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# UDP / Datagram Sockets#

    Stability: 2 - Stable

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 object

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 object. 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:

    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` 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:

    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.

    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 object or string. Message to be sent
- `offset` Integer. Offset in the buffer where the message starts.
- `length` Integer. Number of bytes in the message.
- `port` 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)#

- `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` 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` 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: Socket object

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: Socket object

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`.