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10 <title>mod_http2 - Apache HTTP Server Version 2.5</title>
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21 <p class="apache">Apache HTTP Server Version 2.5</p>
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25 <a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.5</a> > <a href="./">Modules</a></div>
26 <div id="page-content">
27 <div id="preamble"><h1>Apache Module mod_http2</h1>
29 <p><span>Available Languages: </span><a href="../en/mod/mod_http2.html" title="English"> en </a></p>
31 <table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Support for the HTTP/2 transport layer</td></tr>
32 <tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
33 <tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>http2_module</td></tr>
34 <tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_http2.c</td></tr>
35 <tr><th><a href="module-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.17 and later</td></tr></table>
38 <p>This module provides HTTP/2 (<a href="https://tools.ietf.org/html/rfc7540">RFC 7540</a>)
39 support for the Apache HTTP Server.</p>
41 <p>This module relies on <a href="http://nghttp2.org/">libnghttp2</a>
42 to provide the core http/2 engine.</p>
44 <div class="warning"><h3>Warning</h3>
45 <p>This module is experimental. Its behaviors, directives, and
46 defaults are subject to more change from release to
47 release relative to other standard modules. Users are encouraged to
48 consult the "CHANGES" file for potential updates.</p>
51 <p>You must enable HTTP/2 via <code class="directive"><a href="../mod/core.html#protocols">Protocols</a></code>
52 in order to use the functionality described in this document. The
53 HTTP/2 protocol <a href="https://http2.github.io/faq/#does-http2-require-encryption">does not require</a> the use of encryption so two schemes are available:
54 <code>h2</code> (HTTP/2 over TLS) and <code>h2c</code> (HTTP/2 over TCP).</p>
56 <p>Two useful configuration schemes are:</p>
58 <div class="note"><h3>HTTP/2 in a VirtualHost context (TLS only)</h3>
59 <pre class="prettyprint lang-config">Protocols h2 http/1.1</pre>
61 <p>Allows HTTP/2 negotiation (h2) via TLS ALPN in a secure
62 <code class="directive"><a href="../mod/core.html#virtualhost"><VirtualHost></a></code>.
63 HTTP/2 preamble checking (Direct mode, see <code class="directive"><a href="#h2direct">H2Direct</a></code>) is disabled by default for <code>h2</code>.</p>
66 <div class="note"><h3>HTTP/2 in a Server context (TLS and cleartext)</h3>
67 <pre class="prettyprint lang-config">Protocols h2 h2c http/1.1</pre>
69 <p>Allows HTTP/2 negotiation (h2) via TLS ALPN for secure
70 <code class="directive"><a href="../mod/core.html#virtualhost"><VirtualHost></a></code>. Allows
71 HTTP/2 cleartext negotiation (h2c) upgrading from an initial HTTP/1.1
72 connection or via HTTP/2 preamble checking (Direct mode, see
73 <code class="directive"><a href="#h2direct">H2Direct</a></code>).</p>
76 <p>Refer to the official <a href="https://http2.github.io/faq">HTTP/2 FAQ</a>
77 for any doubt about the protocol.</p>
80 <div id="quickview"><h3>Topics</h3>
82 <li><img alt="" src="../images/down.gif" /> <a href="#how-it-works">How it works</a></li>
83 </ul><h3 class="directives">Directives</h3>
85 <li><img alt="" src="../images/down.gif" /> <a href="#h2direct">H2Direct</a></li>
86 <li><img alt="" src="../images/down.gif" /> <a href="#h2maxsessionstreams">H2MaxSessionStreams</a></li>
87 <li><img alt="" src="../images/down.gif" /> <a href="#h2maxworkeridleseconds">H2MaxWorkerIdleSeconds</a></li>
88 <li><img alt="" src="../images/down.gif" /> <a href="#h2maxworkers">H2MaxWorkers</a></li>
89 <li><img alt="" src="../images/down.gif" /> <a href="#h2minworkers">H2MinWorkers</a></li>
90 <li><img alt="" src="../images/down.gif" /> <a href="#h2moderntlsonly">H2ModernTLSOnly</a></li>
91 <li><img alt="" src="../images/down.gif" /> <a href="#h2push">H2Push</a></li>
92 <li><img alt="" src="../images/down.gif" /> <a href="#h2pushdiarysize">H2PushDiarySize</a></li>
93 <li><img alt="" src="../images/down.gif" /> <a href="#h2pushpriority">H2PushPriority</a></li>
94 <li><img alt="" src="../images/down.gif" /> <a href="#h2serializeheaders">H2SerializeHeaders</a></li>
95 <li><img alt="" src="../images/down.gif" /> <a href="#h2sessionextrafiles">H2SessionExtraFiles</a></li>
96 <li><img alt="" src="../images/down.gif" /> <a href="#h2streammaxmemsize">H2StreamMaxMemSize</a></li>
97 <li><img alt="" src="../images/down.gif" /> <a href="#h2tlscooldownsecs">H2TLSCoolDownSecs</a></li>
98 <li><img alt="" src="../images/down.gif" /> <a href="#h2tlswarmupsize">H2TLSWarmUpSize</a></li>
99 <li><img alt="" src="../images/down.gif" /> <a href="#h2upgrade">H2Upgrade</a></li>
100 <li><img alt="" src="../images/down.gif" /> <a href="#h2windowsize">H2WindowSize</a></li>
102 <h3>Bugfix checklist</h3><ul class="seealso"><li><a href="https://www.apache.org/dist/httpd/CHANGES_2.4">httpd changelog</a></li><li><a href="https://bz.apache.org/bugzilla/buglist.cgi?bug_status=__open__&list_id=144532&product=Apache%20httpd-2&query_format=specific&order=changeddate%20DESC%2Cpriority%2Cbug_severity&component=mod_http2">Known issues</a></li><li><a href="https://bz.apache.org/bugzilla/enter_bug.cgi?product=Apache%20httpd-2&component=mod_http2">Report a bug</a></li></ul><h3>See also</h3>
104 <li><a href="#comments_section">Comments</a></li></ul></div>
105 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
106 <div class="section">
107 <h2><a name="how-it-works" id="how-it-works">How it works</a></h2>
109 <h3><a name="dimensioning" id="dimensioning">HTTP/2 Dimensioning</a></h3>
111 Enabling HTTP/2 on your Apache Server has impact on the resource
112 consumption and if you have a busy site, you may need to consider
113 carefully the implications.
116 The first noticeable thing after enabling HTTP/2 is that your server
117 processes will start additional threads. The reason for this is that
118 HTTP/2 gives all requests that it receives to its own <em>Worker</em>
119 threads for processing, collects the results and streams them out
123 In the current implementation, these workers use a separate thread
124 pool from the MPM workers that you might be familiar with. This is
125 just how things are right now and not intended to be like this forever.
126 (It might be forever for the 2.4.x release line, though.) So, HTTP/2
127 workers, or shorter H2Workers, will not show up in <code class="module"><a href="../mod/mod_status.html">mod_status</a></code>. They
128 are also not counted against directives such as <code class="directive"><a href="../mod/mpm_common.html#threadsperchild">ThreadsPerChild</a></code>. However
129 they take <code class="directive"><a href="../mod/mpm_common.html#threadsperchild">ThreadsPerChild</a></code>
130 as default if you have not configured something
131 else via <code class="directive"><a href="#h2minworkers">H2MinWorkers</a></code> and
132 <code class="directive"><a href="#h2maxworkers">H2MaxWorkers</a></code>.
135 Another thing to watch out for is is memory consumption. Since HTTP/2
136 keeps more state on the server to manage all the open request, priorities
137 for and dependencies between them, it will always need more memory
138 than HTTP/1.1 processing. There are three directives which steer the
139 memory footprint of a HTTP/2 connection:
140 <code class="directive"><a href="#h2maxsessionstreams">H2MaxSessionStreams</a></code>,
141 <code class="directive"><a href="#h2windowsize">H2WindowSize</a></code> and
142 <code class="directive"><a href="#h2streammaxmemsize">H2StreamMaxMemSize</a></code>.
145 <code class="directive"><a href="#h2maxsessionstreams">H2MaxSessionStreams</a></code> limits the
146 number of parallel requests that a client can make on a HTTP/2 connection.
147 It depends on your site how many you should allow. The default is 100 which
148 is plenty and unless you run into memory problems, I would keep it this
149 way. Most requests that browsers send are GETs without a body, so they
150 use up only a little bit of memory until the actual processing starts.
153 <code class="directive"><a href="#h2windowsize">H2WindowSize</a></code> controls how much
154 the client is allowed to send as body of a request, before it waits
155 for the server to encourage more. Or, the other way around, it is the
156 amount of request body data the server needs to be able to buffer. This
160 And last, but not least, <code class="directive"><a href="#h2streammaxmemsize">H2StreamMaxMemSize</a></code>
161 controls how much response data shall be buffered. The request sits in
162 a H2Worker thread and is producing data, the HTTP/2 connection tries
163 to send this to the client. If the client does not read fast enough,
164 the connection will buffer this amount of data and then suspend the
168 If you serve a lot of static files, <code class="directive"><a href="#h2sessionextrafiles">H2SessionExtraFiles</a></code>
169 is of interest. This tells the server how many file handles per
170 HTTP/2 connection it is allowed to waste for better performance. Because
171 when a request produces a static file as the response, the file handle
172 gets passed around and is buffered and not the file contents. That allows
173 to serve many large files without wasting memory or copying data
174 unnecessarily. However file handles are a limited resource for a process,
175 and if too many are used this way, requests may fail under load as
176 the amount of open handles has been exceeded.
180 <h3><a name="misdirected" id="misdirected">Multiple Hosts and Misdirected Requests</a></h3>
182 Many sites use the same TLS certificate for multiple virtual hosts. The
183 certificate either has a wildcard name, such as '*.example.org' or carries
184 several alternate names. Browsers using HTTP/2 will recognize that and reuse
185 an already opened connection for such hosts.
188 While this is great for performance, it comes at a price: such vhosts
189 need more care in their configuration. The problem is that you will have
190 multiple requests for multiple hosts on the same TLS connection. And that
191 makes renegotiation impossible, in face the HTTP/2 standard forbids it.
194 So, if you have several virtual hosts using the same certificate and
195 want to use HTTP/2 for them, you need to make sure that all vhosts have
196 exactly the same SSL configuration. You need the same protocol,
197 ciphers and settings for client verification.
200 If you mix things, Apache httpd will detect it and return a special
201 response code, 421 Misdirected Request, to the client.
205 <h3><a name="envvars" id="envvars">Environment Variables</a></h3>
207 This module can be configured to provide HTTP/2 related information
208 as additional environment variables to the SSI and CGI namespace, as well
209 as in custom log configurations (see <code>%{VAR_NAME}e</code>).
212 <table class="bordered">
215 <th><a name="table3">Variable Name:</a></th>
217 <th>Description:</th>
219 <tr><td><code>HTTP2</code></td><td>flag</td><td>HTTP/2 is being used.</td></tr>
220 <tr><td><code>H2PUSH</code></td><td>flag</td><td>HTTP/2 Server Push is enabled for this connection and also supported by the client.</td></tr>
221 <tr><td><code>H2_PUSH</code></td><td>flag</td><td>alternate name for <code>H2PUSH</code></td></tr>
222 <tr><td><code>H2_PUSHED</code></td><td>string</td><td>empty or <code>PUSHED</code> for a request being pushed by the server.</td></tr>
223 <tr><td><code>H2_PUSHED_ON</code></td><td>number</td><td>HTTP/2 stream number that triggered the push of this request.</td></tr>
224 <tr><td><code>H2_STREAM_ID</code></td><td>number</td><td>HTTP/2 stream number of this request.</td></tr>
225 <tr><td><code>H2_STREAM_TAG</code></td><td>string</td><td>HTTP/2 process unique stream identifier, consisting of connection id and stream id separated by <code>-</code>.</td></tr>
230 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
231 <div class="directive-section"><h2><a name="H2Direct" id="H2Direct">H2Direct</a> <a name="h2direct" id="h2direct">Directive</a></h2>
232 <table class="directive">
233 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>H2 Direct Protocol Switch</td></tr>
234 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2Direct on|off</code></td></tr>
235 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2Direct on for h2c, off for h2 protocol</code></td></tr>
236 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
237 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
238 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
241 This directive toggles the usage of the HTTP/2 Direct Mode. This
242 should be used inside a
243 <code class="directive"><a href="../mod/core.html#virtualhost"><VirtualHost></a></code>
244 section to enable direct HTTP/2 communication for that virtual host.
247 Direct communication means that if the first bytes received by the
248 server on a connection match the HTTP/2 preamble, the HTTP/2
249 protocol is switched to immediately without further negotiation.
250 This mode is defined in RFC 7540 for the cleartext (h2c) case. Its
251 use on TLS connections not mandated by the standard.
254 When a server/vhost does not have h2 or h2c enabled via
255 <code class="directive"><a href="../mod/core.html#protocols">Protocols</a></code>,
256 the connection is never inspected for a HTTP/2 preamble.
257 <code class="directive">H2Direct</code>
258 does not matter then. This is important for connections that
259 use protocols where an initial read might hang indefinitely, such
263 For clients that have out-of-band knowledge about a server
264 supporting h2c, direct HTTP/2 saves the client from having to
265 perform an HTTP/1.1 upgrade, resulting in better performance
266 and avoiding the Upgrade restrictions on request bodies.
269 This makes direct h2c attractive for server to server communication
270 as well, when the connection can be trusted or is secured by other means.
272 <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2Direct on</pre>
276 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
277 <div class="directive-section"><h2><a name="H2MaxSessionStreams" id="H2MaxSessionStreams">H2MaxSessionStreams</a> <a name="h2maxsessionstreams" id="h2maxsessionstreams">Directive</a></h2>
278 <table class="directive">
279 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Maximum number of active streams per HTTP/2 session.</td></tr>
280 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2MaxSessionStreams <em>n</em></code></td></tr>
281 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2MaxSessionStreams 100</code></td></tr>
282 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
283 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
284 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
287 This directive sets the maximum number of active streams per HTTP/2 session (e.g. connection)
288 that the server allows. A stream is active if it is not <code>idle</code> or
289 <code>closed</code> according to RFC 7540.
291 <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2MaxSessionStreams 20</pre>
295 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
296 <div class="directive-section"><h2><a name="H2MaxWorkerIdleSeconds" id="H2MaxWorkerIdleSeconds">H2MaxWorkerIdleSeconds</a> <a name="h2maxworkeridleseconds" id="h2maxworkeridleseconds">Directive</a></h2>
297 <table class="directive">
298 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Maximum number of seconds h2 workers remain idle until shut down.</td></tr>
299 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2MaxWorkerIdleSeconds <em>n</em></code></td></tr>
300 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2MaxWorkerIdleSeconds 600</code></td></tr>
301 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
302 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
303 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
306 This directive sets the maximum number of seconds a h2 worker may
307 idle until it shuts itself down. This only happens while the number of
308 h2 workers exceeds <code class="directive"><a href="#h2minworkers">H2MinWorkers</a></code>.
310 <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2MaxWorkerIdleSeconds 20</pre>
314 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
315 <div class="directive-section"><h2><a name="H2MaxWorkers" id="H2MaxWorkers">H2MaxWorkers</a> <a name="h2maxworkers" id="h2maxworkers">Directive</a></h2>
316 <table class="directive">
317 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Maximum number of worker threads to use per child process.</td></tr>
318 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2MaxWorkers <em>n</em></code></td></tr>
319 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
320 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
321 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
324 This directive sets the maximum number of worker threads to spawn
325 per child process for HTTP/2 processing. If this directive is not used,
326 <code class="module"><a href="../mod/mod_http2.html">mod_http2</a></code> will chose a value suitable for the <code>mpm</code>
329 <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2MaxWorkers 20</pre>
333 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
334 <div class="directive-section"><h2><a name="H2MinWorkers" id="H2MinWorkers">H2MinWorkers</a> <a name="h2minworkers" id="h2minworkers">Directive</a></h2>
335 <table class="directive">
336 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Minimal number of worker threads to use per child process.</td></tr>
337 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2MinWorkers <em>n</em></code></td></tr>
338 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
339 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
340 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
343 This directive sets the minimum number of worker threads to spawn
344 per child process for HTTP/2 processing. If this directive is not used,
345 <code class="module"><a href="../mod/mod_http2.html">mod_http2</a></code> will chose a value suitable for the <code>mpm</code>
348 <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2MinWorkers 10</pre>
352 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
353 <div class="directive-section"><h2><a name="H2ModernTLSOnly" id="H2ModernTLSOnly">H2ModernTLSOnly</a> <a name="h2moderntlsonly" id="h2moderntlsonly">Directive</a></h2>
354 <table class="directive">
355 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Require HTTP/2 connections to be "modern TLS" only</td></tr>
356 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2ModernTLSOnly on|off</code></td></tr>
357 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2ModernTLSOnly on</code></td></tr>
358 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
359 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
360 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
361 <tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.18 and later.</td></tr>
364 This directive toggles the security checks on HTTP/2 connections
365 in TLS mode (https:). This can be used server wide or for specific
366 <code class="directive"><a href="../mod/core.html#virtualhost"><VirtualHost></a></code>s.
369 The security checks require that the TSL protocol is at least
370 TLSv1.2 and that none of the ciphers listed in RFC 7540, Appendix A
371 is used. These checks will be extended once new security requirements
375 The name stems from the
376 <a href="https://wiki.mozilla.org/Security/Server_Side_TLS">Security/Server Side TLS</a>
377 definitions at mozilla where "modern compatibility" is defined. Mozilla Firefox and
378 other browsers require modern compatibility for HTTP/2 connections. As everything
379 in OpSec, this is a moving target and can be expected to evolve in the future.
382 One purpose of having these checks in <code class="module"><a href="../mod/mod_http2.html">mod_http2</a></code> is to enforce this
383 security level for all connections, not only those from browsers. The other
384 purpose is to prevent the negotiation of HTTP/2 as a protocol should
385 the requirements not be met.
388 Ultimately, the security of the TLS connection is determined by the
389 server configuration directives for <code class="module"><a href="../mod/mod_ssl.html">mod_ssl</a></code>.
391 <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2ModernTLSOnly off</pre>
395 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
396 <div class="directive-section"><h2><a name="H2Push" id="H2Push">H2Push</a> <a name="h2push" id="h2push">Directive</a></h2>
397 <table class="directive">
398 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>H2 Server Push Switch</td></tr>
399 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2Push on|off</code></td></tr>
400 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2Push on</code></td></tr>
401 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
402 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
403 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
404 <tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.18 and later.</td></tr>
407 This directive toggles the usage of the HTTP/2 server push
408 protocol feature. This should be used inside a
409 <code class="directive"><a href="../mod/core.html#virtualhost"><VirtualHost></a></code>
410 section to enable direct HTTP/2 communication for that virtual host.
413 The HTTP/2 protocol allows the server to push other resources to
414 a client when it asked for a particular one. This is helpful
415 if those resources are connected in some way and the client can
416 be expected to ask for it anyway. The pushing then saves the
417 time it takes the client to ask for the resources itself. On the
418 other hand, pushing resources the client never needs or already
419 has is a waste of bandwidth.
422 Server pushes are detected by inspecting the <code>Link</code> headers of
423 responses (see https://tools.ietf.org/html/rfc5988 for the
424 specification). When a link thus specified has the <code>rel=preload</code>
425 attribute, it is treated as a resource to be pushed.
428 Link headers in responses are either set by the application or
429 can be configured via <code class="module"><a href="../mod/mod_headers.html">mod_headers</a></code> as:
431 <div class="example"><h3>mod_headers example</h3><pre class="prettyprint lang-config"><Location /index.html>
432 Header add Link "</css/site.css>;rel=preload"
433 Header add Link "</images/logo.jpg>;rel=preload"
434 </Location></pre>
437 As the example shows, there can be several link headers added
438 to a response, resulting in several pushes being triggered. There
439 are no checks in the module to avoid pushing the same resource
440 twice or more to one client. Use with care.
443 HTTP/2 server pushes are enabled by default. This directive
444 allows it to be switch off on all resources of this server/virtual
447 <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2Push off</pre>
450 Last but not least, pushes happen only when the client signals
451 its willingness to accept those. Most browsers do, some, like Safari 9,
452 do not. Also, pushes also only happen for resources from the same
453 <em>authority</em> as the original response is for.
457 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
458 <div class="directive-section"><h2><a name="H2PushDiarySize" id="H2PushDiarySize">H2PushDiarySize</a> <a name="h2pushdiarysize" id="h2pushdiarysize">Directive</a></h2>
459 <table class="directive">
460 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>H2 Server Push Diary Size</td></tr>
461 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2PushDiarySize <em>n</em></code></td></tr>
462 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2PushDiarySize 256</code></td></tr>
463 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
464 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
465 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
466 <tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.19 and later.</td></tr>
469 This directive toggles the maximum number of HTTP/2 server pushes
470 that are remembered per HTTP/2 connection. This can be used inside the
471 <code class="directive"><a href="../mod/core.html#virtualhost"><VirtualHost></a></code>
472 section to influence the number for all connections to that virtual host.
475 The push diary records a digest (currently using a 64 bit number) of pushed
476 resources (their URL) to avoid duplicate pushes on the same connection.
477 These value are not persisted, so clients opening a new connection
478 will experience known pushes again. There is ongoing work to enable
479 a client to disclose a digest of the resources it already has, so
480 the diary maybe initialized by the client on each connection setup.
483 If the maximum size is reached, newer entries replace the oldest
484 ones. A diary entry uses 8 bytes, letting a
485 default diary with 256 entries consume around 2 KB of memory.
488 A size of 0 will effectively disable the push diary.
492 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
493 <div class="directive-section"><h2><a name="H2PushPriority" id="H2PushPriority">H2PushPriority</a> <a name="h2pushpriority" id="h2pushpriority">Directive</a></h2>
494 <table class="directive">
495 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>H2 Server Push Priority</td></tr>
496 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2PushPriority <em>mime-type</em> [after|before|interleaved] [weight]</code></td></tr>
497 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2PushPriority * After 16</code></td></tr>
498 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
499 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
500 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
501 <tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.18 and later. For having an
502 effect, a nghttp2 library version 1.5.0 or newer is necessary.</td></tr>
505 This directive defines the priority handling of pushed responses
506 based on the content-type of the response. This is usually defined
507 per server config, but may also appear in a virtual host.
510 HTTP/2 server pushes are always related to a client request. Each
511 such request/response pairs, or <em>streams</em> have a dependency
512 and a weight, together defining the <em>priority</em> of a stream.
515 When a stream <em>depends</em> on another, say X depends on Y,
516 then Y gets all bandwidth before X gets any. Note that this
517 does not mean that Y will block X. If Y has no data to send,
518 all bandwidth allocated to Y can be used by X.
521 When a stream has more than one dependant, say X1 and X2 both
522 depend on Y, the <em>weight</em> determines the bandwidth
523 allocation. If X1 and X2 have the same weight, they both get
524 half of the available bandwidth. If the weight of X1 is twice
525 as large as that for X2, X1 gets twice the bandwidth of X2.
528 Ultimately, every stream depends on the <em>root</em> stream which
529 gets all the bandwidth available, but never sends anything. So all
530 its bandwidth is distributed by weight among its children. Which
531 either have data to send or distribute the bandwidth to their
532 own children. And so on. If none of the children have data
533 to send, that bandwidth get distributed somewhere else according
537 The purpose of this priority system is to always make use of
538 available bandwidth while allowing precedence and weight
539 to be given to specific streams. Since, normally, all streams
540 are initiated by the client, it is also the one that sets
544 Only when such a stream results in a PUSH, gets the server to
545 decide what the <em>initial</em> priority of such a pushed
546 stream is. In the examples below, X is the client stream. It
547 depends on Y and the server decides to PUSH streams P1 and P2
551 The default priority rule is:
553 <div class="example"><h3>Default Priority Rule</h3><pre class="prettyprint lang-config">H2PushPriority * After 16</pre>
556 which reads as 'Send a pushed stream of any content-type
557 depending on the client stream with weight 16'. And so P1
558 and P2 will be send after X and, as they have equal weight,
559 share bandwidth equally among themselves.
561 <div class="example"><h3>Interleaved Priority Rule</h3><pre class="prettyprint lang-config">H2PushPriority text/css Interleaved 256</pre>
564 which reads as 'Send any CSS resource on the same dependency and
565 weight as the client stream'. If P1 has content-type 'text/css',
566 it will depend on Y (as does X) and its effective weight will be
567 calculated as <code>P1ew = Xw * (P1w / 256)</code>. With P1w being
568 256, this will make the effective weight the same as the weight
569 of X. If both X and P1 have data to send, bandwidth will be allocated
573 With Pw specified as 512, a pushed, interleaved stream would
574 get double the weight of X. With 128 only half as much. Note that
575 effective weights are always capped at 256.
577 <div class="example"><h3>Before Priority Rule</h3><pre class="prettyprint lang-config">H2PushPriority application/json Before</pre>
580 This says that any pushed stream of content type 'application/json'
581 should be send out <em>before</em> X. This makes P1 dependent
582 on Y and X dependent on P1. So, X will be stalled as long as
583 P1 has data to send. The effective weight is inherited from the
584 client stream. Specifying a weight is not allowed.
587 Be aware that the effect of priority specifications is limited
588 by the available server resources. If a server does not have
589 workers available for pushed streams, the data for the stream
590 may only ever arrive when other streams have been finished.
593 Last, but not least, there are some specifics of the syntax
594 to be used in this directive:
597 <li>'*' is the only special content-type that matches all others.
598 'image/*' will not work.</li>
599 <li>The default dependency is 'After'. </li>
600 <li>There are also default weights: for 'After' it is 16, 'interleaved' is 256.
603 <div class="example"><h3>Shorter Priority Rules</h3><pre class="prettyprint lang-config">H2PushPriority application/json 32 # an After rule
604 H2PushPriority image/jpeg before # weight inherited
605 H2PushPriority text/css interleaved # weight 256 default</pre>
609 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
610 <div class="directive-section"><h2><a name="H2SerializeHeaders" id="H2SerializeHeaders">H2SerializeHeaders</a> <a name="h2serializeheaders" id="h2serializeheaders">Directive</a></h2>
611 <table class="directive">
612 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Serialize Request/Response Processing Switch</td></tr>
613 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2SerializeHeaders on|off</code></td></tr>
614 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2SerializeHeaders off</code></td></tr>
615 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
616 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
617 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
620 This directive toggles if HTTP/2 requests shall be serialized in
621 HTTP/1.1 format for processing by <code>httpd</code> core or if
622 received binary data shall be passed into the <code>request_rec</code>s
626 Serialization will lower performance, but gives more backward
627 compatibility in case custom filters/hooks need it.
629 <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2SerializeHeaders on</pre>
633 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
634 <div class="directive-section"><h2><a name="H2SessionExtraFiles" id="H2SessionExtraFiles">H2SessionExtraFiles</a> <a name="h2sessionextrafiles" id="h2sessionextrafiles">Directive</a></h2>
635 <table class="directive">
636 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Number of Extra File Handles</td></tr>
637 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2SessionExtraFiles <em>n</em></code></td></tr>
638 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
639 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
640 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
643 This directive sets maximum number of <em>extra</em> file handles
644 a HTTP/2 session is allowed to use. A file handle is counted as
645 <em>extra</em> when it is transferred from a h2 worker thread to
646 the main HTTP/2 connection handling. This commonly happens when
647 serving static files.
649 Depending on the processing model configured on the server, the
650 number of connections times number of active streams may exceed
651 the number of file handles for the process. On the other hand,
652 converting every file into memory bytes early results in too
653 many buffer writes. This option helps to mitigate that.
655 The number of file handles used by a server process is then in
658 <pre>(h2_connections * extra_files) + (h2_max_worker)</pre>
659 <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2SessionExtraFiles 10</pre>
662 If nothing is configured, the module tries to make a conservative
663 guess how many files are safe to use. This depends largely on the
668 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
669 <div class="directive-section"><h2><a name="H2StreamMaxMemSize" id="H2StreamMaxMemSize">H2StreamMaxMemSize</a> <a name="h2streammaxmemsize" id="h2streammaxmemsize">Directive</a></h2>
670 <table class="directive">
671 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Maximum amount of output data buffered per stream.</td></tr>
672 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2StreamMaxMemSize <em>bytes</em></code></td></tr>
673 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2StreamMaxMemSize 65536</code></td></tr>
674 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
675 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
676 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
679 This directive sets the maximum number of outgoing data bytes buffered in memory
680 for an active streams. This memory is not allocated per stream as such. Allocations
681 are counted against this limit when they are about to be done. Stream processing
682 freezes when the limit has been reached and will only continue when buffered data
683 has been sent out to the client.
685 <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2StreamMaxMemSize 128000</pre>
689 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
690 <div class="directive-section"><h2><a name="H2TLSCoolDownSecs" id="H2TLSCoolDownSecs">H2TLSCoolDownSecs</a> <a name="h2tlscooldownsecs" id="h2tlscooldownsecs">Directive</a></h2>
691 <table class="directive">
692 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td /></tr>
693 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2TLSCoolDownSecs <em>seconds</em></code></td></tr>
694 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2TLSCoolDownSecs 1</code></td></tr>
695 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
696 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
697 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
698 <tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.18 and later.</td></tr>
701 This directive sets the number of seconds of idle time on a TLS
702 connection before the TLS write size falls back to small (~1300 bytes)
704 This can be used server wide or for specific
705 <code class="directive"><a href="../mod/core.html#virtualhost"><VirtualHost></a></code>s.
708 See <code class="directive"><a href="#h2tlswarmupsize">H2TLSWarmUpSize</a></code> for a
709 description of TLS warmup. <code class="directive">H2TLSCoolDownSecs</code> reflects the fact
710 that connections may deteriorate over time (and TCP flow adjusts)
711 for idle connections as well. It is beneficial to overall performance
712 to fall back to the pre-warmup phase after a number of seconds that
713 no data has been sent.
716 In deployments where connections can be considered reliable, this
717 timer can be disabled by setting it to 0.
720 The following example sets the seconds to zero, effectively disabling
721 any cool down. Warmed up TLS connections stay on maximum record
724 <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2TLSCoolDownSecs 0</pre>
728 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
729 <div class="directive-section"><h2><a name="H2TLSWarmUpSize" id="H2TLSWarmUpSize">H2TLSWarmUpSize</a> <a name="h2tlswarmupsize" id="h2tlswarmupsize">Directive</a></h2>
730 <table class="directive">
731 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td /></tr>
732 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2TLSWarmUpSize <em>amount</em></code></td></tr>
733 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2TLSWarmUpSize 1048576</code></td></tr>
734 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
735 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
736 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
737 <tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.18 and later.</td></tr>
740 This directive sets the number of bytes to be sent in small
741 TLS records (~1300 bytes) until doing maximum sized writes (16k)
742 on https: HTTP/2 connections.
743 This can be used server wide or for specific
744 <code class="directive"><a href="../mod/core.html#virtualhost"><VirtualHost></a></code>s.
747 Measurements by <a href="https://www.igvita.com">google performance
748 labs</a> show that best performance on TLS connections is reached,
749 if initial record sizes stay below the MTU level, to allow a
750 complete record to fit into an IP packet.
753 While TCP adjust its flow-control and window sizes, longer TLS
754 records can get stuck in queues or get lost and need retransmission.
755 This is of course true for all packets. TLS however needs the
756 whole record in order to decrypt it. Any missing bytes at the end
757 will stall usage of the received ones.
760 After a sufficient number of bytes have been send successfully,
761 the TCP state of the connection is stable and maximum TLS record
762 sizes (16 KB) can be used for optimal performance.
765 In deployments where servers are reached locally or over reliable
766 connections only, the value might be decreased with 0 disabling
767 any warmup phase altogether.
770 The following example sets the size to zero, effectively disabling
773 <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2TLSWarmUpSize 0</pre>
777 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
778 <div class="directive-section"><h2><a name="H2Upgrade" id="H2Upgrade">H2Upgrade</a> <a name="h2upgrade" id="h2upgrade">Directive</a></h2>
779 <table class="directive">
780 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>H2 Upgrade Protocol Switch</td></tr>
781 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2Upgrade on|off</code></td></tr>
782 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2Upgrade on for h2c, off for h2 protocol</code></td></tr>
783 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
784 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
785 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
788 This directive toggles the usage of the HTTP/1.1 Upgrade method
789 for switching to HTTP/2. This
790 should be used inside a
791 <code class="directive"><a href="../mod/core.html#virtualhost"><VirtualHost></a></code>
792 section to enable Upgrades to HTTP/2 for that virtual host.
795 This method of switching protocols is defined in HTTP/1.1 and
796 uses the "Upgrade" header (thus the name) to announce willingness
797 to use another protocol. This may happen on any request of a
801 This method of protocol switching is enabled by default on cleartext
802 (potential h2c) connections and disabled on TLS (potential h2),
803 as mandated by RFC 7540.
806 Please be aware that Upgrades are only accepted for requests
807 that carry no body. POSTs and PUTs with content will never
808 trigger an upgrade to HTTP/2.
809 See <code class="directive"><a href="#h2direct">H2Direct</a></code> for an
810 alternative to Upgrade.
813 This mode only has an effect when h2 or h2c is enabled via
814 the <code class="directive"><a href="../mod/core.html#protocols">Protocols</a></code>.
816 <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2Upgrade on</pre>
820 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
821 <div class="directive-section"><h2><a name="H2WindowSize" id="H2WindowSize">H2WindowSize</a> <a name="h2windowsize" id="h2windowsize">Directive</a></h2>
822 <table class="directive">
823 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Size of Stream Window for upstream data.</td></tr>
824 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2WindowSize <em>bytes</em></code></td></tr>
825 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2WindowSize 65535</code></td></tr>
826 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
827 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
828 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
831 This directive sets the size of the window that is used for flow control
832 from client to server and limits the amount of data the server has to buffer.
833 The client will stop sending on a stream once the limit has been reached until
834 the server announces more available space (as it has processed some of the data).
836 This limit affects only request bodies, not its meta data such as headers. Also,
837 it has no effect on response bodies as the window size for those are managed
840 <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2WindowSize 128000</pre>
845 <div class="bottomlang">
846 <p><span>Available Languages: </span><a href="../en/mod/mod_http2.html" title="English"> en </a></p>
847 </div><div class="top"><a href="#page-header"><img src="../images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our <a href="http://httpd.apache.org/lists.html">mailing lists</a>.</div>
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