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<div id="preamble"><h1>Apache Module mod_proxy_beacon</h1>
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<table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Dynamic Balancer membership where backends announce themselves
to the reverse proxy over unicast UDP datagrams</td></tr>
<tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="module-dict.html#ModuleIdentifier">Module&nbsp;Identifier:</a></th><td>proxy_beacon_module</td></tr>
<tr><th><a href="module-dict.html#SourceFile">Source&nbsp;File:</a></th><td>mod_proxy_beacon.c</td></tr>
<tr><th><a href="module-dict.html#Compatibility">Compatibility:</a></th><td>Available in Apache 2.5 and later</td></tr></table>
<h3>Summary</h3>
<p>This module lets backend servers <em>announce themselves</em> to a
front-end reverse proxy, which then adds each announcing backend as a live
member (worker) of a <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> balancer. When a
backend stops announcing, the proxy takes it out of rotation. This provides
self-registering, self-healing balancer membership without editing the proxy
configuration or driving the <code>balancer-manager</code> by hand.</p>
<p>Communication uses plain <strong>unicast UDP</strong> datagrams (not
multicast, which is filtered on most networks and does not traverse the
public Internet). The data flows from backend to proxy:</p>
<ul>
<li>The reverse proxy binds a UDP socket and <em>receives</em> on a
stable address (<code class="directive">ProxyBeaconListen</code>).</li>
<li>Each backend periodically <em>sends</em> a short announcement datagram
to the proxy (<code class="directive">ProxyBeaconAddress</code>), advertising
its own routable URL
(<code class="directive">ProxyBeaconAdvertise</code>).</li>
</ul>
<p>Datagrams are fire-and-forget: a lost announcement is recovered by the
next periodic one, and reordering is rejected by a per-backend timestamp
check, so no connection, reconnect, or framing layer is needed.</p>
<p>On the proxy, <code class="directive">ProxyBeaconBalancer</code> names the balancer
that announced backends are added to. Membership changes are applied using
the same internal mechanism as the <code>balancer-manager</code> web
interface, so a backend added this way behaves exactly like a statically
configured or manually added
<code class="directive"><a href="../mod/mod_proxy.html#balancermember">BalancerMember</a></code>, and is visible and
editable in the <code>balancer-manager</code>.</p>
<p>This module <em>requires</em> the service of
<code class="module"><a href="../mod/mod_watchdog.html">mod_watchdog</a></code> and <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code>. The
background work (listening, publishing, adding and evicting members) runs in
a single <code class="module"><a href="../mod/mod_watchdog.html">mod_watchdog</a></code> child process, so it is not available
under the <code>prefork</code> MPM behaviour where that singleton cannot
run.</p>
<div class="warning"><h3>Authentication</h3>
<p>Any host that can reach the proxy's receive port could otherwise announce
an arbitrary backend URL and cause the proxy to send client traffic to it
(and a UDP source address is trivially spoofable).
<code class="directive">ProxyBeaconSecret</code> is therefore <strong>required</strong>:
it must be set to the same value on the proxy and on every backend, and the
server fails to start if any participating server omits it. Announcements are
authenticated with a keyed message-authentication code (MAC) and a timestamp,
and the proxy drops any announcement that is not validly signed and recent.
There is no unauthenticated mode.</p>
</div>
<div class="note"><h3>Confidentiality</h3>
<p>Announcements are authenticated but not encrypted; the payload is
operational metadata (backend URLs), not secret data. Transport
confidentiality (e.g. DTLS) is not currently provided and would be a separate
future layer.</p>
</div>
</div>
<div id="quickview"><h3>Topics</h3>
<ul id="topics">
<li><img alt="" src="../images/down.gif"> <a href="#examples">Usage example</a></li>
</ul><h3 class="directives">Directives</h3>
<ul id="toc">
<li><img alt="" src="../images/down.gif"> <a href="#proxybeaconaddress">ProxyBeaconAddress</a></li>
<li><img alt="" src="../images/down.gif"> <a href="#proxybeaconadvertise">ProxyBeaconAdvertise</a></li>
<li><img alt="" src="../images/down.gif"> <a href="#proxybeaconbalancer">ProxyBeaconBalancer</a></li>
<li><img alt="" src="../images/down.gif"> <a href="#proxybeaconinterval">ProxyBeaconInterval</a></li>
<li><img alt="" src="../images/down.gif"> <a href="#proxybeaconlisten">ProxyBeaconListen</a></li>
<li><img alt="" src="../images/down.gif"> <a href="#proxybeaconmaxskew">ProxyBeaconMaxSkew</a></li>
<li><img alt="" src="../images/down.gif"> <a href="#proxybeaconsecret">ProxyBeaconSecret</a></li>
<li><img alt="" src="../images/down.gif"> <a href="#proxybeacontimeout">ProxyBeaconTimeout</a></li>
</ul>
<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__&amp;list_id=144532&amp;product=Apache%20httpd-2&amp;query_format=specific&amp;order=changeddate%20DESC%2Cpriority%2Cbug_severity&amp;component=mod_proxy_beacon">Known issues</a></li><li><a href="https://bz.apache.org/bugzilla/enter_bug.cgi?product=Apache%20httpd-2&amp;component=mod_proxy_beacon">Report a bug</a></li></ul><h3>See also</h3>
<ul class="seealso">
<li><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code></li>
<li><code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code></li>
<li><code class="module"><a href="../mod/mod_proxy_hcheck.html">mod_proxy_hcheck</a></code></li>
<li><code class="module"><a href="../mod/mod_watchdog.html">mod_watchdog</a></code></li>
</ul></div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif"></a></div>
<div class="section">
<h2 id="examples">Usage example <a title="Permanent link" href="#examples" class="permalink">&para;</a></h2>
<p>The following pair of configurations sets up a self-registering balancer.
The backends require no knowledge of each other and the proxy needs no
pre-declared <code class="directive"><a href="../mod/mod_proxy.html#balancermember">BalancerMember</a></code>
entries &mdash; only an empty balancer with room to grow.</p>
<p>On the <strong>reverse proxy</strong>:</p>
<pre class="prettyprint lang-config"># Receive backend announcements on the cluster network interface (UDP).
ProxyBeaconListen 0.0.0.0:5555
ProxyBeaconSecret "a-long-random-shared-cluster-secret"
ProxyBeaconBalancer cluster
# A backend is dropped from rotation if it does not announce for 30 seconds.
ProxyBeaconTimeout 30
# An initially-empty balancer with spare slots for the dynamic members.
&lt;Proxy balancer://cluster&gt;
ProxySet growth=16
&lt;/Proxy&gt;
ProxyPass "/" "balancer://cluster/"
ProxyPassReverse "/" "balancer://cluster/"</pre>
<p>On each <strong>backend</strong> server:</p>
<pre class="prettyprint lang-config"># Announce this backend's routable origin to the proxy every 10 seconds (UDP).
ProxyBeaconAddress proxy.example.com:5555
ProxyBeaconAdvertise http://10.0.0.5:8080
ProxyBeaconSecret "a-long-random-shared-cluster-secret"
ProxyBeaconInterval 10</pre>
<p>When a backend starts it begins sending announcements. The proxy
verifies each announcement against the shared secret, adds
<code>http://10.0.0.5:8080</code> as a member of
<code>balancer://cluster</code>, and enables it. If that backend later stops
announcing for longer than <code class="directive">ProxyBeaconTimeout</code>, the proxy
disables the member (taking it out of rotation); a subsequent announcement
re-enables it.</p>
<div class="note">
<p>A backend added at runtime occupies one of the balancer's growth slots
for the lifetime of the server process; it is disabled rather than removed
when it stops announcing, matching the behaviour of the
<code>balancer-manager</code> (which can add, but not remove, workers at
runtime). Size <code>growth</code> for the maximum number of backends you
expect to register.</p>
</div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif"></a></div>
<div class="directive-section"><h2 id="proxybeaconaddress"><span id="ProxyBeaconAddress">ProxyBeaconAddress</span> Directive <a title="Permanent link" href="#proxybeaconaddress" class="permalink">&para;</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Address of the reverse proxy to which a backend sends its
announcements</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>ProxyBeaconAddress <em>address:port</em></code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_proxy_beacon</td></tr>
</table>
<p>The <code class="directive">ProxyBeaconAddress</code> directive marks a server as an
announcement <em>sender</em> (a backend). It sends UDP datagrams to the
proxy's <code class="directive">ProxyBeaconListen</code> address given by
<em>address:port</em>, e.g. <code>proxy.example.com:5555</code> (a leading
scheme such as <code>tcp://</code> is accepted and ignored). Because UDP is
connectionless, a backend may be started before the proxy is available:
early datagrams are simply dropped and the next interval retries.</p>
<p>Use <code class="directive">ProxyBeaconAdvertise</code> to specify the routable URL
the backend announces. <code class="directive">ProxyBeaconAddress</code> and
<code class="directive">ProxyBeaconListen</code> are mutually exclusive on the same
server.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif"></a></div>
<div class="directive-section"><h2 id="proxybeaconadvertise"><span id="ProxyBeaconAdvertise">ProxyBeaconAdvertise</span> Directive <a title="Permanent link" href="#proxybeaconadvertise" class="permalink">&para;</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>The routable URL a backend announces to the reverse proxy</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>ProxyBeaconAdvertise <em>url</em></code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_proxy_beacon</td></tr>
</table>
<p>The <code class="directive">ProxyBeaconAdvertise</code> directive sets the backend's
own reachable origin (for example <code>http://10.0.0.5:8080</code>) that the
proxy will add as a <code class="directive"><a href="../mod/mod_proxy.html#balancermember">BalancerMember</a></code>.
It must be a full <code>scheme://host[:port]</code> URL that the proxy can
reach &mdash; not the local listen address &mdash; and is validated when the
configuration is parsed.</p>
<p>This directive is used on a backend, alongside
<code class="directive">ProxyBeaconAddress</code>. If it is omitted, the backend still
sends a heartbeat but advertises no URL, so the proxy logs the
announcement without adding a member.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif"></a></div>
<div class="directive-section"><h2 id="proxybeaconbalancer"><span id="ProxyBeaconBalancer">ProxyBeaconBalancer</span> Directive <a title="Permanent link" href="#proxybeaconbalancer" class="permalink">&para;</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Name of the balancer that announced backends are added to</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>ProxyBeaconBalancer <em>name</em></code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_proxy_beacon</td></tr>
</table>
<p>The <code class="directive">ProxyBeaconBalancer</code> directive names the balancer,
on the reverse proxy, into which announced backends are inserted as members.
Give the bare balancer name (for example <code>cluster</code> for
<code>balancer://cluster</code>); a leading <code>balancer://</code> is
accepted and stripped.</p>
<p>The named balancer must exist and have spare capacity. Declare it with a
<code class="directive"><a href="../mod/mod_proxy.html#<proxy>">&lt;Proxy&gt;</a></code> block and a
<code>growth</code> setting (or rely on
<code class="directive"><a href="../mod/mod_proxy.html#balancergrowth">BalancerGrowth</a></code>) so there are free
slots for the dynamically added members. This directive is used together
with <code class="directive">ProxyBeaconListen</code>.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif"></a></div>
<div class="directive-section"><h2 id="proxybeaconinterval"><span id="ProxyBeaconInterval">ProxyBeaconInterval</span> Directive <a title="Permanent link" href="#proxybeaconinterval" class="permalink">&para;</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>How often a backend publishes its announcement</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>ProxyBeaconInterval <em>interval</em></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>ProxyBeaconInterval 5</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_proxy_beacon</td></tr>
</table>
<p>The <code class="directive">ProxyBeaconInterval</code> directive sets how frequently
a backend (a <code class="directive">ProxyBeaconAddress</code> server) publishes its
announcement. It uses the
<a href="directive-dict.html#Syntax">time-interval</a> directive syntax and
defaults to seconds; the default is 5 seconds.</p>
<p>The interval must be meaningfully smaller than the proxy's
<code class="directive">ProxyBeaconTimeout</code>, so that the occasional lost or
delayed announcement does not cause a healthy backend to be evicted.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif"></a></div>
<div class="directive-section"><h2 id="proxybeaconlisten"><span id="ProxyBeaconListen">ProxyBeaconListen</span> Directive <a title="Permanent link" href="#proxybeaconlisten" class="permalink">&para;</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Address on which the reverse proxy receives backend
beacons</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>ProxyBeaconListen [<em>address</em>][:<em>port</em>]</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_proxy_beacon</td></tr>
</table>
<p>The <code class="directive">ProxyBeaconListen</code> directive marks a server as
the beacon <em>receiver</em> (the reverse proxy). It binds a UDP socket to
the given address, e.g. <code>0.0.0.0:5555</code> to receive on all
interfaces. A leading scheme (such as <code>tcp://</code>) is accepted and
ignored.</p>
<p>The address and port are both optional and, when omitted, are inherited
from this server's own address and port (its <code class="directive"><a href="../mod/core.html#listen">Listen</a></code>/<code class="directive"><a href="../mod/core.html#servername">ServerName</a></code>). With no argument at all, the beacon
listener binds the server's own address and port; given just an address it
inherits the port, and so on. Because UDP and TCP are independent port
spaces, binding the beacon socket to the server's port does <em>not</em>
collide with the server's TCP listener &mdash; letting the beacon channel
share the service endpoint, which also identifies the proxy to backends by
its real address. (The listener binds in an unprivileged child, so a
privileged port such as 80 or 443 cannot be shared this way; use the
server's port only when it is non-privileged.)</p>
<p>Backends send to this address via
<code class="directive">ProxyBeaconAddress</code>. The directive should be used
together with <code class="directive">ProxyBeaconBalancer</code>; without it,
announcements are received and logged but no members are added.
<code class="directive">ProxyBeaconListen</code> and
<code class="directive">ProxyBeaconAddress</code> are mutually exclusive on the same
server.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif"></a></div>
<div class="directive-section"><h2 id="proxybeaconmaxskew"><span id="ProxyBeaconMaxSkew">ProxyBeaconMaxSkew</span> Directive <a title="Permanent link" href="#proxybeaconmaxskew" class="permalink">&para;</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Maximum allowed age of a signed announcement</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>ProxyBeaconMaxSkew <em>interval</em></code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_proxy_beacon</td></tr>
</table>
<p>The <code class="directive">ProxyBeaconMaxSkew</code> directive sets the anti-replay
window used when <code class="directive">ProxyBeaconSecret</code> is configured: the
proxy rejects any announcement whose signed timestamp differs from the
current time by more than this amount, in either direction. It uses the
<a href="directive-dict.html#Syntax">time-interval</a> directive syntax and
defaults to seconds.</p>
<p>If unset, the default is 30 seconds. A larger window tolerates greater
clock skew between hosts; a smaller window bounds the freshness check. Note
that the per-backend strictly-increasing-timestamp check (see
<code class="directive">ProxyBeaconSecret</code>) blocks replays regardless of this
window. This directive is used on the proxy.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif"></a></div>
<div class="directive-section"><h2 id="proxybeaconsecret"><span id="ProxyBeaconSecret">ProxyBeaconSecret</span> Directive <a title="Permanent link" href="#proxybeaconsecret" class="permalink">&para;</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Pre-shared secret used to authenticate announcements</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>ProxyBeaconSecret <em>secret</em></code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_proxy_beacon</td></tr>
</table>
<p>The <code class="directive">ProxyBeaconSecret</code> directive sets a pre-shared
cluster secret. It must be configured with the <em>same</em> value on the
reverse proxy and on every backend. The backend (sender) signs each
announcement with a keyed message-authentication code (a SipHash MAC) derived
from the secret, together with a timestamp; the proxy (receiver) recomputes the MAC and
checks the timestamp, dropping any announcement that is forged, tampered
with, or replayed. Replayed messages are caught two ways: a freshness window
(<code class="directive">ProxyBeaconMaxSkew</code>) rejects old timestamps, and a
per-backend check rejects any announcement whose timestamp does not strictly
advance, so a captured-and-resent message (for example, one replayed to keep
a dead backend from being evicted) is dropped.</p>
<p>This directive is <strong>required</strong> on every server that
participates in the beacon channel &mdash; the proxy
(<code class="directive">ProxyBeaconListen</code>) and every backend
(<code class="directive">ProxyBeaconAddress</code>). If any such server omits it, the
server fails to start; there is no unauthenticated mode.</p>
<p>Every announcement must carry a valid, recent MAC or it is rejected. If the
secrets on the proxy and a backend differ, that backend's announcements are
silently rejected (and logged), which appears as the backend never joining
the balancer.</p>
<p>Because the secret is stored in the configuration file, restrict that
file's permissions as you would for a private key.</p>
<div class="note"><h3>Clock synchronisation</h3>
<p>The timestamp-based replay protection compares the announcement's time
against the proxy's clock, so the proxy and backends must have reasonably
synchronised clocks (for example via NTP). See
<code class="directive">ProxyBeaconMaxSkew</code>.</p>
</div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif"></a></div>
<div class="directive-section"><h2 id="proxybeacontimeout"><span id="ProxyBeaconTimeout">ProxyBeaconTimeout</span> Directive <a title="Permanent link" href="#proxybeacontimeout" class="permalink">&para;</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>How long the proxy waits, without an announcement, before a backend
is taken out of rotation</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>ProxyBeaconTimeout <em>interval</em></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>ProxyBeaconTimeout 0</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_proxy_beacon</td></tr>
</table>
<p>The <code class="directive">ProxyBeaconTimeout</code> directive sets how long the
reverse proxy will wait for an announcement from a backend before disabling
that backend's balancer member (taking it out of rotation). A later
announcement from the same backend re-enables it. It uses the
<a href="directive-dict.html#Syntax">time-interval</a> directive syntax and
defaults to seconds.</p>
<p>The default, <code>0</code>, disables eviction entirely: backends are
added when they announce but are never automatically removed. Set this to a
small multiple of the backends' <code class="directive">ProxyBeaconInterval</code> to
enable self-healing membership. This directive is used on the proxy.</p>
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