docs/manual/mod/mod_policy.xml

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

<name>mod_policy</name>
<description>HTTP protocol compliance enforcement.</description>
<status>Extension</status>
<sourcefile>mod_policy.c</sourcefile>
<identifier>policy_module</identifier>

<summary>
    <p>The HTTP protocol recommends that clients should be "liberal in
    what they accept", and servers "strict with what they send". In some
    cases it can be difficult to detect when a server or an application
    has been misconfigured, is serving uncacheable content or is behaving
    suboptimally, as an HTTP client might be compensating for the server.
    These problems can potentially lead to excessive bandwidth
    consumption, or a server outage under load.</p>

    <p>The <module>mod_policy</module> module consists of a set of
    filters that test servers for HTTP protocol compliance. These
    tests allow the server administrator to log violations of, or
    outright reject responses where certain defined conditions exist.</p>

    <p>This could be used as a way to set minimum HTTP protocol compliance
    criteria for a restful application. Alternatively, a reverse proxy or
    cache could be configured to protect itself from misconfigured origin
    servers or unexpectedly uncacheable content, or as a mechanism to
    detect configuration mistakes within the server itself.</p>

</summary>
<seealso><a href="../filter.html">Filters</a></seealso>
<seealso><a href="../compliance.html">HTTP Protocol Compliance</a></seealso>

<section id="actions">
    <title>Actions</title>

    <p>If a policy is violated, one of the following actions can be
    taken:</p>

    <dl>
    <dt><strong>ignore</strong></dt>
    <dd>The policy check will be ignored for the given URL space, even
    if the filter is present.</dd>

    <dt><strong>log</strong></dt>
    <dd>The policy check will be executed, and if a violation is detected
    a warning will be logged to the server error_log, and a
    <code>Warning</code> header added to the response for the benefit of
    the client.</dd>

    <dt><strong>enforce</strong></dt>
    <dd>The policy check will be executed, and if a violation is detected
    an error will be logged to the server error_log, a
    <code>Warning</code> header added to the response, and a <code>502
    Bad Gateway</code> will be returned to the client. Optional links to
    explanatory documentation can be added to each error message,
    detailing the origin of each policy.</dd>

    </dl>

    <p>It is also possible to selectively disable all policies for a
    given URL space, should the need arise, using the
    <directive module="mod_policy">PolicyFilter</directive> directive.</p>

    <p>Alternatively, the
    <directive module="mod_policy">PolicyEnvironment</directive>
    directive can be used to specify an environment variable, which if
    present, will cause the policies to be selectively downgraded or
    bypassed.</p>

</section>

<section id="tests">
    <title>Policy Tests</title>

    <p>The following policy filters are available:</p>

    <dl>
    <dt><strong><a href="../compliance.html#policytype">POLICY_TYPE</a>
    </strong>: Enforce valid content types</dt>
    <dd>Content types that are syntactically invalid or blank can be detected
	and the request rejected. Types can be restricted to a specific list
	containing optional wildcards ? and *.</dd>

    <dt><strong><a href="../compliance.html#policylength">POLICY_LENGTH</a>
    </strong>: Enforce the presence of a Content-Length</dt>
    <dd>The length of responses can be specified in one of three ways, by
    specifying an explicit length in advance, using chunked encoding to set
    the length, or by setting no length at all and terminating the request
    when complete. The absence of a specific content length can affect the
    cacheability of the response, and prevents the use of keepalive during
    HTTP/1.0 requests. This policy enforces the presence of an explicit
    content length on the response.</dd>

    <dt><strong><a href="../compliance.html#policykeepalive">POLICY_KEEPALIVE
    </a></strong>: Enforce the option to keepalive</dt>
    <dd>Less restrictive than the POLICY_LENGTH test, this policy enforces the
    possibility that the response can be kept alive. If the response doesn't
    have a protocol defined zero length, and the response isn't already an
    error, and the response has neither a Content-Length or is declared
    HTTP/1.1 and lacks Content-Encoding: chunked, then this response will be
    rejected.</dd>

    <dt><strong><a href="../compliance.html#policyvary">POLICY_VARY</a>
    </strong>: Enforce the absence of certain headers within Vary headers</dt>
    <dd>If the Vary header contains any of the headers specified, this policy
    will reject the request. The typical case is the presence of the User-Agent
    within Vary, which is likely to cause a denial of service condition to a
    cache.</dd>

    <dt><strong><a href="../compliance.html#policyvalidation">
    POLICY_VALIDATION</a></strong>: Enforce the presence of Etag and/or
    Last-Modified</dt>
    <dd>The ability for a cache to determine whether a cached entity can be
    refreshed is dependent on whether a valid Etag and/or Last-Modified header
    is present to revalidate against. The absence of both headers, or the
    invalid syntax of a header will cause this policy to be rejected.</dd>

    <dt><strong><a href="../compliance.html#policyconditional">
    POLICY_CONDITIONAL</a></strong>: Enforce correct operation of conditional
    requests</dt>
    <dd>When conditional headers are present in the request, a server should
    respond with a <code>304 Not Modified</code> or <code>412 Precondition
    Failed</code> response where appropriate. A server may ignore conditional
    headers, and this affects the efficiency of the HTTP caching mechanism.
    This policy rejects requests where a conditional header is present, and
    a 304 or 412 response code was expected, but a 2xx response was seen
    instead.</dd>

    <dt><strong><a href="../compliance.html#policynocache">POLICY_NOCACHE</a>
    </strong>: Enforce cacheable responses</dt>
    <dd>When a response is encountered that declares itself explicitly
    uncacheable, the request is rejected. A response is considered
    uncacheable if it specifies any of the following:
    <ul><li><code>Cache-Control: no-cache</code></li>
    <li><code>Pragma: no-cache</code></li>
    <li><code>Cache-Control: no-store</code></li>
    <li><code>Cache-Control: private</code></li>
    </ul></dd>

    <dt><strong><a href="../compliance.html#policymaxage">POLICY_MAXAGE</a>
    </strong>: Enforce a minimum maxage</dt>
    <dd>When a response is encountered where the freshness lifetime is less
    than the given value, or the freshness lifetime is heuristic, the request
    is rejected. A response is checked in the following order:
    <ul><li>If <code>s-maxage</code> is present but too small; or</li>
    <li>If <code>max-age</code> is present but too small; or</li>
    <li>If <code>Expires</code> is present and invalid; or</li>
    <li><code>Date</code> is present and invalid; or</li>
    <li><code>Expires</code> minus Date is too small; or</li>
    <li>No <code>s-maxage</code>, <code>maxage</code>, or
    <code>Expires</code>/<code>Date</code> declared at all</li>
    </ul></dd>

    <dt><strong><a href="../compliance.html#policyversion">POLICY_VERSION</a>
    </strong>: Enforce a minimum HTTP version within a request</dt>
    <dd>When a request is encountered with an HTTP version number less than
    the required minimum version, the request is rejected. The following
    version numbers are recognised:
    <ul><li><code>HTTP/1.1</code></li>
    <li><code>HTTP/1.0</code></li>
    <li><code>HTTP/0.9</code></li>
    </ul></dd>

    </dl>

</section>

<section id="example">
    <title>Example Configuration</title>

    <p>A typical configuration protecting a server serving static content
    might be as follows:</p>

    <highlight language="config">
&lt;Location "/"&gt;
  SetOutputFilter POLICY_TYPE;POLICY_LENGTH;POLICY_KEEPALIVE;POLICY_VARY;POLICY_VALIDATION; \
    POLICY_CONDITIONAL;POLICY_NOCACHE;POLICY_MAXAGE;POLICY_VERSION

  # content type must be present and valid, but can be anything
  PolicyType enforce */*

  # reject if no explicitly declared content length
  PolicyLength enforce

  # covered by the policy length filter
  PolicyKeepalive ignore

  # reject if User-Agent appears within Vary headers
  PolicyVary enforce User-Agent

  # we want to enforce validation
  PolicyValidation enforce

  # non-functional conditional responses should be rejected
  PolicyConditional enforce

  # no-cache responses should be rejected
  PolicyNocache enforce

  # maxage must be at least a day
  PolicyMaxage enforce 86400

  # request version can be anything
  PolicyVersion ignore HTTP/1.1
&lt;/Location&gt;

# suppress policy protection for server-status
&lt;Location "/server-status"&gt;
  PolicyFilter off
&lt;/Location&gt;
    </highlight>

</section>

<directivesynopsis>

<name>PolicyFilter</name>
<description>Enable or disable policies for the given URL space.</description>
<syntax>PolicyFilter <var>on|off</var></syntax>
<default>on</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyFilter is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>Master switch to enable or disable policies for a given URL space.</p>

    <example><title>Example</title>
    <highlight language="config">
# enabled by default
&lt;Location "/"&gt;
  PolicyFilter on
&lt;/Location&gt;

# suppress policy protection for server-status
&lt;Location "/server-status"&gt;
  PolicyFilter off
&lt;/Location&gt;
</highlight>
    </example>
</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyEnvironment</name>
<description>Override policies based on an environment variable.</description>
<syntax>PolicyEnvironment <var>variable</var> <var>log-value</var> <var>ignore-value</var></syntax>
<default>none</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyEnvironment is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>Downgrade policies to logging only or ignored based on the presence
    of an environment variable. If the given variable is present and equal
    to the log-value, enforced policies will be logged instead. If the given
    variable is present and equal to the ignore-value, all policies will
    be ignored.</p>

    <example><title>Example</title>
    <highlight language="config">
# downgrade if POLICY_CONTROL was present
PolicyEnvironment POLICY_CONTROL log ignore
    </highlight>
    </example>
</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyConditional</name>
<description>Enable the conditional request policy.</description>
<syntax>PolicyConditional <var>ignore|log|enforce</var></syntax>
<default>ignore</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyConditional is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>When logged or enforced, a response that should have been conditional
    but wasn't will be rejected.</p>

    <example><title>Example</title>
    <highlight language="config">
# non-functional conditional responses should be rejected
PolicyConditional enforce
    </highlight>
    </example>
</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyConditionalURL</name>
<description>URL describing the conditional request policy.</description>
<syntax>PolicyConditionalURL <var>url</var></syntax>
<default>none</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyConditionalURL is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>Specify the URL of the documentation describing the conditional
    request policy, to appear within error messages.</p>
</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyLength</name>
<description>Enable the content length policy.</description>
<syntax>PolicyLength <var>ignore|log|enforce</var></syntax>
<default>ignore</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyLength is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>When logged or enforced, a response that lacks an explicit
    <code>Content-Length</code> header will be rejected.</p>

    <example><title>Example</title>
    <highlight language="config">
# missing Content-Length header should be rejected
PolicyLength enforce
    </highlight>
    </example>
</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyLengthURL</name>
<description>URL describing the content length policy.</description>
<syntax>PolicyLengthURL <var>url</var></syntax>
<default>none</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyLengthURL is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>Specify the URL of the documentation describing the content length
    policy, to appear within error messages.</p>
</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyKeepalive</name>
<description>Enable the keepalive policy.</description>
<syntax>PolicyKeepalive <var>ignore|log|enforce</var></syntax>
<default>ignore</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyKeepalive is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>When logged or enforced, a response that lacks both an explicit
    <code>Content-Length</code> header and a <code>Transfer-Encoding</code>
    of <code>chunked</code> will be rejected.</p>

    <example><title>Example</title>
    <highlight language="config">
# missing Content-Length or Transfer-Encoding should be rejected
PolicyKeepalive enforce
</highlight>
    </example>
</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyKeepaliveURL</name>
<description>URL describing the keepalive policy.</description>
<syntax>PolicyKeepaliveURL <var>url</var></syntax>
<default>none</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyKeepaliveURL is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>Specify the URL of the documentation describing the keepalive
    policy, to appear within error messages.</p>
</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyType</name>
<description>Enable the content type policy.</description>
<syntax>PolicyType <var>ignore|log|enforce</var> <var>type</var> [ <var>type</var> [ ... ]]</syntax>
<default>ignore</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyType is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>When logged or enforced, a response that lacks a <code>Content-Type</code>
    header, where the <code>Content-Type</code> header is malformed, or where the
    header does not match the given pattern or patterns will be rejected.</p>

    <example><title>Example</title>
    <highlight language="config">
# enforce json or XML
PolicyType enforce application/json text/xml
    </highlight>
    </example>

    <example><title>Example</title>
    <highlight language="config">
# malformed content type should be rejected
PolicyType enforce */*
    </highlight>
    </example>

</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyTypeURL</name>
<description>URL describing the content type policy.</description>
<syntax>PolicyTypeURL <var>url</var></syntax>
<default>none</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyTypeURL is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>Specify the URL of the documentation describing the content type
    policy, to appear within error messages.</p>
</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyVary</name>
<description>Enable the Vary policy.</description>
<syntax>PolicyVary <var>ignore|log|enforce</var> <var>header</var> [ <var>header</var> [ ... ]]</syntax>
<default>ignore</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyVary is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>When logged or enforced, a response that contains a <code>Vary</code>
    header which in turn contains one of the headers listed, will be
    rejected.</p>

    <example><title>Example</title>
    <highlight language="config">
# reject responses with "User-Agent" listed in the Vary header
PolicyVary enforce User-Agent
    </highlight>
    </example>

</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyVaryURL</name>
<description>URL describing the content type policy.</description>
<syntax>PolicyVaryURL <var>url</var></syntax>
<default>none</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyVaryURL is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>Specify the URL of the documentation describing the vary policy, to
    appear within error messages.</p>
</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyValidation</name>
<description>Enable the validation policy.</description>
<syntax>PolicyValidation <var>ignore|log|enforce</var></syntax>
<default>ignore</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyValidation is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>When logged or enforced, a response that lacks either a valid
    <code>ETag</code> header or a <code>Last-Modified</code> header, or where
    either header is syntactically incorrect, will be rejected.</p>

    <example><title>Example</title>
    <highlight language="config">
# no ETag or Last-Modified will be rejected
PolicyValidation enforce
    </highlight>
    </example>

</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyValidationURL</name>
<description>URL describing the content type policy.</description>
<syntax>PolicyValidationURL <var>url</var></syntax>
<default>none</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyValidationURL is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>Specify the URL of the documentation describing the validation policy, to
    appear within error messages.</p>
</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyNocache</name>
<description>Enable the caching no-cache policy.</description>
<syntax>PolicyNocache <var>ignore|log|enforce</var></syntax>
<default>ignore</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyNocache is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>When logged or enforced, a response that defines itself uncacheable
    using the <code>Cache-Control</code> or <code>Pragma</code> headers will
    be rejected.</p>

    <example><title>Example</title>
    <highlight language="config">
# Cache-Control: no-cache will be rejected
PolicyNocache enforce
    </highlight>
    </example>

</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyNocacheURL</name>
<description>URL describing the caching no-cache policy.</description>
<syntax>PolicyNocacheURL <var>url</var></syntax>
<default>none</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyNocacheURL is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>Specify the URL of the documentation describing the caching no-cache
    policy, to appear within error messages.</p>
</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyMaxage</name>
<description>Enable the caching minimum max-age policy.</description>
<syntax>PolicyMaxage <var>ignore|log|enforce</var> <var>age</var></syntax>
<default>ignore</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyMaxage is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>When logged or enforced, a response that lacks an explicit freshness
    lifetime defined with <code>max-age</code>, <code>s-maxage</code> or an
    <code>Expires</code> header, or where the explicit freshness lifetime is
    smaller than the given value, will be rejected.</p>

    <example><title>Example</title>
    <highlight language="config">
# reject responses with a freshness lifetime shorter than a day
PolicyMaxage enforce 86400
    </highlight>
    </example>

</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyMaxageURL</name>
<description>URL describing the caching minimum freshness lifetime policy.</description>
<syntax>PolicyMaxageURL <var>url</var></syntax>
<default>none</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyMaxageURL is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>Specify the URL of the documentation describing the caching minimum
    freshness lifetime policy, to appear within error messages.</p>
</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyVersion</name>
<description>Enable the version policy.</description>
<syntax>PolicyVersion <var>ignore|log|enforce</var> <var>HTTP/0.9|HTTP/1.0|HTTP/1.1</var></syntax>
<default>ignore</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyVersion is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>When logged or enforced, a request with a version lower than specified
    will be rejected.</p>

    <example><title>Example</title>
    <highlight language="config">
# reject requests with an HTTP version older than HTTP/1.1
PolicyVersion enforce HTTP/1.1
    </highlight>
    </example>

</usage>

</directivesynopsis>

<directivesynopsis>

<name>PolicyVersionURL</name>
<description>URL describing the minimum request HTTP version policy.</description>
<syntax>PolicyVersionURL <var>url</var></syntax>
<default>none</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>
<compatibility>PolicyVersionURL is only available in Apache 2.5.0 and
later.</compatibility>

<usage>
    <p>Specify the URL of the documentation describing the minimum request
    HTTP version policy, to appear within error messages.</p>
</usage>

</directivesynopsis>

</modulesynopsis>