTP status codes that indicate a more specific cause of a 502 error:
- 502.1 - CGI application timeout.
- 502.2 - Bad gateway.
|
| 503 (Service Unavailable) |
IIS defines the following HTTP status codes that indicate a more specific cause of a 503 error:
- 503.0 - Application pool unavailable.
- 503.2 - Concurrent request limit exceeded.
|
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