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+# UDP / Datagram Sockets
+
+    Stability: 2 - Stable
+
+<!-- name=dgram -->
+
+The `dgram` module provides an implementation of UDP Datagram sockets.
+
+```js
+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
+```
+
+## Class: dgram.Socket
+
+The `dgram.Socket` object is an [`EventEmitter`][] that encapsulates the
+datagram functionality.
+
+New instances of `dgram.Socket` are created using [`dgram.createSocket()`][].
+The `new` keyword is not to be used to create `dgram.Socket` instances.
+
+### Event: 'close'
+
+The `'close'` event is emitted after a socket is closed with [`close()`][].
+Once triggered, no new `'message'` events will be emitted on this socket.
+
+### Event: 'error'
+
+* `exception` {Error}
+
+The `'error'` event is emitted whenever any error occurs. The event handler
+function is passed a single Error object.
+
+### Event: 'listening'
+
+The `'listening'` event is emitted whenever a socket begins listening for
+datagram messages. This occurs as soon as UDP sockets are created.
+
+### Event: 'message'
+
+* `msg` {Buffer} - The message
+* `rinfo` {Object} - Remote address information
+
+The `'message'` event is emitted when a new datagram is available on a socket.
+The event handler function is passed two arguments: `msg` and `rinfo`. The
+`msg` argument is a [`Buffer`][] and `rinfo` is an object with the sender's
+address information provided by the `address`, `family` and `port` properties:
+
+```js
+socket.on('message', (msg, rinfo) => {
+  console.log('Received %d bytes from %s:%d\n',
+              msg.length, rinfo.address, rinfo.port);
+});
+```
+
+### socket.addMembership(multicastAddress[, multicastInterface])
+
+* `multicastAddress` {String}
+* `multicastInterface` {String}, Optional
+
+Tells the kernel to join a multicast group at the given `multicastAddress`
+using the `IP_ADD_MEMBERSHIP` socket option. If the `multicastInterface`
+argument is not specified, the operating system will try to add membership to
+all valid networking interfaces.
+
+### socket.address()
+
+Returns an object containing the address information for a socket.
+For UDP sockets, this object will contain `address`, `family` and `port`
+properties.
+
+### socket.bind([port][, address][, callback])
+
+* `port` {Number} - Integer, Optional
+* `address` {String}, Optional
+* `callback` {Function} with no parameters, Optional. Called when
+  binding is complete.
+
+For UDP sockets, causes the `dgram.Socket` to listen for datagram messages on a
+named `port` and optional `address`. If `port` is not specified, the operating
+system will attempt to bind to a random port. If `address` is not specified,
+the operating system will attempt to listen on all addresses.  Once binding is
+complete, a `'listening'` event is emitted and the optional `callback` function
+is called.
+
+Note that specifying both a `'listening'` event listener and passing a
+`callback` to the `socket.bind()` method is not harmful but not very
+useful.
+
+A bound datagram socket keeps the Node.js process running to receive
+datagram messages.
+
+If binding fails, an `'error'` event is generated. In rare case (e.g.
+attempting to bind with a closed socket), an [`Error`][] may be thrown.
+
+Example of a UDP server listening on port 41234:
+
+```js
+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
+```
+
+### socket.bind(options[, callback])
+
+* `options` {Object} - Required. Supports the following properties:
+  * `port` {Number} - Required.
+  * `address` {String} - Optional.
+  * `exclusive` {Boolean} - Optional.
+* `callback` {Function} - Optional.
+
+For UDP sockets, causes the `dgram.Socket` to listen for datagram messages on a
+named `port` and optional `address` that are passed as properties of an
+`options` object passed as the first argument. If `port` is not specified, the
+operating system will attempt to bind to a random port. If `address` is not
+specified, the operating system will attempt to listen on all addresses.  Once
+binding is complete, a `'listening'` event is emitted and the optional
+`callback` function is called.
+
+The `options` object may contain an additional `exclusive` property that is
+use when using `dgram.Socket` objects with the [`cluster`] module. When
+`exclusive` is set to `false` (the default), cluster workers will use the same
+underlying socket handle allowing connection handling duties to be shared.
+When `exclusive` is `true`, however, the handle is not shared and attempted
+port sharing results in an error.
+
+An example socket listening on an exclusive port is shown below.
+
+```js
+socket.bind({
+  address: 'localhost',
+  port: 8000,
+  exclusive: true
+});
+```
+
+### socket.close([callback])
+
+Close the underlying socket and stop listening for data on it. If a callback is
+provided, it is added as a listener for the [`'close'`][] event.
+
+### socket.dropMembership(multicastAddress[, multicastInterface])
+
+* `multicastAddress` {String}
+* `multicastInterface` {String}, Optional
+
+Instructs the kernel to leave a multicast group at `multicastAddress` using the
+`IP_DROP_MEMBERSHIP` 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.
+
+If `multicastInterface` is not specified, the operating system will attempt to
+drop membership on all valid interfaces.
+
+### socket.send(msg, [offset, length,] port, address[, callback])
+
+* `msg` {Buffer|String|Array} Message to be sent
+* `offset` {Number} Integer. Optional. Offset in the buffer where the message starts.
+* `length` {Number} Integer. Optional. Number of bytes in the message.
+* `port` {Number} Integer. Destination port.
+* `address` {String} Destination hostname or IP address.
+* `callback` {Function} Called when the message has been sent. Optional.
+
+Broadcasts a datagram on the socket. The destination `port` and `address` must
+be specified.
+
+The `msg` argument contains the message to be sent.
+Depending on its type, different behavior can apply. If `msg` is a `Buffer`,
+the `offset` and `length` specify the offset within the `Buffer` where the
+message begins and the number of bytes in the message, respectively.
+If `msg` is a `String`, then it is automatically converted to a `Buffer`
+with `'utf8'` encoding. With messages that
+contain  multi-byte characters, `offset` and `length` will be calculated with
+respect to [byte length][] and not the character position.
+If `msg` is an array, `offset` and `length` must not be specified.
+
+The `address` argument is a string. If the value of `address` is a host name,
+DNS will be used to resolve the address of the host. If the `address` is not
+specified or is an empty string, `'127.0.0.1'` or `'::1'` will be used instead.
+
+If the socket has not been previously bound with a call to `bind`, the socket
+is assigned a random port number and is bound to the "all interfaces" address
+(`'0.0.0.0'` for `udp4` sockets, `'::0'` for `udp6` sockets.)
+
+An optional `callback` function  may be specified to as a way of reporting
+DNS errors or for determining when it is safe to reuse the `buf` object.
+Note that DNS lookups delay the time to send for at least one tick of the
+Node.js event loop.
+
+The only way to know for sure that the datagram has been sent is by using a
+`callback`. If an error occurs and a `callback` is given, the error will be
+passed as the first argument to the `callback`. If a `callback` is not given,
+the error is emitted as an `'error'` event on the `socket` object.
+
+Offset and length are optional, but if you specify one you would need to
+specify the other. Also, they are supported only when the first
+argument is a `Buffer`.
+
+Example of sending a UDP packet to a random port on `localhost`;
+
+```js
+const dgram = require('dgram');
+const message = new Buffer('Some bytes');
+const client = dgram.createSocket('udp4');
+client.send(message, 41234, 'localhost', (err) => {
+  client.close();
+});
+```
+
+Example of sending a UDP packet composed of multiple buffers to a random port on `localhost`;
+
+```js
+const dgram = require('dgram');
+const buf1 = new Buffer('Some ');
+const buf2 = new Buffer('bytes');
+const client = dgram.createSocket('udp4');
+client.send([buf1, buf2], 41234, 'localhost', (err) => {
+  client.close();
+});
+```
+
+Sending multiple buffers might be faster or slower depending on your
+application and operating system: benchmark it. Usually it is faster.
+
+**A Note about UDP datagram size**
+
+The maximum size of an `IPv4/v6` datagram depends on the `MTU`
+(_Maximum Transmission Unit_) and on the `Payload Length` field size.
+
+- The `Payload Length` field is `16 bits` wide, which means that a normal
+  payload exceed 64K octets _including_ 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.
+
+- The `MTU` is the largest size a given link layer technology can support for
+  datagram messages. For any link, `IPv4` mandates a minimum `MTU` of `68`
+  octets, while the recommended `MTU` for IPv4 is `576` (typically recommended
+  as the `MTU` for dial-up type applications), whether they arrive whole or in
+  fragments.
+
+  For `IPv6`, the minimum `MTU` is `1280` octets, however, the mandatory minimum
+  fragment reassembly buffer size is `1500` octets. The value of `68` octets is
+  very small, since most current link layer technologies, like Ethernet, have a
+  minimum `MTU` of `1500`.
+
+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 `MTU` will
+not work because the packet will get silently dropped without informing the
+source that the data did not reach its intended recipient.
+
+### socket.setBroadcast(flag)
+
+* `flag` {Boolean}
+
+Sets or clears the `SO_BROADCAST` socket option.  When set to `true`, UDP
+packets may be sent to a local interface's broadcast address.
+
+### socket.setMulticastLoopback(flag)
+
+* `flag` {Boolean}
+
+Sets or clears the `IP_MULTICAST_LOOP` socket option.  When set to `true`,
+multicast packets will also be received on the local interface.
+
+### socket.setMulticastTTL(ttl)
+
+* `ttl` {Number} Integer
+
+Sets the `IP_MULTICAST_TTL` 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.
+
+The argument passed to to `socket.setMulticastTTL()` is a number of hops
+between 0 and 255. The default on most systems is `1` but can vary.
+
+### socket.setTTL(ttl)
+
+* `ttl` {Number} Integer
+
+Sets the `IP_TTL` 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.
+
+The argument to `socket.setTTL()` is a number of hops between 1 and 255.
+The default on most systems is 64 but can vary.
+
+### socket.ref()
+
+By default, binding a socket will cause it to block the Node.js process from
+exiting as long as the socket is open. The `socket.unref()` method can be used
+to exclude the socket from the reference counting that keeps the Node.js
+process active. The `socket.ref()` method adds the socket back to the reference
+counting and restores the default behavior.
+
+Calling `socket.ref()` multiples times will have no additional effect.
+
+The `socket.ref()` method returns a reference to the socket so calls can be
+chained.
+
+### socket.unref()
+
+By default, binding a socket will cause it to block the Node.js process from
+exiting as long as the socket is open. The `socket.unref()` 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.
+
+Calling `socket.unref()` multiple times will have no addition effect.
+
+The `socket.unref()` method returns a reference to the socket so calls can be
+chained.
+
+### Change to asynchronous `socket.bind()` behavior
+
+As of Node.js v0.10, [`dgram.Socket#bind()`][] changed to an asynchronous
+execution model. Legacy code that assumes synchronous behavior, as in the
+following example:
+
+```js
+const s = dgram.createSocket('udp4');
+s.bind(1234);
+s.addMembership('224.0.0.114');
+```
+
+Must be changed to pass a callback function to the [`dgram.Socket#bind()`][]
+function:
+
+```js
+const s = dgram.createSocket('udp4');
+s.bind(1234, () => {
+  s.addMembership('224.0.0.114');
+});
+```
+
+## `dgram` module functions
+
+### dgram.createSocket(options[, callback])
+
+* `options` {Object}
+* `callback` {Function} Attached as a listener to `'message'` events.
+* Returns: {dgram.Socket}
+
+Creates a `dgram.Socket` object. The `options` argument is an object that
+should contain a `type` field of either `udp4` or `udp6` and an optional
+boolean `reuseAddr` field.
+
+When `reuseAddr` is `true` [`socket.bind()`][] will reuse the address, even if
+another process has already bound a socket on it. `reuseAddr` defaults to
+`false`. An optional `callback` function can be passed specified which is added
+as a listener for `'message'` events.
+
+Once the socket is created, calling [`socket.bind()`][] will instruct the
+socket to begin listening for datagram messages. When `address` and `port` are
+not passed to  [`socket.bind()`][] the method will bind the socket to the "all
+interfaces" address on a random port (it does the right thing for both `udp4`
+and `udp6` sockets). The bound address and port can be retrieved using
+[`socket.address().address`][] and [`socket.address().port`][].
+
+### dgram.createSocket(type[, callback])
+
+* `type` {String} - Either 'udp4' or 'udp6'
+* `callback` {Function} - Attached as a listener to `'message'` events.
+  Optional
+* Returns: {dgram.Socket}
+
+Creates a `dgram.Socket` object of the specified `type`. The `type` argument
+can be either `udp4` or `udp6`. An optional `callback` function can be passed
+which is added as a listener for `'message'` events.
+
+Once the socket is created, calling [`socket.bind()`][] will instruct the
+socket to begin listening for datagram messages. When `address` and `port` are
+not passed to  [`socket.bind()`][] the method will bind the socket to the "all
+interfaces" address on a random port (it does the right thing for both `udp4`
+and `udp6` sockets). The bound address and port can be retrieved using
+[`socket.address().address`][] and [`socket.address().port`][].
+
+[`EventEmitter`]: events.html
+[`Buffer`]: buffer.html
+[`'close'`]: #dgram_event_close
+[`addMembership()`]: #dgram_socket_addmembership_multicastaddress_multicastinterface
+[`close()`]: #dgram_socket_close_callback
+[`dgram.createSocket()`]: #dgram_dgram_createsocket_options_callback
+[`dgram.Socket#bind()`]: #dgram_socket_bind_options_callback
+[`Error`]: errors.html#errors_class_error
+[`socket.address().address`]: #dgram_socket_address
+[`socket.address().port`]: #dgram_socket_address
+[`socket.bind()`]: #dgram_socket_bind_port_address_callback
+[byte length]: buffer.html#buffer_class_method_buffer_bytelength_string_encoding