More s/code/module/

    change between releases.

    </p>

  </summary>

  <seealso><a href="../mod/mod_http2.html">mod_http2</a></seealso>

  <seealso><module>mod_http2</module></seealso>

  <section id="protocol">

    <title>The HTTP/2 protocol</title>

  <section id="implementation">

    <title>HTTP/2 in Apache httpd</title>

    <p>The HTTP/2 protocol is implemented by its own httpd module, aptly named

    <a href="../mod/mod_http2.html">mod_http2</a>. It implements the complete set

    <module>mod_http2</module>. It implements the complete set

    of features described by RFC 7540 and supports HTTP/2 over cleartext (http:), as

    well as secure (https:) connections. The cleartext variant is named '<code>h2c</code>', 

    the secure one '<code>h2</code>'. For <code>h2c</code> it allows the <em>direct</em>

  <section id="building">

    <title>Build httpd with HTTP/2 support</title>

    <p><a href="../mod/mod_http2.html">mod_http2</a> uses the library of <a href="https://nghttp2.org">nghttp2</a>

    as its implementation base. In order to build <code>mod_http2</code> you need at least version 1.2.1 of

    <p><module>mod_http2</module> uses the library of <a href="https://nghttp2.org">nghttp2</a>

    as its implementation base. In order to build <module>mod_http2</module> you need at least version 1.2.1 of

    <code>libnghttp2</code> installed on your system.</p>

    <p>When you <code>./configure</code> you Apache httpd source tree, you need to give it 

    '<code>--enable-http2</code>' as additional argument to trigger the build of the module.

    to <code>configure</code>.</p>

    <p>While that should do the trick for most, they are people who might prefer a statically

    linked <code>nghttp2</code> in this module. For those, the option <code>--enable-nghttp2-staticlib-deps</code>

    exists. It works quite similar to how one statically links openssl to mod_ssl.</p>

    exists. It works quite similar to how one statically links openssl to <module>mod_ssl</module>.</p>

    <p>Speaking of SSL, you need to be aware that most browsers will speak HTTP/2 only on <code>https:</code>

    URLs, so you need a server with SSL support. But not only that, you will need a SSL library

    that supports the <code>ALPN</code> extension. If OpenSSL is the library you use, you need

  <section id="basic-config">

    <title>Basic Configuration</title>

    <p>When you have a <code>httpd</code> built with <code>mod_http2</code> you need some

    <p>When you have a <code>httpd</code> built with <module>mod_http2</module> you need some

    basic configuration for it becoming active. The first thing, as with every Apache module,

    is that you need to load it:</p>

    <highlight language="config">

    <p>This allows only HTTP/1 on connections, except SSL connections to <code>test.example.org</code>

    which offer HTTP/2.</p>

    <note><title>Choose a strong SSLCipherSuite</title>

     <p>The <directive module="mod_ssl">SSLCipherSuite</directive> needs to be configured with a strong TLS cipher suite. The current version of mod_http2 does not enforce any cipher but most clients do so. Pointing a browser to a <code>h2</code> enabled server with a inappropriate cipher suite will force it to simply refuse and fall back to HTTP 1.1. This is a common mistake that is done while configuring httpd for HTTP/2 the first time, so please keep it in mind to avoid long debugging sessions! If you want to be sure about the cipher suite to choose please avoid the ones listed in the <a href="http://httpwg.org/specs/rfc7540.html#BadCipherSuites">HTTP/2 TLS blacklist</a>.</p>

    <p>The <directive module="mod_ssl">SSLCipherSuite</directive> needs to be configured with

    a strong TLS cipher suite. The current version of <directive>mod_http2</directive> does not enforce any cipher but most

    clients do so. Pointing a browser to a <code>h2</code> enabled server with a inappropriate

    cipher suite will force it to simply refuse and fall back to HTTP 1.1. This is a common mistake

    that is done while configuring httpd for HTTP/2 the first time, so please keep it in mind to avoid

    long debugging sessions! If you want to be sure about the cipher suite to choose please avoid

    the ones listed in the <a href="http://httpwg.org/specs/rfc7540.html#BadCipherSuites">HTTP/2 TLS blacklist</a>.</p>

    </note>

    <p>The order of protocols mentioned is also relevant. By default, the first one is the 

    most preferred protocol. When a client offers multiple choices, the one most to the 

    ordering will decide.</p>

    <p>A last thing: the protocols you configure are not checked for correctness

    or spelling. You can mention protocols that do not exist, so there is no need

    to guard <code>Protocols</code> with any <code>IfModule</code> checks.</p>

    to guard <directive module="core">Protocols</directive> with any

    <directive type="section" module="core">IfModule</directive> checks.</p>

    <p>For more advanced tips on configuration, see the <a href="../mod/mod_http2.html#dimensioning">

    modules section about dimensioning</a> and <a href="../mod/mod_http2.html#misdirected">

    how to manage multiple hosts with the same certificate</a>.</p>

        <li><a href="https://nghttp2.org/documentation/h2load-howto.html">h2load</a> - useful to stress-test your server.</li>

    </ul>

    <p>Chrome offers detailed HTTP/2 logs on its connections via the 

    <a href="chrome://net-internals/#http2">special net-internals page</a>. There is also an interesting extension for <a href="https://chrome.google.com/webstore/detail/http2-and-spdy-indicator/mpbpobfflnpcgagjijhmgnchggcjblin?hl=en">Chrome</a> and <a href="https://addons.mozilla.org/en-us/firefox/addon/spdy-indicator/">Firefox</a> to visualize when your browser is using HTTP/2.</p>

    <a href="chrome://net-internals/#http2">special net-internals page</a>. There is also an

    interesting extension for <a href="https://chrome.google.com/webstore/detail/http2-and-spdy-indicator/mpbpobfflnpcgagjijhmgnchggcjblin?hl=en">Chrome</a>

    and <a href="https://addons.mozilla.org/en-us/firefox/addon/spdy-indicator/">Firefox</a>

    to visualize when your browser is using HTTP/2.</p>

  </section>

  <section id="push">

    <p>To summarize: there is no one good strategy on how to make best use of this 

    feature of HTTP/2 and everyone is still experimenting. So, how do you experiment

    with it in Apache httpd?</p>

    <p><code>mod_http2</code> inspect response header for <code>Link</code> headers

    <p><module>mod_http2</module> inspect response header for <code>Link</code> headers

    in a certain format:</p>

    <highlight language="config">

Link </xxx.css>;rel=preload, </xxx.js>; rel=preload

    <p>There are people thinking about how a client can tell a server what it

    already has, so PUSHes for those things can be avoided, but this is all

    highly experimental right now.</p>

    <p>Another experimental draft that has been implemented in <code>mod_http2</code>

    <p>Another experimental draft that has been implemented in <module>mod_http2</module>

    is the <a href="https://tools.ietf.org/html/draft-ruellan-http-accept-push-policy-00">

    Accept-Push-Policy Header Field</a> where a client can, for each request, define

    what kind of PUSHes it accepts.</p>

H2EarlyHints on

    </highlight>

    <p>(It is not enabled by default since some older browser tripped on such responses.)</p>

    <p>If this feature is on, you can the directive <code>H2PushResource</code> to 

    <p>If this feature is on, you can the directive <directive module="mod_http2">H2PushResource</directive> to 

    trigger early hints and resource PUSHes:</p>

    <highlight language="config">

<Location /xxx.html>

    as the server <em>starts</em> processing the request. This may be much early than

    the time the first response headers have been determined, depending on your web

    application.</p>

    <p>If <code>H2Push</code> is enabled, this will also start the PUSH right after the

    103 response. If <code>H2Push</code> is disabled however, the 103 response will be send

    <p>If <directive module="mod_http2">H2Push</directive> is enabled, this will also start the PUSH right after the

    103 response. If <directive module="mod_http2">H2Push</directive> is disabled however, the 103 response will be send

    nevertheless to the client.</p>

  </section>