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  <section id="vcl-backends">
<span id="vcl-backend-7"></span><h1>VCL-backends<a class="headerlink" href="#vcl-backends" title="Link to this heading">¶</a></h1>
<section id="configuring-backends">
<h2>Configuring Backends<a class="headerlink" href="#configuring-backends" title="Link to this heading">¶</a></h2>
<dl class="field-list simple">
<dt class="field-odd">Manual section<span class="colon">:</span></dt>
<dd class="field-odd"><p>7</p>
</dd>
</dl>
<section id="backend-definition">
<span id="id1"></span><h3>Backend definition<a class="headerlink" href="#backend-definition" title="Link to this heading">¶</a></h3>
<p>A backend declaration creates and initialises a named backend object.
A declaration start with the keyword <code class="docutils literal notranslate"><span class="pre">backend</span></code> followed by the name of the
backend. The actual declaration is in curly brackets, in a key/value fashion.:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">backend</span> <span class="n">name</span> <span class="p">{</span>
    <span class="o">.</span><span class="n">attribute1</span> <span class="o">=</span> <span class="n">value</span><span class="p">;</span>
    <span class="o">.</span><span class="n">attribute2</span> <span class="o">=</span> <span class="n">value</span><span class="p">;</span>
    <span class="p">[</span><span class="o">...</span><span class="p">]</span>
<span class="p">}</span>
</pre></div>
</div>
<p>If there is a backend named <code class="docutils literal notranslate"><span class="pre">default</span></code> it will be used unless another
backend is explicitly set.  If no backend is named <code class="docutils literal notranslate"><span class="pre">default</span></code> the first
backend in the VCL program becomes the default.</p>
<p>If you only use dynamic backends created by VMODs, an empty, always failing
(503) backend can be specified:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">backend</span> <span class="n">default</span> <span class="n">none</span><span class="p">;</span>
</pre></div>
</div>
<p>A backend must be specified with either a <code class="docutils literal notranslate"><span class="pre">.host</span></code> or a <code class="docutils literal notranslate"><span class="pre">.path</span></code> attribute, but
not both.  All other attributes have default values.</p>
</section>
<section id="attribute-host">
<h3>Attribute <code class="docutils literal notranslate"><span class="pre">.host</span></code><a class="headerlink" href="#attribute-host" title="Link to this heading">¶</a></h3>
<p>To specify a networked backend <code class="docutils literal notranslate"><span class="pre">.host</span></code> takes either a numeric
IPv4/IPv6 address or a domain name which resolves to <em>at most</em>
one IPv4 and one IPv6 address:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">.</span><span class="n">host</span> <span class="o">=</span> <span class="s2">&quot;127.0.0.1&quot;</span><span class="p">;</span>

<span class="o">.</span><span class="n">host</span> <span class="o">=</span> <span class="s2">&quot;example.com:8081&quot;</span><span class="p">;</span>

<span class="o">.</span><span class="n">host</span> <span class="o">=</span> <span class="s2">&quot;example.com:http&quot;</span><span class="p">;</span>
</pre></div>
</div>
</section>
<section id="attribute-port">
<h3>Attribute <code class="docutils literal notranslate"><span class="pre">.port</span></code><a class="headerlink" href="#attribute-port" title="Link to this heading">¶</a></h3>
<p>The TCP port number or service name can be specified as part of
<code class="docutils literal notranslate"><span class="pre">.host</span></code> as above or separately using the <code class="docutils literal notranslate"><span class="pre">.port</span></code> attribute:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">.</span><span class="n">port</span> <span class="o">=</span> <span class="s2">&quot;8081&quot;</span><span class="p">;</span>

<span class="o">.</span><span class="n">port</span> <span class="o">=</span> <span class="s2">&quot;http&quot;</span><span class="p">;</span>
</pre></div>
</div>
</section>
<section id="attribute-path">
<h3>Attribute <code class="docutils literal notranslate"><span class="pre">.path</span></code><a class="headerlink" href="#attribute-path" title="Link to this heading">¶</a></h3>
<p>The absolute path to a Unix(4) domain socket of a local backend:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">.</span><span class="n">path</span> <span class="o">=</span> <span class="s2">&quot;/var/run/http.sock&quot;</span><span class="p">;</span>
</pre></div>
</div>
<p>or, where available, <code class="docutils literal notranslate"><span class="pre">&#64;</span></code> followed by the name of an abstract socket
of a local backend:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">.</span><span class="n">path</span> <span class="o">=</span> <span class="s2">&quot;@mybackend&quot;</span><span class="p">;</span>
</pre></div>
</div>
<p>A warning will be issued if the uds-socket does not exist when the
VCL is loaded.  This makes it possible to start the UDS-listening peer,
or set the socket file’s permissions afterwards.</p>
<p>If the uds-socket socket does not exist or permissions deny access,
connection attempts will fail.</p>
</section>
<section id="attribute-host-header">
<h3>Attribute <code class="docutils literal notranslate"><span class="pre">.host_header</span></code><a class="headerlink" href="#attribute-host-header" title="Link to this heading">¶</a></h3>
<p>A host header to add to probes and regular backend requests if they have no such header:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">.</span><span class="n">host_header</span> <span class="o">=</span> <span class="s2">&quot;Host: example.com&quot;</span><span class="p">;</span>
</pre></div>
</div>
</section>
<section id="timeout-attributes">
<h3>Timeout Attributes<a class="headerlink" href="#timeout-attributes" title="Link to this heading">¶</a></h3>
<p>These attributes control how patient <cite>varnishd</cite> is during backend fetches:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">.</span><span class="n">connect_timeout</span> <span class="o">=</span> <span class="mf">1.4</span><span class="n">s</span><span class="p">;</span>
<span class="o">.</span><span class="n">first_byte_timeout</span> <span class="o">=</span> <span class="mi">20</span><span class="n">s</span><span class="p">;</span>
<span class="o">.</span><span class="n">between_bytes_timeout</span> <span class="o">=</span> <span class="mi">10</span><span class="n">s</span><span class="p">;</span>
</pre></div>
</div>
<p>The default values comes parameters with the same names, see <a class="reference internal" href="varnishd.html#varnishd-1"><span class="std std-ref">varnishd</span></a>.</p>
</section>
<section id="attribute-max-connections">
<h3>Attribute <code class="docutils literal notranslate"><span class="pre">.max_connections</span></code><a class="headerlink" href="#attribute-max-connections" title="Link to this heading">¶</a></h3>
<p>Limit how many simultaneous connections varnish can open to the backend:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">.</span><span class="n">max_connections</span> <span class="o">=</span> <span class="mi">1000</span><span class="p">;</span>
</pre></div>
</div>
</section>
<section id="attribute-proxy-header">
<h3>Attribute <code class="docutils literal notranslate"><span class="pre">.proxy_header</span></code><a class="headerlink" href="#attribute-proxy-header" title="Link to this heading">¶</a></h3>
<p>Send a PROXY protocol header to the backend with the <code class="docutils literal notranslate"><span class="pre">client.ip</span></code> and
<code class="docutils literal notranslate"><span class="pre">server.ip</span></code> values:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">.</span><span class="n">proxy_header</span> <span class="o">=</span> <span class="mi">2</span><span class="p">;</span>
</pre></div>
</div>
<p>Legal values are one and two, depending which version of the PROXY protocol you want.</p>
<p><em>Notice</em> this setting will lead to backend connections being used
for a single request only (subject to future improvements). Thus,
extra care should be taken to avoid running into failing backend
connections with EADDRNOTAVAIL due to no local ports being
available. Possible options are:</p>
<blockquote>
<div><ul class="simple">
<li><p>Use additional backend connections to extra IP addresses or TCP ports</p></li>
<li><p>Increase the number of available ports (Linux sysctl <code class="docutils literal notranslate"><span class="pre">net.ipv4.ip_local_port_range</span></code>)</p></li>
<li><p>Reuse backend connection ports early (Linux sysctl <code class="docutils literal notranslate"><span class="pre">net.ipv4.tcp_tw_reuse</span></code>)</p></li>
</ul>
</div></blockquote>
</section>
<section id="attribute-preamble">
<h3>Attribute <code class="docutils literal notranslate"><span class="pre">.preamble</span></code><a class="headerlink" href="#attribute-preamble" title="Link to this heading">¶</a></h3>
<p>Send a BLOB on all newly opened connections to the backend:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">.</span><span class="n">preamble</span> <span class="o">=</span> <span class="p">:</span><span class="n">SGVsbG8gV29ybGRcbgo</span><span class="o">=</span><span class="p">:;</span>
</pre></div>
</div>
</section>
<section id="attribute-via">
<span id="backend-definition-via"></span><h3>Attribute <code class="docutils literal notranslate"><span class="pre">.via</span></code><a class="headerlink" href="#attribute-via" title="Link to this heading">¶</a></h3>
<p>Name of another <em>proxy</em> backend through which to make the connection
to the <em>destination</em> backend using the <a class="reference external" href="https://raw.githubusercontent.com/haproxy/haproxy/master/doc/proxy-protocol.txt">PROXY2</a> protocol, for example:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">backend</span> <span class="n">proxy</span> <span class="p">{</span>
  <span class="o">.</span><span class="n">path</span> <span class="o">=</span> <span class="s2">&quot;/path/to/proxy2_endpoint&quot;</span><span class="p">;</span>
<span class="p">}</span>
<span class="n">backend</span> <span class="n">destination</span> <span class="p">{</span>
  <span class="o">.</span><span class="n">host</span> <span class="o">=</span> <span class="s2">&quot;1.2.3.4&quot;</span><span class="p">;</span>
  <span class="o">.</span><span class="n">via</span> <span class="o">=</span> <span class="n">proxy</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>The <em>proxy</em> backend can also use a <code class="docutils literal notranslate"><span class="pre">.host</span></code>/<code class="docutils literal notranslate"><span class="pre">.port</span></code> definition
rather than <code class="docutils literal notranslate"><span class="pre">.path</span></code>.</p>
<p>Use of the <code class="docutils literal notranslate"><span class="pre">.path</span></code> attribute for the <em>destination</em> backend is not
supported.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">.via</span></code> attribute is unrelated to <code class="docutils literal notranslate"><span class="pre">.proxy_header</span></code>. If both are
used, a second header is sent as per <code class="docutils literal notranslate"><span class="pre">.proxy_header</span></code> specification.</p>
<p>As of this release, the <em>proxy</em> backend used with <code class="docutils literal notranslate"><span class="pre">.via</span></code> can not be
a director, it can not itself use <code class="docutils literal notranslate"><span class="pre">.via</span></code> (error: <em>Can not stack .via
backends</em>) and the protocol is fixed to <a class="reference external" href="https://raw.githubusercontent.com/haproxy/haproxy/master/doc/proxy-protocol.txt">PROXY2</a>.</p>
<p>Implementation detail:</p>
<p>If <code class="docutils literal notranslate"><span class="pre">.via</span> <span class="pre">=</span> <span class="pre">&lt;proxy&gt;</span></code> is used, a <a class="reference external" href="https://raw.githubusercontent.com/haproxy/haproxy/master/doc/proxy-protocol.txt">PROXY2</a> preamble is created with
the <em>destination</em> backend’s address information as <code class="docutils literal notranslate"><span class="pre">dst_addr</span></code>/<code class="docutils literal notranslate"><span class="pre">dst_port</span></code> and, optionally, other TLV attributes. The connection is
then made to the <em>proxy</em> backend’s endpoint (<code class="docutils literal notranslate"><span class="pre">path</span></code> or <code class="docutils literal notranslate"><span class="pre">host</span></code>/<code class="docutils literal notranslate"><span class="pre">port</span></code>). This is technically equivalent to specifying a <code class="docutils literal notranslate"><span class="pre">backend</span>
<span class="pre">destination_via_proxy</span></code> with a <code class="docutils literal notranslate"><span class="pre">.preamble</span></code> attribute containing the
appropriate <a class="reference external" href="https://raw.githubusercontent.com/haproxy/haproxy/master/doc/proxy-protocol.txt">PROXY2</a> preamble for the <em>destination</em> backend.</p>
</section>
<section id="attribute-authority">
<h3>Attribute <code class="docutils literal notranslate"><span class="pre">.authority</span></code><a class="headerlink" href="#attribute-authority" title="Link to this heading">¶</a></h3>
<p>The HTTP authority to use when connecting to this backend. If unset,
<code class="docutils literal notranslate"><span class="pre">.host_header</span></code> or <code class="docutils literal notranslate"><span class="pre">.host</span></code> are used.</p>
<p><code class="docutils literal notranslate"><span class="pre">.authority</span> <span class="pre">=</span> <span class="pre">&quot;&quot;</span></code> disables sending an authority.</p>
<p>As of this release, the attribute is only used by <code class="docutils literal notranslate"><span class="pre">.via</span></code> connections
as a <code class="docutils literal notranslate"><span class="pre">PP2_TYPE_AUTHORITY</span></code> Type-Length-Value (TLV) in the <a class="reference external" href="https://raw.githubusercontent.com/haproxy/haproxy/master/doc/proxy-protocol.txt">PROXY2</a>
preamble.</p>
</section>
<section id="attribute-probe">
<h3>Attribute <code class="docutils literal notranslate"><span class="pre">.probe</span></code><a class="headerlink" href="#attribute-probe" title="Link to this heading">¶</a></h3>
<p>Please see <a class="reference internal" href="vcl-probe.html#vcl-probe-7"><span class="std std-ref">VCL-probe</span></a>.</p>
</section>
<section id="see-also">
<h3>SEE ALSO<a class="headerlink" href="#see-also" title="Link to this heading">¶</a></h3>
<ul class="simple">
<li><p><a class="reference internal" href="varnishd.html#varnishd-1"><span class="std std-ref">varnishd</span></a></p></li>
<li><p><a class="reference internal" href="vcl.html#vcl-7"><span class="std std-ref">VCL</span></a></p></li>
<li><p><a class="reference internal" href="vcl-probe.html#vcl-probe-7"><span class="std std-ref">VCL-probe</span></a></p></li>
<li><p><a class="reference internal" href="vmod_directors.html#vmod-directors-3"><span class="std std-ref">VMOD directors - Varnish Directors Module</span></a></p></li>
<li><p><a class="reference internal" href="vmod_std.html#vmod-std-3"><span class="std std-ref">VMOD std - Varnish Standard Module</span></a></p></li>
</ul>
</section>
<section id="history">
<h3>HISTORY<a class="headerlink" href="#history" title="Link to this heading">¶</a></h3>
<p>VCL was developed by Poul-Henning Kamp in cooperation with Verdens
Gang AS, Redpill Linpro and Varnish Software.  This manual page is
written by Per Buer, Poul-Henning Kamp, Martin Blix Grydeland,
Kristian Lyngstøl, Lasse Karstensen and others.</p>
</section>
<section id="copyright">
<h3>COPYRIGHT<a class="headerlink" href="#copyright" title="Link to this heading">¶</a></h3>
<p>This document is licensed under the same license as Varnish
itself. See LICENSE for details.</p>
<ul class="simple">
<li><p>Copyright (c) 2006 Verdens Gang AS</p></li>
<li><p>Copyright (c) 2006-2021 Varnish Software AS</p></li>
</ul>
</section>
</section>
</section>

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    <h3><a href="../index.html">Table of Contents</a></h3>
    <ul>
<li><a class="reference internal" href="#">VCL-backends</a><ul>
<li><a class="reference internal" href="#configuring-backends">Configuring Backends</a><ul>
<li><a class="reference internal" href="#backend-definition">Backend definition</a></li>
<li><a class="reference internal" href="#attribute-host">Attribute <code class="docutils literal notranslate"><span class="pre">.host</span></code></a></li>
<li><a class="reference internal" href="#attribute-port">Attribute <code class="docutils literal notranslate"><span class="pre">.port</span></code></a></li>
<li><a class="reference internal" href="#attribute-path">Attribute <code class="docutils literal notranslate"><span class="pre">.path</span></code></a></li>
<li><a class="reference internal" href="#attribute-host-header">Attribute <code class="docutils literal notranslate"><span class="pre">.host_header</span></code></a></li>
<li><a class="reference internal" href="#timeout-attributes">Timeout Attributes</a></li>
<li><a class="reference internal" href="#attribute-max-connections">Attribute <code class="docutils literal notranslate"><span class="pre">.max_connections</span></code></a></li>
<li><a class="reference internal" href="#attribute-proxy-header">Attribute <code class="docutils literal notranslate"><span class="pre">.proxy_header</span></code></a></li>
<li><a class="reference internal" href="#attribute-preamble">Attribute <code class="docutils literal notranslate"><span class="pre">.preamble</span></code></a></li>
<li><a class="reference internal" href="#attribute-via">Attribute <code class="docutils literal notranslate"><span class="pre">.via</span></code></a></li>
<li><a class="reference internal" href="#attribute-authority">Attribute <code class="docutils literal notranslate"><span class="pre">.authority</span></code></a></li>
<li><a class="reference internal" href="#attribute-probe">Attribute <code class="docutils literal notranslate"><span class="pre">.probe</span></code></a></li>
<li><a class="reference internal" href="#see-also">SEE ALSO</a></li>
<li><a class="reference internal" href="#history">HISTORY</a></li>
<li><a class="reference internal" href="#copyright">COPYRIGHT</a></li>
</ul>
</li>
</ul>
</li>
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