UDP / Datagram Sockets

The dgram module provides an implementation of UDP Datagram sockets.

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'

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.

• msg <Buffer> - The message
• rinfo <Object> - Remote address information
  • address <String> The sender address
  • family <String> The address family ('IPv4' or 'IPv6')
  • port <Number> The sender port
  • size <Number> The message size

socket.addMembership(multicastAddress[, multicastInterface])

• multicastAddress <String>
• multicastInterface <String>, Optional

Tells the kernel to join a multicast group at the given multicastAddress and multicastInterface using the IP_ADD_MEMBERSHIP socket option. If the multicastInterface argument is not specified, the operating system will choose one interface and will add membership to it. To add membership to every available interface, call addMembership multiple times, once per interface.

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 or is 0, 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:

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> - Optional.
  • 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 or is 0, 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.

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.

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.

An example socket listening on an exclusive port is shown below.

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(buf, offset, length, port, address[, callback])

• buf <Buffer> | <String> Message to be sent
• offset <Number> Integer. Offset in the buffer where the message starts.
• length <Number> Integer. 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 buf argument is a Buffer object containing the message. The offset and length specify the offset within the Buffer where the message begins and the number of bytes in the message, respectively. With messages that contain multi-byte characters, offset and length will be calculated with respect to byte length and not the character position.

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, '0.0.0.0' or '::0' 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.

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.

Example of sending a UDP packet to a random port on localhost;

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();
});

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)

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:

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:

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. The optional callback function 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.