apache/docs/manual/mod/mod_log_debug.xml

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<modulesynopsis metafile="mod_log_debug.xml.meta">

<name>mod_log_debug</name>
<description>Additional configurable debug logging</description>
<status>Experimental</status>
<sourcefile>mod_log_debug.c</sourcefile>
<identifier>log_debug_module</identifier>
<compatibility>Available in Apache 2.3.14 and later</compatibility>

<section id="examples"><title>Examples</title>

    <ol>
      <li>
        Log message after request to /foo/* is processed:

        <highlight language="config">
&lt;Location "/foo/"&gt;
&nbsp;&nbsp;LogMessage "/foo/ has been requested"
&lt;/Location&gt;
        </highlight>
      </li>

      <li>
        Log message if request to /foo/* is processed in a sub-request:
        <highlight language="config">
&lt;Location "/foo/"&gt;
&nbsp;&nbsp;LogMessage "subrequest to /foo/" hook=type_checker "expr=-T %{IS_SUBREQ}"
&lt;/Location&gt;
        </highlight>

        The default log_transaction hook is not executed for sub-requests,
        therefore we have to use a different hook.
      </li>


      <li>
        Log message if an IPv6 client causes a request timeout:
        <highlight language="config">
            LogMessage "IPv6 timeout from %{REMOTE_ADDR}" "expr=-T %{IPV6} &amp;&amp; %{REQUEST_STATUS} = 408"
        </highlight>
        Note the placing of the double quotes for the <code>expr=</code> argument.
      </li>

      <li>
        Log the value of the "X-Foo" request environment variable in each
        stage of the request:
        <highlight language="config">
&lt;Location "/"&gt;
&nbsp;&nbsp;LogMessage "%{reqenv:X-Foo}" hook=all
&lt;/Location&gt;
        </highlight>
        Together with microsecond time stamps in the error log,
        <code>hook=all</code> also lets you determine the times spent
        in the different parts of the request processing.
      </li>

    </ol>
</section>

<directivesynopsis>
<name>LogMessage</name>
<description>Log user-defined message to error log
</description>
<syntax>LogMessage <var>message</var>
[hook=<var>hook</var>] [expr=<var>expression</var>]
</syntax>
<default>Unset</default>
<contextlist><context>directory</context>
</contextlist>

<usage>
    <p>This directive causes a user defined message to be logged to the
    error log. The message can use variables and functions from the
    <a href="../expr.html">ap_expr syntax</a>. References to HTTP headers
    will not cause header names to be added to the Vary header. The
    messages are logged at loglevel info.</p>

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

    <table border="1" style="zebra">
    <columnspec><column width="1"/></columnspec>
    <tr><th>Name</th></tr>
    <tr><td><code>translate_name</code></td></tr>
    <tr><td><code>type_checker</code></td></tr>
    <tr><td><code>quick_handler</code></td></tr>
    <tr><td><code>map_to_storage</code></td></tr>
    <tr><td><code>check_access</code></td></tr>
    <tr><td><code>check_access_ex</code></td></tr>
    <tr><td><code>insert_filter</code></td></tr>
    <tr><td><code>check_authn</code></td></tr>
    <tr><td><code>check_authz</code></td></tr>
    <tr><td><code>fixups</code></td></tr>
    <tr><td><code>handler</code></td></tr>
    <tr><td><code>log_transaction</code></td></tr>
    </table>

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

    <p>The optional expression allows to restrict the message if a
    condition is met. The details of the expression syntax are described in
    the <a href="../expr.html">ap_expr documentation</a>. References to HTTP
    headers will not cause the header names to be added to the Vary header.</p>

</usage>

</directivesynopsis>

</modulesynopsis>