Apache HTTP Server Version 2.1

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# Error Messages - Frequently Asked Questions

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## Error Messages

- Invalid argument: core_output_filter: writing data to the network
- AcceptEx failed
- Premature end of script headers
- Permission denied

### Invalid argument: core_output_filter: writing data to the network

Apache uses the `sendfile` syscall on platforms where it is available in order to speed sending of responses. Unfortunately, on some systems, Apache will detect the presence of `sendfile` at compile-time, even when it does not work properly. This happens most frequently when using network or other non-standard file-system.

Symptoms of this problem include the above message in the error log and zero-length responses to non-zero-sized files. The problem generally occurs only for static files, since dynamic content usually does not make use of `sendfile`.

To fix this problem, simply use the `EnableSendfile` directive to disable `sendfile` for all or part of your server. Also see the `EnableMMAP`, which can help with similar problems.

### AcceptEx Failed

If you get error messages related to the `AcceptEx` syscall on win32, see the `Win32DisableAcceptEx` directive.

### Premature end of script headers

Most problems with CGI scripts result in this message written in the error log together with an `Internal Server Error` delivered to the browser. A guide to helping debug this type of problem is available in the CGI tutorial.

### Permission denied

A `Permission denied` error in the `error_log`, accompanied by a `Forbidden` message to the client usually indicates a problem with your filesystem permissions, rather than a problem in the Apache HTTP Server configuration files. Check to make sure that the `User` and `Group` running the child processes has adequate permission to access the files in question. Also check that the directory and all parent directories are at least searchable for that user and group (i.e., `chmod +x`).

Recent releases of Fedora Core and other Linux distributions using SELinux have additional access restrictions beyond those used by the basic filesystem. Violations of these restrictions will also result in a `Permission denied` message. See the Fedora SELinux FAQ and Apache SELinux Policy Document.

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