Apache HTTP Server Version 2.4

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# Apache Module mod_log_debug

Description: | Additional configurable debug logging
Status: | Experimental
Module Identifier: | log_debug_module
Source File: | mod_log_debug.c
Compatibility: | Available in Apache 2.3.14 and later

### Topics

- Examples

### Directives

### Bugfix checklist

- httpd changelog
- Known issues
- Report a bug

## Examples

1. Log message after request to /foo/* is processed:

       <Location "/foo/">
         LogMessage "/foo/ has been requested"
       </Location>

2. Log message if request to /foo/* is processed in a sub-request:

       <Location "/foo/">
         LogMessage "subrequest to /foo/" hook=type_checker "expr=-T %{IS_SUBREQ}"
       </Location>

   The default log_transaction hook is not executed for sub-requests, therefore we have to use a different hook.
3. Log message if an IPv6 client causes a request timeout:

       LogMessage "IPv6 timeout from %{REMOTE_ADDR}" "expr=-T %{IPV6} && %{REQUEST_STATUS} = 408"

   Note the placing of the double quotes for the `expr=` argument.
4. Log the value of the "X-Foo" request environment variable in each stage of the request:

       <Location "/">
         LogMessage "%{reqenv:X-Foo}" hook=all
       </Location>

   Together with microsecond time stamps in the error log, `hook=all` also lets you determine the times spent in the different parts of the request processing.

## LogMessage Directive

Description: | Log user-defined message to error log
Syntax: | `LogMessage message [hook=hook] [expr=expression]`
Default: | `Unset`
Context: | directory
Status: | Experimental
Module: | mod_log_debug

This directive causes a user defined message to be logged to the error log. The message can use variables and functions from the ap_expr syntax. References to HTTP headers will not cause header names to be added to the Vary header. The messages are logged at loglevel info.

The hook specifies before which phase of request processing the message will be logged. The following hooks are supported:

Name
`pre_translate_name`
`translate_name`
`type_checker`
`quick_handler`
`map_to_storage`
`check_access`
`check_access_ex`
`insert_filter`
`check_authn`
`check_authz`
`fixups`
`handler`
`log_transaction`

The default is `log_transaction`. The special value `all` is also supported, causing a message to be logged at each phase. Not all hooks are executed for every request.

The optional expression allows to restrict the message if a condition is met. The details of the expression syntax are described in the ap_expr documentation. References to HTTP headers will not cause the header names to be added to the Vary header.