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<h2>Table of Contents</h2>
<ul>
<li><a href="#dgram_udp_datagram_sockets">UDP / Datagram Sockets</a><ul>
<li><a href="#dgram_class_dgram_socket">Class: dgram.Socket</a><ul>
<li><a href="#dgram_event_close">Event: 'close'</a></li>
<li><a href="#dgram_event_error">Event: 'error'</a></li>
<li><a href="#dgram_event_listening">Event: 'listening'</a></li>
<li><a href="#dgram_event_message">Event: 'message'</a></li>
<li><a href="#dgram_socket_addmembership_multicastaddress_multicastinterface">socket.addMembership(multicastAddress[, multicastInterface])</a></li>
<li><a href="#dgram_socket_address">socket.address()</a></li>
<li><a href="#dgram_socket_bind_port_address_callback">[socket.bind([port][, address][, callback])]</a></li>
<li><a href="#dgram_socket_bind_options_callback">socket.bind(options[, callback])</a></li>
<li><a href="#dgram_socket_close_callback">socket.close([callback])</a></li>
<li><a href="#dgram_socket_dropmembership_multicastaddress_multicastinterface">socket.dropMembership(multicastAddress[, multicastInterface])</a></li>
<li><a href="#dgram_socket_send_buf_offset_length_port_address_callback">socket.send(buf, offset, length, port, address[, callback])</a></li>
<li><a href="#dgram_socket_setbroadcast_flag">socket.setBroadcast(flag)</a></li>
<li><a href="#dgram_socket_setmulticastloopback_flag">socket.setMulticastLoopback(flag)</a></li>
<li><a href="#dgram_socket_setmulticastttl_ttl">socket.setMulticastTTL(ttl)</a></li>
<li><a href="#dgram_socket_setttl_ttl">socket.setTTL(ttl)</a></li>
<li><a href="#dgram_socket_ref">socket.ref()</a></li>
<li><a href="#dgram_socket_unref">socket.unref()</a></li>
<li><a href="#dgram_change_to_asynchronous_socket_bind_behavior">Change to asynchronous <code>socket.bind()</code> behavior</a></li>
</ul>
</li>
<li><a href="#dgram_dgram_module_functions"><code>dgram</code> module functions</a><ul>
<li><a href="#dgram_dgram_createsocket_options_callback">dgram.createSocket(options[, callback])</a></li>
</ul>
</li>
<li><a href="#dgram_dgram_createsocket_type_callback">dgram.createSocket(type[, callback])</a></li>
</ul>
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<div id="apicontent">
<h1>UDP / Datagram Sockets<span><a class="mark" href="#dgram_udp_datagram_sockets" id="dgram_udp_datagram_sockets">#</a></span></h1>
<pre class="api_stability_2">Stability: 2 - Stable</pre><!-- name=dgram -->
<p>The <code>dgram</code> module provides an implementation of UDP Datagram sockets.
</p>
<pre><code>const dgram = require('dgram');
const server = dgram.createSocket('udp4');
server.on('error', (err) => {
console.log(`server error:\n${err.stack}`);
server.close();
});
server.on('message', (msg, rinfo) => {
console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
});
server.on('listening', () => {
var address = server.address();
console.log(`server listening ${address.address}:${address.port}`);
});
server.bind(41234);
// server listening 0.0.0.0:41234</code></pre>
<h2>Class: dgram.Socket<span><a class="mark" href="#dgram_class_dgram_socket" id="dgram_class_dgram_socket">#</a></span></h2>
<p>The <code>dgram.Socket</code> object is an <a href="events.html"><code>EventEmitter</code></a> that encapsulates the
datagram functionality.
</p>
<p>New instances of <code>dgram.Socket</code> are created using <a href="#dgram_dgram_createsocket_options_callback"><code>dgram.createSocket()</code></a>.
The <code>new</code> keyword is not to be used to create <code>dgram.Socket</code> instances.
</p>
<h3>Event: 'close'<span><a class="mark" href="#dgram_event_close" id="dgram_event_close">#</a></span></h3>
<p>The <code>'close'</code> event is emitted after a socket is closed with <a href="#dgram_socket_close_callback"><code>close()</code></a>.
Once triggered, no new <code>'message'</code> events will be emitted on this socket.
</p>
<h3>Event: 'error'<span><a class="mark" href="#dgram_event_error" id="dgram_event_error">#</a></span></h3>
<div class="signature"><ul>
<li><code>exception</code> Error object</li>
</div></ul>
<p>The <code>'error'</code> event is emitted whenever any error occurs. The event handler
function is passed a single Error object.
</p>
<h3>Event: 'listening'<span><a class="mark" href="#dgram_event_listening" id="dgram_event_listening">#</a></span></h3>
<p>The <code>'listening'</code> event is emitted whenever a socket begins listening for
datagram messages. This occurs as soon as UDP sockets are created.
</p>
<h3>Event: 'message'<span><a class="mark" href="#dgram_event_message" id="dgram_event_message">#</a></span></h3>
<div class="signature"><ul>
<li><code>msg</code> Buffer object. The message</li>
<li><code>rinfo</code> Object. Remote address information</li>
</div></ul>
<p>The <code>'message'</code> event is emitted when a new datagram is available on a socket.
The event handler function is passed two arguments: <code>msg</code> and <code>rinfo</code>. The
<code>msg</code> argument is a <a href="buffer.html"><code>Buffer</code></a> and <code>rinfo</code> is an object with the sender's
address information provided by the <code>address</code>, <code>family</code> and <code>port</code> properties:
</p>
<pre><code>socket.on('message', (msg, rinfo) => {
console.log('Received %d bytes from %s:%d\n',
msg.length, rinfo.address, rinfo.port);
});</code></pre>
<h3>socket.addMembership(multicastAddress[, multicastInterface])<span><a class="mark" href="#dgram_socket_addmembership_multicastaddress_multicastinterface" id="dgram_socket_addmembership_multicastaddress_multicastinterface">#</a></span></h3>
<div class="signature"><ul>
<li><code>multicastAddress</code> String</li>
<li><code>multicastInterface</code> String, Optional</li>
</div></ul>
<p>Tells the kernel to join a multicast group at the given <code>multicastAddress</code>
using the <code>IP_ADD_MEMBERSHIP</code> socket option. If the <code>multicastInterface</code>
argument is not specified, the operating system will try to add membership to
all valid networking interfaces.
</p>
<h3>socket.address()<span><a class="mark" href="#dgram_socket_address" id="dgram_socket_address">#</a></span></h3>
<p>Returns an object containing the address information for a socket.
For UDP sockets, this object will contain <code>address</code>, <code>family</code> and <code>port</code>
properties.
</p>
<h3>[socket.bind([port][, address][, callback])]<span><a class="mark" href="#dgram_socket_bind_port_address_callback" id="dgram_socket_bind_port_address_callback">#</a></span></h3>
<div class="signature"><ul>
<li><code>port</code> Integer, Optional</li>
<li><code>address</code> String, Optional</li>
<li><code>callback</code> Function with no parameters, Optional. Called when
binding is complete.</li>
</div></ul>
<p>For UDP sockets, causes the <code>dgram.Socket</code> to listen for datagram messages on a
named <code>port</code> and optional <code>address</code>. If <code>port</code> is not specified, the operating
system will attempt to bind to a random port. If <code>address</code> is not specified,
the operating system will attempt to listen on all addresses. Once binding is
complete, a <code>'listening'</code> event is emitted and the optional <code>callback</code> function
is called.
</p>
<p>Note that specifying both a <code>'listening'</code> event listener and passing a
<code>callback</code> to the <code>socket.bind()</code> method is not harmful but not very
useful.
</p>
<p>A bound datagram socket keeps the Node.js process running to receive
datagram messages.
</p>
<p>If binding fails, an <code>'error'</code> event is generated. In rare case (e.g.
attempting to bind with a closed socket), an <a href="errors.html#errors_class_error"><code>Error</code></a> may be thrown.
</p>
<p>Example of a UDP server listening on port 41234:
</p>
<pre><code>const dgram = require('dgram');
const server = dgram.createSocket('udp4');
server.on('error', (err) => {
console.log(`server error:\n${err.stack}`);
server.close();
});
server.on('message', (msg, rinfo) => {
console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
});
server.on('listening', () => {
var address = server.address();
console.log(`server listening ${address.address}:${address.port}`);
});
server.bind(41234);
// server listening 0.0.0.0:41234</code></pre>
<h3>socket.bind(options[, callback])<span><a class="mark" href="#dgram_socket_bind_options_callback" id="dgram_socket_bind_options_callback">#</a></span></h3>
<div class="signature"><ul>
<li><code>options</code> <span class="type">Object</span> - Required. Supports the following properties:<ul>
<li><code>port</code> <span class="type">Number</span> - Required.</li>
<li><code>address</code> <span class="type">String</span> - Optional.</li>
<li><code>exclusive</code> <span class="type">Boolean</span> - Optional.</li>
</ul>
</li>
<li><code>callback</code> <span class="type">Function</span> - Optional.</li>
</div></ul>
<p>For UDP sockets, causes the <code>dgram.Socket</code> to listen for datagram messages on a
named <code>port</code> and optional <code>address</code> that are passed as properties of an
<code>options</code> object passed as the first argument. If <code>port</code> is not specified, the
operating system will attempt to bind to a random port. If <code>address</code> is not
specified, the operating system will attempt to listen on all addresses. Once
binding is complete, a <code>'listening'</code> event is emitted and the optional
<code>callback</code> function is called.
</p>
<p>The <code>options</code> object may contain an additional <code>exclusive</code> property that is
use when using <code>dgram.Socket</code> objects with the [<code>cluster</code>] module. When
<code>exclusive</code> is set to <code>false</code> (the default), cluster workers will use the same
underlying socket handle allowing connection handling duties to be shared.
When <code>exclusive</code> is <code>true</code>, however, the handle is not shared and attempted
port sharing results in an error.
</p>
<p>An example socket listening on an exclusive port is shown below.
</p>
<pre><code>socket.bind({
address: 'localhost',
port: 8000,
exclusive: true
});</code></pre>
<h3>socket.close([callback])<span><a class="mark" href="#dgram_socket_close_callback" id="dgram_socket_close_callback">#</a></span></h3>
<p>Close the underlying socket and stop listening for data on it. If a callback is
provided, it is added as a listener for the <a href="#dgram_event_close"><code>'close'</code></a> event.
</p>
<h3>socket.dropMembership(multicastAddress[, multicastInterface])<span><a class="mark" href="#dgram_socket_dropmembership_multicastaddress_multicastinterface" id="dgram_socket_dropmembership_multicastaddress_multicastinterface">#</a></span></h3>
<div class="signature"><ul>
<li><code>multicastAddress</code> String</li>
<li><code>multicastInterface</code> String, Optional</li>
</div></ul>
<p>Instructs the kernel to leave a multicast group at <code>multicastAddress</code> using the
<code>IP_DROP_MEMBERSHIP</code> socket option. This method is automatically called by the
kernel when the socket is closed or the process terminates, so most apps will
never have reason to call this.
</p>
<p>If <code>multicastInterface</code> is not specified, the operating system will attempt to
drop membership on all valid interfaces.
</p>
<h3>socket.send(buf, offset, length, port, address[, callback])<span><a class="mark" href="#dgram_socket_send_buf_offset_length_port_address_callback" id="dgram_socket_send_buf_offset_length_port_address_callback">#</a></span></h3>
<div class="signature"><ul>
<li><code>buf</code> Buffer object or string. Message to be sent</li>
<li><code>offset</code> Integer. Offset in the buffer where the message starts.</li>
<li><code>length</code> Integer. Number of bytes in the message.</li>
<li><code>port</code> Integer. Destination port.</li>
<li><code>address</code> String. Destination hostname or IP address.</li>
<li><code>callback</code> Function. Called when the message has been sent. Optional.</li>
</div></ul>
<p>Broadcasts a datagram on the socket. The destination <code>port</code> and <code>address</code> must
be specified.
</p>
<p>The <code>buf</code> argument is a <a href="buffer.html"><code>Buffer</code></a> object containing the message. The <code>offset</code>
and <code>length</code> specify the offset within the <code>Buffer</code> where the message begins
and the number of bytes in the message, respectively. With messages that
contain multi-byte characters, <code>offset</code> and <code>length</code> will be calculated with
respect to <a href="buffer.html#buffer_class_method_buffer_bytelength_string_encoding">byte length</a> and not the character position.
</p>
<p>The <code>address</code> argument is a string. If the value of <code>address</code> is a host name,
DNS will be used to resolve the address of the host. If the <code>address</code> is not
specified or is an empty string, <code>'0.0.0.0'</code> or <code>'::0'</code> will be used instead.
It is possible, depending on the network configuration, that these defaults
may not work; accordingly, it is best to be explicit about the destination
address.
</p>
<p>If the socket has not been previously bound with a call to <code>bind</code>, the socket
is assigned a random port number and is bound to the "all interfaces" address
(<code>'0.0.0.0'</code> for <code>udp4</code> sockets, <code>'::0'</code> for <code>udp6</code> sockets.)
</p>
<p>An optional <code>callback</code> function may be specified to as a way of reporting
DNS errors or for determining when it is safe to reuse the <code>buf</code> object.
Note that DNS lookups delay the time to send for at least one tick of the
Node.js event loop.
</p>
<p>The only way to know for sure that the datagram has been sent is by using a
<code>callback</code>. If an error occurs and a <code>callback</code> is given, the error will be
passed as the first argument to the <code>callback</code>. If a <code>callback</code> is not given,
the error is emitted as an <code>'error'</code> event on the <code>socket</code> object.
</p>
<p>Example of sending a UDP packet to a random port on <code>localhost</code>;
</p>
<pre><code>const dgram = require('dgram');
const message = new Buffer('Some bytes');
const client = dgram.createSocket('udp4');
client.send(message, 0, message.length, 41234, 'localhost', (err) => {
client.close();
});</code></pre>
<p><strong>A Note about UDP datagram size</strong>
</p>
<p>The maximum size of an <code>IPv4/v6</code> datagram depends on the <code>MTU</code>
(<em>Maximum Transmission Unit</em>) and on the <code>Payload Length</code> field size.
</p>
<ul>
<li><p>The <code>Payload Length</code> field is <code>16 bits</code> wide, which means that a normal
payload exceed 64K octets <em>including</em> the internet header and data
(65,507 bytes = 65,535 − 8 bytes UDP header − 20 bytes IP header);
this is generally true for loopback interfaces, but such long datagram
messages are impractical for most hosts and networks.</p>
</li>
<li><p>The <code>MTU</code> is the largest size a given link layer technology can support for
datagram messages. For any link, <code>IPv4</code> mandates a minimum <code>MTU</code> of <code>68</code>
octets, while the recommended <code>MTU</code> for IPv4 is <code>576</code> (typically recommended
as the <code>MTU</code> for dial-up type applications), whether they arrive whole or in
fragments.</p>
<p>For <code>IPv6</code>, the minimum <code>MTU</code> is <code>1280</code> octets, however, the mandatory minimum
fragment reassembly buffer size is <code>1500</code> octets. The value of <code>68</code> octets is
very small, since most current link layer technologies, like Ethernet, have a
minimum <code>MTU</code> of <code>1500</code>.</p>
</li>
</ul>
<p>It is impossible to know in advance the MTU of each link through which
a packet might travel. Sending a datagram greater than the receiver <code>MTU</code> will
not work because the packet will get silently dropped without informing the
source that the data did not reach its intended recipient.
</p>
<h3>socket.setBroadcast(flag)<span><a class="mark" href="#dgram_socket_setbroadcast_flag" id="dgram_socket_setbroadcast_flag">#</a></span></h3>
<div class="signature"><ul>
<li><code>flag</code> Boolean</li>
</div></ul>
<p>Sets or clears the <code>SO_BROADCAST</code> socket option. When set to <code>true</code>, UDP
packets may be sent to a local interface's broadcast address.
</p>
<h3>socket.setMulticastLoopback(flag)<span><a class="mark" href="#dgram_socket_setmulticastloopback_flag" id="dgram_socket_setmulticastloopback_flag">#</a></span></h3>
<div class="signature"><ul>
<li><code>flag</code> Boolean</li>
</div></ul>
<p>Sets or clears the <code>IP_MULTICAST_LOOP</code> socket option. When set to <code>true</code>,
multicast packets will also be received on the local interface.
</p>
<h3>socket.setMulticastTTL(ttl)<span><a class="mark" href="#dgram_socket_setmulticastttl_ttl" id="dgram_socket_setmulticastttl_ttl">#</a></span></h3>
<div class="signature"><ul>
<li><code>ttl</code> Integer</li>
</div></ul>
<p>Sets the <code>IP_MULTICAST_TTL</code> socket option. While TTL generally stands for
"Time to Live", in this context it specifies the number of IP hops that a
packet is allowed to travel through, specifically for multicast traffic. Each
router or gateway that forwards a packet decrements the TTL. If the TTL is
decremented to 0 by a router, it will not be forwarded.
</p>
<p>The argument passed to to <code>socket.setMulticastTTL()</code> is a number of hops
between 0 and 255. The default on most systems is <code>1</code> but can vary.
</p>
<h3>socket.setTTL(ttl)<span><a class="mark" href="#dgram_socket_setttl_ttl" id="dgram_socket_setttl_ttl">#</a></span></h3>
<div class="signature"><ul>
<li><code>ttl</code> Integer</li>
</div></ul>
<p>Sets the <code>IP_TTL</code> socket option. While TTL generally stands for "Time to Live",
in this context it specifies the number of IP hops that a packet is allowed to
travel through. Each router or gateway that forwards a packet decrements the
TTL. If the TTL is decremented to 0 by a router, it will not be forwarded.
Changing TTL values is typically done for network probes or when multicasting.
</p>
<p>The argument to <code>socket.setTTL()</code> is a number of hops between 1 and 255.
The default on most systems is 64 but can vary.
</p>
<h3>socket.ref()<span><a class="mark" href="#dgram_socket_ref" id="dgram_socket_ref">#</a></span></h3>
<p>By default, binding a socket will cause it to block the Node.js process from
exiting as long as the socket is open. The <code>socket.unref()</code> method can be used
to exclude the socket from the reference counting that keeps the Node.js
process active. The <code>socket.ref()</code> method adds the socket back to the reference
counting and restores the default behavior.
</p>
<p>Calling <code>socket.ref()</code> multiples times will have no additional effect.
</p>
<p>The <code>socket.ref()</code> method returns a reference to the socket so calls can be
chained.
</p>
<h3>socket.unref()<span><a class="mark" href="#dgram_socket_unref" id="dgram_socket_unref">#</a></span></h3>
<p>By default, binding a socket will cause it to block the Node.js process from
exiting as long as the socket is open. The <code>socket.unref()</code> method can be used
to exclude the socket from the reference counting that keeps the Node.js
process active, allowing the process to exit even if the socket is still
listening.
</p>
<p>Calling <code>socket.unref()</code> multiple times will have no addition effect.
</p>
<p>The <code>socket.unref()</code> method returns a reference to the socket so calls can be
chained.
</p>
<h3>Change to asynchronous <code>socket.bind()</code> behavior<span><a class="mark" href="#dgram_change_to_asynchronous_socket_bind_behavior" id="dgram_change_to_asynchronous_socket_bind_behavior">#</a></span></h3>
<p>As of Node.js v0.10, <a href="#dgram_socket_bind_options_callback"><code>dgram.Socket#bind()</code></a> changed to an asynchronous
execution model. Legacy code that assumes synchronous behavior, as in the
following example:
</p>
<pre><code>const s = dgram.createSocket('udp4');
s.bind(1234);
s.addMembership('224.0.0.114');</code></pre>
<p>Must be changed to pass a callback function to the <a href="#dgram_socket_bind_options_callback"><code>dgram.Socket#bind()</code></a>
function:
</p>
<pre><code>const s = dgram.createSocket('udp4');
s.bind(1234, () => {
s.addMembership('224.0.0.114');
});</code></pre>
<h2><code>dgram</code> module functions<span><a class="mark" href="#dgram_dgram_module_functions" id="dgram_dgram_module_functions">#</a></span></h2>
<h3>dgram.createSocket(options[, callback])<span><a class="mark" href="#dgram_dgram_createsocket_options_callback" id="dgram_dgram_createsocket_options_callback">#</a></span></h3>
<ul>
<li><code>options</code> Object</li>
<li><code>callback</code> Function. Attached as a listener to <code>'message'</code> events.</li>
<li>Returns: Socket object</li>
</ul>
<p>Creates a <code>dgram.Socket</code> object. The <code>options</code> argument is an object that
should contain a <code>type</code> field of either <code>udp4</code> or <code>udp6</code> and an optional
boolean <code>reuseAddr</code> field.
</p>
<p>When <code>reuseAddr</code> is <code>true</code> <a href="#dgram_socket_bind_port_address_callback"><code>socket.bind()</code></a> will reuse the address, even if
another process has already bound a socket on it. <code>reuseAddr</code> defaults to
<code>false</code>. An optional <code>callback</code> function can be passed specified which is added
as a listener for <code>'message'</code> events.
</p>
<p>Once the socket is created, calling <a href="#dgram_socket_bind_port_address_callback"><code>socket.bind()</code></a> will instruct the
socket to begin listening for datagram messages. When <code>address</code> and <code>port</code> are
not passed to <a href="#dgram_socket_bind_port_address_callback"><code>socket.bind()</code></a> the method will bind the socket to the "all
interfaces" address on a random port (it does the right thing for both <code>udp4</code>
and <code>udp6</code> sockets). The bound address and port can be retrieved using
<a href="#dgram_socket_address"><code>socket.address().address</code></a> and <a href="#dgram_socket_address"><code>socket.address().port</code></a>.
</p>
<h2>dgram.createSocket(type[, callback])<span><a class="mark" href="#dgram_dgram_createsocket_type_callback" id="dgram_dgram_createsocket_type_callback">#</a></span></h2>
<div class="signature"><ul>
<li><code>type</code> String. Either 'udp4' or 'udp6'</li>
<li><code>callback</code> Function. Attached as a listener to <code>'message'</code> events.
Optional</li>
<li>Returns: Socket object</li>
</div></ul>
<p>Creates a <code>dgram.Socket</code> object of the specified <code>type</code>. The <code>type</code> argument
can be either <code>udp4</code> or <code>udp6</code>. An optional <code>callback</code> function can be passed
which is added as a listener for <code>'message'</code> events.
</p>
<p>Once the socket is created, calling <a href="#dgram_socket_bind_port_address_callback"><code>socket.bind()</code></a> will instruct the
socket to begin listening for datagram messages. When <code>address</code> and <code>port</code> are
not passed to <a href="#dgram_socket_bind_port_address_callback"><code>socket.bind()</code></a> the method will bind the socket to the "all
interfaces" address on a random port (it does the right thing for both <code>udp4</code>
and <code>udp6</code> sockets). The bound address and port can be retrieved using
<a href="#dgram_socket_address"><code>socket.address().address</code></a> and <a href="#dgram_socket_address"><code>socket.address().port</code></a>.
</p>
</div>
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