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<title>HTTP Protocol Compliance - Apache HTTP Server Version 2.5</title>
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<p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/quickreference.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p>
<p class="apache">Apache HTTP Server Version 2.5</p>
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<div class="up"><a href="./"><img title="&lt;-" alt="&lt;-" src="./images/left.gif" /></a></div>
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<a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs/">Documentation</a> &gt; <a href="./">Version 2.5</a></div><div id="page-content"><div id="preamble"><h1>HTTP Protocol Compliance</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="./en/compliance.html" title="English">&nbsp;en&nbsp;</a> |
<a href="./fr/compliance.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a></p>
</div>
<p>This document describes the mechanism to set a policy for HTTP
protocol compliance for a given URL space by the origin servers or
applications behind that URL space.</p>
<p>For those who may have received an error message from a rejected
policy, and need to know what the policy rejection means and what
they might do to fix the error, each policy is described below.</p>
</div>
<div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#intro">Enforcing HTTP Protocol Compliance in Apache 2</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#policyconditional">Conditional Request Policy</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#policylength">Content-Length Policy</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#policytype">Content-Type Policy</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#policykeepalive">Keepalive Policy</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#policymaxage">Freshness Lifetime / Maxage Policy</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#policynocache">No Cache Policy</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#policyvalidation">Validation Policy</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#policyvary">Vary Header Policy</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#policyversion">Protocol Version Policy</a></li>
</ul><h3>See also</h3><ul class="seealso"><li><a href="filter.html">Filters</a></li><li><a href="#comments_section">Comments</a></li></ul></div>
<div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
<div class="section">
<h2><a name="intro" id="intro">Enforcing HTTP Protocol Compliance in Apache 2</a></h2>
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_policy.html">mod_policy</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_policy.html#policyconditional">PolicyConditional</a></code></li><li><code class="directive"><a href="./mod/mod_policy.html#policylength">PolicyLength</a></code></li><li><code class="directive"><a href="./mod/mod_policy.html#policykeepalive">PolicyKeepalive</a></code></li><li><code class="directive"><a href="./mod/mod_policy.html#policytype">PolicyType</a></code></li><li><code class="directive"><a href="./mod/mod_policy.html#policyvary">PolicyVary</a></code></li><li><code class="directive"><a href="./mod/mod_policy.html#policyvalidation">PolicyValidation</a></code></li><li><code class="directive"><a href="./mod/mod_policy.html#policynocache">PolicyNocache</a></code></li><li><code class="directive"><a href="./mod/mod_policy.html#policymaxage">PolicyMaxage</a></code></li><li><code class="directive"><a href="./mod/mod_policy.html#policyversion">PolicyVersion</a></code></li></ul></td></tr></table>
<p>The HTTP protocol follows the <strong>robustness principle</strong>
as described in <a href="http://tools.ietf.org/html/rfc1122">RFC1122</a>,
which states <strong>"Be liberal in what you accept, and conservative in
what you send"</strong>. As a result of this principle, HTTP clients will
compensate for and recover from incorrect or misconfigured responses, or
responses that are uncacheable.</p>
<p>As a website is scaled up to face greater and greater traffic loads,
suboptimal or misconfigured applications or server configurations can
threaten both the stability and scalability of the website, as well as
the hosting costs associated with it. A website can also scale up to face
greater configuration complexity, and it can be increasingly difficult to
detect and keep track of suboptimally configured URL spaces on a given
server.</p>
<p>Eventually a point is reached where the principle "conservative in
what you send" needs to be enforced by the server administrator.</p>
<p>The <code class="module"><a href="./mod/mod_policy.html">mod_policy</a></code> module provides a set of filters
which can be applied to a server, allowing key features of the HTTP
protocol to be explicitly tested, and non compliant responses logged as
warnings, or rejected outright as an error. Each filter can be applied
separately, allowing the administrator to pick and choose which policies
should be enforced depending on the circumstances of their environment.
</p>
<p>The filters might be placed in testing and staging environments for
the benefit of application and website developers, or may be applied
to production servers to protect infrastructure from systems outside
the administrator's direct control.</p>
<p class="figure">
<img src="images/compliance-reverse-proxy.png" width="666" height="239" alt="Enforcing HTTP protocol compliance for an application server" />
</p>
<p>In the above example, an Apache httpd server has been placed between
the application server and the internet at large, and configured to cache
responses from the application server. The <code class="module"><a href="./mod/mod_policy.html">mod_policy</a></code>
filters have been added to enforce support for cacheable content and
conditional requests, ensuring that both <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> and
public caches on the internet are fully able to cache content created
by the restful application server efficiently.</p>
<p class="figure">
<img src="images/compliance-static.png" width="469" height="239" alt="Enforcing HTTP protocol compliance in a static server" />
</p>
<p>In the above simpler example, a static server serving highly cacheable
content has a set of policies applied to ensure that the server configuration
conforms to a minimum level of compliance.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
<div class="section">
<h2><a name="policyconditional" id="policyconditional">Conditional Request Policy</a></h2>
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_policy.html">mod_policy</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_policy.html#policyconditional">PolicyConditional</a></code></li></ul></td></tr></table>
<p>This policy will be rejected if the server does not correctly respond
to a conditional request with the appropriate status code.</p>
<p>Conditional requests form the mechanism by which an HTTP cache makes
stale content fresh again, and particularly for content with short freshness
lifetimes, lack of support for conditional requests can add avoidable load
to the server.</p>
<p>Most specifically, the existence of any of following headers in the
request makes the request conditional:</p>
<dl>
<dt><code>If-Match</code></dt>
<dd>If the provided ETag in the <code>If-Match</code> header does not match
the ETag of the response, the server should return
<code>412 Precondition Failed</code>. Full details of how to handle an
<code>If-Match</code> header can be found in
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.24">
RFC2616 section 14.24</a>.</dd>
<dt><code>If-None-Match</code></dt>
<dd>If the provided ETag in the <code>If-None-Match</code> header matches
the ETag of the response, the server should return either
<code>304 Not Modified</code> for GET/HEAD requests, or
<code>412 Precondition Failed</code> for other methods. Full details of how
to handle an <code>If-None-Match</code> header can be found in
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.26">
RFC2616 section 14.26</a>.</dd>
<dt><code>If-Modified-Since</code></dt>
<dd>If the provided date in the <code>If-Modified-Since</code> header is
older than the <code>Last-Modified</code> header of the response, the server
should return <code>304 Not Modified</code>. Full details of how to handle an
<code>If-Modified-Since</code> header can be found in
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.25">
RFC2616 section 14.25</a>.</dd>
<dt><code>If-Unmodified-Since</code></dt>
<dd>If the provided date in the <code>If-Modified-Since</code> header is
newer than the <code>Last-Modified</code> header of the response, the server
should return <code>412 Precondition Failed</code>. Full details of how to
handle an <code>If-Unmodified-Since</code> header can be found in
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.28">
RFC2616 section 14.28</a>.</dd>
<dt><code>If-Range</code></dt>
<dd>If the provided ETag or date in the <code>If-Range</code> header matches
the ETag or Last-Modified of the response, and a valid <code>Range</code>
is present, the server should return
<code>206 Partial Response</code>. Full details of how to handle an
<code>If-Range</code> header can be found in
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.27">
RFC2616 section 14.27</a>.</dd>
</dl>
<p>If the response is detected to have been successful (a 2xx response),
but was conditional and one of the responses above was expected instead,
this policy will be rejected. Responses that indicate a redirect or a
failure of some kind (3xx, 4xx, 5xx) will be ignored by this policy.</p>
<p>This policy is implemented by the <strong>POLICY_CONDITIONAL</strong>
filter.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
<div class="section">
<h2><a name="policylength" id="policylength">Content-Length Policy</a></h2>
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_policy.html">mod_policy</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_policy.html#policylength">PolicyLength</a></code></li></ul></td></tr></table>
<p>This policy will be rejected if the server response does not contain
an explicit <code>Content-Length</code> header.</p>
<p>There are a number of ways of determining the length of a response
body, described in full in
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.4">
RFC2616 section 4.4 Message Length</a>.</p>
<p>When the <code>Content-Length</code> header is present, the size of
the body is declared at the start of the response. If this information
is missing, an HTTP cache might choose to ignore the response, as it
does not know in advance whether the response will fit within the
cache's defined limits.</p>
<p>HTTP/1.1 defines the <code>Transfer-Encoding</code> header as an
alternative to <code>Content-Length</code>, allowing the end of the
response to be indicated to the client without the client having to
know the length beforehand. However, when HTTP/1.0 requests are
processed, and no <code>Content-Length</code> is specified, the only
mechanism available to the server to indicate the end of the request
is to drop the connection. In an environment containing load
balancers, this can cause the keepalive mechanism to be bypassed.
</p>
<p>If the response is detected to have been successful (a 2xx response),
and has a response body (this excludes <code>204 No Content</code>), and
the <code>Content-Length</code> header is missing, this policy will be
rejected. Responses that indicate a redirect or a failure of some kind
(3xx, 4xx, 5xx) will be ignored by this policy.</p>
<div class="warning">It should be noted that some modules, such as
<code class="module"><a href="./mod/mod_proxy.html">mod_proxy</a></code>, add their own <code>Content-Length</code>
header should the response be small enough for it to have been possible
to read the response lacking such a header in one go. This may cause
small responses to pass this policy, while larger responses may
fail for the same URL.</div>
<p>This policy is implemented by the <strong>POLICY_LENGTH</strong>
filter.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
<div class="section">
<h2><a name="policytype" id="policytype">Content-Type Policy</a></h2>
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_policy.html">mod_policy</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_policy.html#policytype">PolicyType</a></code></li></ul></td></tr></table>
<p>This policy will be rejected if the server response does not contain
an explicit and syntactically correct <code>Content-Type</code> header
that matches the server defined pattern.</p>
<p>The media type of the body is placed in the <code>Content-Type</code>
header, and the format of the header is described in full in
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.7">
RFC2616 section 3.7 Media Types</a>.</p>
<p>A syntactically valid content type might look as follows:</p>
<div class="example"><p><code>
Content-Type: text/html; charset=iso-8859-1
</code></p></div>
<p>Invalid content types might include:</p>
<div class="example"><p><code>
# invalid<br />
Content-Type: foo<br />
# blank<br />
Content-Type:
</code></p></div>
<p>The server administrator has the option to restrict the policy to one
or more specific types, or could specify a general wildcard type such as
<code>*/*</code>.</p>
<p>This policy is implemented by the <strong>POLICY_TYPE</strong>
filter.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
<div class="section">
<h2><a name="policykeepalive" id="policykeepalive">Keepalive Policy</a></h2>
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_policy.html">mod_policy</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_policy.html#policykeepalive">PolicyKeepalive</a></code></li></ul></td></tr></table>
<p>This policy will be rejected if the server response does not contain
an explicit <code>Content-Length</code> header, or a
<code>Transfer-Encoding</code> of chunked.</p>
<p>There are a number of ways of determining the length of a response
body, described in full in
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.4">
RFC2616 section 4.4 Message Length</a>.</p>
<p>When the <code>Content-Length</code> header is present, the size of
the body is declared at the start of the response. HTTP/1.1 defines the
<code>Transfer-Encoding</code> header as an alternative to
<code>Content-Length</code>, allowing the end of the response to be
indicated to the client without the client having to know the length
beforehand. In the absence of these two mechanisms, the only way for
a server to indicate the end of the request is to drop the connection.
In an environment containing load balancers, this can cause the keepalive
mechanism to be bypassed.
</p>
<p>Most specifically, we follow these rules:</p>
<dl>
<dt>IF</dt>
<dd>we have not marked this connection as errored;</dd>
<dt>and</dt>
<dd>the client isn't expecting 100-continue</dd>
<dt>and</dt>
<dd>the response status does not require a close;</dd>
<dt>and</dt>
<dd>the response body has a defined length due to the status code
being 304 or 204, the request method being HEAD, already having defined
Content-Length or Transfer-Encoding: chunked, or the request version
being HTTP/1.1 and thus capable of being set as chunked</dd>
<dt>THEN</dt>
<dd>we support keepalive.</dd>
</dl>
<div class="warning">The server may choose to turn off keepalive for
various reasons, such as an imminent shutdown, or a Connection: close from
the client, or an HTTP/1.0 client request with a response with no
<code>Content-Length</code>, but for our purposes we only care that
keepalive was possible from the application, not that keepalive actually
took place.</div>
<p>It should also be noted that the Apache httpd server includes a filter
that adds chunked encoding to responses without an explicit content
length. This policy catches those cases where this filter is bypassed or
not in effect.</p>
<p>This policy is implemented by the <strong>POLICY_KEEPALIVE</strong>
filter.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
<div class="section">
<h2><a name="policymaxage" id="policymaxage">Freshness Lifetime / Maxage Policy</a></h2>
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_policy.html">mod_policy</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_policy.html#policymaxage">PolicyMaxage</a></code></li></ul></td></tr></table>
<p>This policy will be rejected if the server response does not have
an explicit <strong>freshness lifetime</strong> at least as long
as the server defined limit, or if the freshness lifetime is
calculated based on a heuristic.</p>
<p>Full details of how a freshness lifetime is calculated is described in
full in
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.2">
RFC2616 section 13.2 Expiration Model</a>.</p>
<p>During the freshness lifetime, a cache does not need to contact the
origin server at all, it can simply pass the cached content as is back
to the client.</p>
<p>When the freshness lifetime is reached, the cache should contact the
origin server in an effort to check whether the content is still fresh,
and if not, replace the content.</p>
<p>When the freshness lifetime is too short, it can result in excessive
load on the server. In addition, should an outage occur that is as long
or longer than the freshness lifetime, all cached content will become
stale, which could cause a thundering herd of traffic when the
server or network returns.</p>
<p>This policy is implemented by the <strong>POLICY_MAXAGE</strong>
filter.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
<div class="section">
<h2><a name="policynocache" id="policynocache">No Cache Policy</a></h2>
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_policy.html">mod_policy</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_policy.html#policynocache">PolicyNocache</a></code></li></ul></td></tr></table>
<p>This policy will be rejected if the server response declares itself
uncacheable using either the <code>Cache-Control</code> or
<code>Pragma</code> headers.</p>
<p>Full details of how content may be declared uncacheable is described in
full in
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9.1">
RFC2616 section 14.9.1 What is Cacheable</a>, and within the definition
for the <code>Pragma</code> header in
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.32">
RFC2616 section 14.32 Pragma</a>.</p>
<p>Most specifically, should any of the following header combinations
exist in the response headers, the response will be rejected:</p>
<ul>
<li><code>Cache-Control: no-cache</code></li>
<li><code>Cache-Control: no-store</code></li>
<li><code>Cache-Control: private</code></li>
<li><code>Pragma: no-cache</code></li>
</ul>
<p>When unexpected, uncacheable content may produce unacceptable levels
of server load, or may incur significant cost. When this policy is enabled,
all server defined uncacheable content will be rejected.</p>
<p>This policy is implemented by the <strong>POLICY_NOCACHE</strong>
filter.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
<div class="section">
<h2><a name="policyvalidation" id="policyvalidation">Validation Policy</a></h2>
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_policy.html">mod_policy</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_policy.html#policyvalidation">PolicyValidation</a></code></li></ul></td></tr></table>
<p>This policy will be rejected if the server response does not contain
either a syntactically correct <code>ETag</code> or
<code>Last-Modified</code> header.</p>
<p>The <code>ETag</code> header is described in full in
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.19">
RFC2616 section 14.19 Etag</a>, and the <code>Last-Modified</code> header
is described in full in
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.29">
RFC2616 section 14.29 Last-Modified</a>.</p>
<p>In addition to being checked present, the headers are checked for
syntax.</p>
<p>An <code>ETag</code> that is not surrounded with quotes, or is not
declared "weak" by prefixing it with a "W/" will cause the policy to be
rejected. A <code>Last-Modified</code> that is not parsed as a valid date
will cause the policy to be rejected.</p>
<p>This policy is implemented by the <strong>POLICY_VALIDATION</strong>
filter.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
<div class="section">
<h2><a name="policyvary" id="policyvary">Vary Header Policy</a></h2>
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_policy.html">mod_policy</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_policy.html#policyvary">PolicyVary</a></code></li></ul></td></tr></table>
<p>This policy will be rejected if the server response contains a
<code>Vary</code> header, and that header in turn contains a header
blacklisted by the administrator.</p>
<p>The <code>Vary</code> header is described in full in
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.44">
RFC2616 section 14.44 Vary</a>.</p>
<p>Some client provided headers, such as <code>User-Agent</code>,
can contain thousands or millions of combinations of values over a period
of time, and if the response is declared cacheable, a cache might attempt
to cache each of these responses separately, filling up the cache and
crowding out other entries in the cache. In this scenario, if so
configured, the policy will reject the response.</p>
<p>This policy is implemented by the <strong>POLICY_VARY</strong>
filter.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
<div class="section">
<h2><a name="policyversion" id="policyversion">Protocol Version Policy</a></h2>
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_policy.html">mod_policy</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_policy.html#policyversion">PolicyVersion</a></code></li></ul></td></tr></table>
<p>This policy will be rejected if the client request was made with a
version number lower than the version of HTTP specified.</p>
<p>This policy is typically used with restful applications where
control over the type of client is desired. This policy can be used
alongside the <code>POLICY_KEEPALIVE</code> filter to ensure that
HTTP/1.0 clients don't cause keepalive connections to be dropped.</p>
<p>Possible minimum versions that could be specified are:</p>
<ul><li><code>HTTP/1.1</code></li>
<li><code>HTTP/1.0</code></li>
<li><code>HTTP/0.9</code></li>
</ul>
<p>This policy is implemented by the <strong>POLICY_VERSON</strong>
filter.</p>
</div></div>
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