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<title>Nine Nines: Listeners</title>
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<h1 class="lined-header"><span>Listeners</span></h1>
<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>
<p>This chapter is specific to Cowboy. Please refer to the <a href="https://ninenines.eu/docs/en/ranch/1.3/guide/listeners/">Ranch User Guide</a> for more information about listeners.</p>
<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>
<h2 id="_clear_tcp_listener">Clear TCP listener</h2>
<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>
<p>The following snippet starts listening for connections on port 8080:</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">start</font></b>(<font color="#009900">_Type</font>, <font color="#009900">_Args</font>) <font color="#990000">-></font>
<font color="#009900">Dispatch</font> <font color="#990000">=</font> <b><font color="#000000">cowboy_router:compile</font></b>([
{<font color="#FF6600">'_'</font>, [{<font color="#FF0000">"/"</font>, <font color="#FF6600">hello_handler</font>, []}]}
]),
{<font color="#FF6600">ok</font>, <font color="#990000">_</font>} <font color="#990000">=</font> <b><font color="#000000">cowboy:start_clear</font></b>(<font color="#FF6600">my_http_listener</font>,
[{<font color="#FF6600">port</font>, <font color="#993399">8080</font>}],
#{<font color="#0000FF">env</font> <font color="#990000">=></font> #{<font color="#0000FF">dispatch</font> <font color="#990000">=></font> <font color="#009900">Dispatch</font>}}
),
<b><font color="#000000">hello_erlang_sup:start_link</font></b>()<font color="#990000">.</font></tt></pre>
</div></div>
<p>The <a href="../getting_started">Getting Started</a> chapter uses a clear TCP listener.</p>
<p>Clients connecting to Cowboy on the clear listener port are expected to use either HTTP/1.1 or HTTP/2.</p>
<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>
<p>Compatibility with HTTP/1.0 is provided by Cowboy's HTTP/1.1 implementation.</p>
<h2 id="_secure_tls_listener">Secure TLS listener</h2>
<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>
<!-- @todo Make a complete list of restrictions.-->
<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 class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">start</font></b>(<font color="#009900">_Type</font>, <font color="#009900">_Args</font>) <font color="#990000">-></font>
<font color="#009900">Dispatch</font> <font color="#990000">=</font> <b><font color="#000000">cowboy_router:compile</font></b>([
{<font color="#FF6600">'_'</font>, [{<font color="#FF0000">"/"</font>, <font color="#FF6600">hello_handler</font>, []}]}
]),
{<font color="#FF6600">ok</font>, <font color="#990000">_</font>} <font color="#990000">=</font> <b><font color="#000000">cowboy:start_tls</font></b>(<font color="#FF6600">my_http_listener</font>,
[
{<font color="#FF6600">port</font>, <font color="#993399">8443</font>},
{<font color="#FF6600">certfile</font>, <font color="#FF0000">"/path/to/certfile"</font>},
{<font color="#FF6600">keyfile</font>, <font color="#FF0000">"/path/to/keyfile"</font>}
],
#{<font color="#0000FF">env</font> <font color="#990000">=></font> #{<font color="#0000FF">dispatch</font> <font color="#990000">=></font> <font color="#009900">Dispatch</font>}}
),
<b><font color="#000000">hello_erlang_sup:start_link</font></b>()<font color="#990000">.</font></tt></pre>
</div></div>
<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>
<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>
<p>Compatibility with HTTP/1.0 is provided by Cowboy's HTTP/1.1 implementation.</p>
<h2 id="_protocol_configuration">Protocol configuration</h2>
<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>
<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>
<!-- @todo Describe good to know options for both protocols?-->
<!-- Maybe do that in separate chapters?-->
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