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<title>Nine Nines: Listeners</title>
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<h1 class="lined-header"><span>Listeners</span></h1>
<div class="paragraph"><p>A listener is a set of processes that listens on a port for
new connections. Incoming connections get handled by Cowboy.
Depending on the connection handshake, one or another protocol
may be used.</p></div>
<div class="paragraph"><p>This chapter is specific to Cowboy. Please refer to the
<a href="http://ninenines.eu/docs/en/ranch/1.3/guide/listeners/">Ranch User Guide</a>
for more information about listeners.</p></div>
<div class="paragraph"><p>Cowboy provides two types of listeners: one listening for
clear TCP connections, and one listening for secure TLS
connections. Both of them support the HTTP/1.1 and HTTP/2
protocols.</p></div>
<div class="sect1">
<h2 id="_clear_tcp_listener">Clear TCP listener</h2>
<div class="sectionbody">
<div class="paragraph"><p>The clear TCP listener will accept connections on the
given port. A typical HTTP server would listen on port 80.
Port 80 requires special permissions on most platforms
however so a common alternative is port 8080.</p></div>
<div class="paragraph"><p>The following snippet starts listening for connections
on port 8080:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-weight: bold"><span style="color: #000000">start</span></span>(<span style="color: #009900">_Type</span>, <span style="color: #009900">_Args</span>) <span style="color: #990000">-></span>
<span style="color: #009900">Dispatch</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_router:compile</span></span>([
{<span style="color: #FF6600">'_'</span>, [{<span style="color: #FF0000">"/"</span>, <span style="color: #FF6600">hello_handler</span>, []}]}
]),
{<span style="color: #FF6600">ok</span>, <span style="color: #990000">_</span>} <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy:start_clear</span></span>(<span style="color: #FF6600">my_http_listener</span>,
[{<span style="color: #FF6600">port</span>, <span style="color: #993399">8080</span>}],
#{<span style="color: #0000FF">env</span> <span style="color: #990000">=></span> #{<span style="color: #0000FF">dispatch</span> <span style="color: #990000">=></span> <span style="color: #009900">Dispatch</span>}}
),
<span style="font-weight: bold"><span style="color: #000000">hello_erlang_sup:start_link</span></span>()<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>The <a href="../getting_started">Getting Started</a> chapter uses a
clear TCP listener.</p></div>
<div class="paragraph"><p>Clients connecting to Cowboy on the clear listener port are
expected to use either HTTP/1.1 or HTTP/2.</p></div>
<div class="paragraph"><p>Cowboy supports both methods of initiating a clear
HTTP/2 connection: through the Upgrade mechanism
(<a href="https://tools.ietf.org/html/rfc7540#section-3.2">RFC 7540 3.2</a>)
or by sending the preface directly
(<a href="https://tools.ietf.org/html/rfc7540#section-3.4">RFC 7540 3.4</a>).</p></div>
<div class="paragraph"><p>Compatibility with HTTP/1.0 is provided by Cowboy’s HTTP/1.1
implementation.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_secure_tls_listener">Secure TLS listener</h2>
<div class="sectionbody">
<div class="paragraph"><p>The secure TLS listener will accept connections on the
given port. A typical HTTPS server would listen on port 443.
Port 443 requires special permissions on most platforms
however so a common alternative is port 8443.</p></div>
<div class="paragraph"><p>The function provided by Cowboy will ensure that the TLS
options given are following the HTTP/2 RFC with regards
to security. For example some TLS extensions or ciphers
may be disabled. This also applies to HTTP/1.1 connections
on this listener. If this is not desirable, Ranch can be
used directly to setup a custom listener.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-weight: bold"><span style="color: #000000">start</span></span>(<span style="color: #009900">_Type</span>, <span style="color: #009900">_Args</span>) <span style="color: #990000">-></span>
<span style="color: #009900">Dispatch</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_router:compile</span></span>([
{<span style="color: #FF6600">'_'</span>, [{<span style="color: #FF0000">"/"</span>, <span style="color: #FF6600">hello_handler</span>, []}]}
]),
{<span style="color: #FF6600">ok</span>, <span style="color: #990000">_</span>} <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy:start_tls</span></span>(<span style="color: #FF6600">my_http_listener</span>,
[
{<span style="color: #FF6600">port</span>, <span style="color: #993399">8443</span>},
{<span style="color: #FF6600">certfile</span>, <span style="color: #FF0000">"/path/to/certfile"</span>},
{<span style="color: #FF6600">keyfile</span>, <span style="color: #FF0000">"/path/to/keyfile"</span>}
],
#{<span style="color: #0000FF">env</span> <span style="color: #990000">=></span> #{<span style="color: #0000FF">dispatch</span> <span style="color: #990000">=></span> <span style="color: #009900">Dispatch</span>}}
),
<span style="font-weight: bold"><span style="color: #000000">hello_erlang_sup:start_link</span></span>()<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Clients connecting to Cowboy on the secure listener are
expected to use the ALPN TLS extension to indicate what
protocols they understand. Cowboy always prefers HTTP/2
over HTTP/1.1 when both are supported. When neither are
supported by the client, or when the ALPN extension was
missing, Cowboy expects HTTP/1.1 to be used.</p></div>
<div class="paragraph"><p>Cowboy also advertises HTTP/2 support through the older
NPN TLS extension for compatibility. Note however that
this support will likely not be enabled by default when
Cowboy 2.0 gets released.</p></div>
<div class="paragraph"><p>Compatibility with HTTP/1.0 is provided by Cowboy’s HTTP/1.1
implementation.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_protocol_configuration">Protocol configuration</h2>
<div class="sectionbody">
<div class="paragraph"><p>The HTTP/1.1 and HTTP/2 protocols share the same semantics;
only their framing differs. The first is a text protocol and
the second a binary protocol.</p></div>
<div class="paragraph"><p>Cowboy doesn’t separate the configuration for HTTP/1.1 and
HTTP/2. Everything goes into the same map. Many options are
shared.</p></div>
</div>
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<h3>
Cowboy
2.0
User Guide
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