Table of Contents

• UDP/datagram sockets
  • Class: dgram.Socket
    • Event: 'close'
    • Event: 'connect'
    • Event: 'error'
    • Event: 'listening'
    • Event: 'message'
    • socket.addMembership(multicastAddress[, multicastInterface])
    • socket.addSourceSpecificMembership(sourceAddress, groupAddress[, multicastInterface])
    • socket.address()
    • socket.bind([port][, address][, callback])
    • socket.bind(options[, callback])
    • socket.close([callback])
    • socket.connect(port[, address][, callback])
    • socket.disconnect()
    • socket.dropMembership(multicastAddress[, multicastInterface])
    • socket.dropSourceSpecificMembership(sourceAddress, groupAddress[, multicastInterface])
    • socket.getRecvBufferSize()
    • socket.getSendBufferSize()
    • socket.ref()
    • socket.remoteAddress()
    • socket.send(msg[, offset, length][, port][, address][, callback])
      • Note about UDP datagram size
    • socket.setBroadcast(flag)
    • socket.setMulticastInterface(multicastInterface)
      • Example: IPv6 outgoing multicast interface
      • Example: IPv4 outgoing multicast interface
      • Call results
    • socket.setMulticastLoopback(flag)
    • socket.setMulticastTTL(ttl)
    • socket.setRecvBufferSize(size)
    • socket.setSendBufferSize(size)
    • socket.setTTL(ttl)
    • socket.unref()
  • dgram module functions
    • dgram.createSocket(options[, callback])
    • dgram.createSocket(type[, callback])

UDP/datagram sockets#

Source Code: lib/dgram.js

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', () => {
  const address = server.address();
  console.log(`server listening ${address.address}:${address.port}`);
});

server.bind(41234);
// Prints: server listening 0.0.0.0:41234

Class: dgram.Socket#

• Extends: <EventEmitter>

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: 'connect'#

The 'connect' event is emitted after a socket is associated to a remote address as a result of a successful connect() call.

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 once the dgram.Socket is addressable and can receive data. This happens either explicitly with socket.bind() or implicitly the first time data is sent using socket.send(). Until the dgram.Socket is listening, the underlying system resources do not exist and calls such as socket.address() and socket.setTTL() will fail.

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.

If the source address of the incoming packet is an IPv6 link-local address, the interface name is added to the address. For example, a packet received on the en0 interface might have the address field set to 'fe80::2618:1234:ab11:3b9c%en0', where '%en0' is the interface name as a zone ID suffix.

socket.addMembership(multicastAddress[, multicastInterface])#

• multicastAddress <string>
• multicastInterface <string>

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.

When called on an unbound socket, this method will implicitly bind to a random port, listening on all interfaces.

When sharing a UDP socket across multiple cluster workers, the socket.addMembership() function must be called only once or an EADDRINUSE error will occur:

const cluster = require('cluster');
const dgram = require('dgram');
if (cluster.isMaster) {
  cluster.fork(); // Works ok.
  cluster.fork(); // Fails with EADDRINUSE.
} else {
  const s = dgram.createSocket('udp4');
  s.bind(1234, () => {
    s.addMembership('224.0.0.114');
  });
}

socket.addSourceSpecificMembership(sourceAddress, groupAddress[, multicastInterface])#

• sourceAddress <string>
• groupAddress <string>
• multicastInterface <string>

Tells the kernel to join a source-specific multicast channel at the given sourceAddress and groupAddress, using the multicastInterface with the IP_ADD_SOURCE_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 socket.addSourceSpecificMembership() multiple times, once per interface.

When called on an unbound socket, this method will implicitly bind to a random port, listening on all interfaces.

socket.address()#

• Returns: <Object>

Returns an object containing the address information for a socket. For UDP sockets, this object will contain address, family and port properties.

This method throws EBADF if called on an unbound socket.

socket.bind([port][, address][, callback])#

• port <integer>
• address <string>
• callback <Function> with no parameters. 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.

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', () => {
  const address = server.address();
  console.log(`server listening ${address.address}:${address.port}`);
});

server.bind(41234);
// Prints: server listening 0.0.0.0:41234

socket.bind(options[, callback])#

• options <Object> Required. Supports the following properties:
  • port <integer>
  • address <string>
  • exclusive <boolean>
  • fd <integer>
• callback <Function>

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.

The options object may contain a fd property. When a fd greater than 0 is set, it will wrap around an existing socket with the given file descriptor. In this case, the properties of port and address will be ignored.

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 used 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])#

• callback <Function> Called when the socket has been closed.

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.connect(port[, address][, callback])#

• port <integer>
• address <string>
• callback <Function> Called when the connection is completed or on error.

Associates the dgram.Socket to a remote address and port. Every message sent by this handle is automatically sent to that destination. Also, the socket will only receive messages from that remote peer. Trying to call connect() on an already connected socket will result in an ERR_SOCKET_DGRAM_IS_CONNECTED exception. If address is not provided, '127.0.0.1' (for udp4 sockets) or '::1' (for udp6 sockets) will be used by default. Once the connection is complete, a 'connect' event is emitted and the optional callback function is called. In case of failure, the callback is called or, failing this, an 'error' event is emitted.

socket.disconnect()#

A synchronous function that disassociates a connected dgram.Socket from its remote address. Trying to call disconnect() on an unbound or already disconnected socket will result in an ERR_SOCKET_DGRAM_NOT_CONNECTED exception.

socket.dropMembership(multicastAddress[, multicastInterface])#

• multicastAddress <string>
• multicastInterface <string>

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.dropSourceSpecificMembership(sourceAddress, groupAddress[, multicastInterface])#

• sourceAddress <string>
• groupAddress <string>
• multicastInterface <string>

Instructs the kernel to leave a source-specific multicast channel at the given sourceAddress and groupAddress using the IP_DROP_SOURCE_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.getRecvBufferSize()#

• Returns: <number> the SO_RCVBUF socket receive buffer size in bytes.

This method throws ERR_SOCKET_BUFFER_SIZE if called on an unbound socket.

socket.getSendBufferSize()#

• Returns: <number> the SO_SNDBUF socket send buffer size in bytes.

This method throws ERR_SOCKET_BUFFER_SIZE if called on an unbound socket.

socket.ref()#

• Returns: <dgram.Socket>

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.remoteAddress()#

• Returns: <Object>

Returns an object containing the address, family, and port of the remote endpoint. This method throws an ERR_SOCKET_DGRAM_NOT_CONNECTED exception if the socket is not connected.

socket.send(msg[, offset, length][, port][, address][, callback])#

• msg <Buffer> | <TypedArray> | <DataView> | <string> | <Array> 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 host name or IP address.
• callback <Function> Called when the message has been sent.

Broadcasts a datagram on the socket. For connectionless sockets, the destination port and address must be specified. Connected sockets, on the other hand, will use their associated remote endpoint, so the port and address arguments must not be set.

The msg argument contains the message to be sent. Depending on its type, different behavior can apply. If msg is a Buffer, any TypedArray or a DataView, 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 address is not provided or otherwise falsy, '127.0.0.1' (for udp4 sockets) or '::1' (for udp6 sockets) will be used by default.

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. 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 both must be set if either are used. They are supported only when the first argument is a Buffer, a TypedArray, or a DataView.

This method throws ERR_SOCKET_BAD_PORT if called on an unbound socket.

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

const dgram = require('dgram');
const message = Buffer.from('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 port on 127.0.0.1;

const dgram = require('dgram');
const buf1 = Buffer.from('Some ');
const buf2 = Buffer.from('bytes');
const client = dgram.createSocket('udp4');
client.send([buf1, buf2], 41234, (err) => {
  client.close();
});

Sending multiple buffers might be faster or slower depending on the application and operating system. Run benchmarks to determine the optimal strategy on a case-by-case basis. Generally speaking, however, sending multiple buffers is faster.

Example of sending a UDP packet using a socket connected to a port on localhost:

const dgram = require('dgram');
const message = Buffer.from('Some bytes');
const client = dgram.createSocket('udp4');
client.connect(41234, 'localhost', (err) => {
  client.send(message, (err) => {
    client.close();
  });
});

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 cannot 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.

This method throws EBADF if called on an unbound socket.

socket.setMulticastInterface(multicastInterface)#

• multicastInterface <string>

All references to scope in this section are referring to IPv6 Zone Indices, which are defined by RFC 4007. In string form, an IP with a scope index is written as 'IP%scope' where scope is an interface name or interface number.

Sets the default outgoing multicast interface of the socket to a chosen interface or back to system interface selection. The multicastInterface must be a valid string representation of an IP from the socket's family.

For IPv4 sockets, this should be the IP configured for the desired physical interface. All packets sent to multicast on the socket will be sent on the interface determined by the most recent successful use of this call.

For IPv6 sockets, multicastInterface should include a scope to indicate the interface as in the examples that follow. In IPv6, individual send calls can also use explicit scope in addresses, so only packets sent to a multicast address without specifying an explicit scope are affected by the most recent successful use of this call.

This method throws EBADF if called on an unbound socket.

Example: IPv6 outgoing multicast interface#

On most systems, where scope format uses the interface name:

const socket = dgram.createSocket('udp6');

socket.bind(1234, () => {
  socket.setMulticastInterface('::%eth1');
});

On Windows, where scope format uses an interface number:

const socket = dgram.createSocket('udp6');

socket.bind(1234, () => {
  socket.setMulticastInterface('::%2');
});

Example: IPv4 outgoing multicast interface#

All systems use an IP of the host on the desired physical interface:

const socket = dgram.createSocket('udp4');

socket.bind(1234, () => {
  socket.setMulticastInterface('10.0.0.2');
});

Call results#

A call on a socket that is not ready to send or no longer open may throw a Not running Error.

If multicastInterface can not be parsed into an IP then an EINVAL System Error is thrown.

On IPv4, if multicastInterface is a valid address but does not match any interface, or if the address does not match the family then a System Error such as EADDRNOTAVAIL or EPROTONOSUP is thrown.

On IPv6, most errors with specifying or omitting scope will result in the socket continuing to use (or returning to) the system's default interface selection.

A socket's address family's ANY address (IPv4 '0.0.0.0' or IPv6 '::') can be used to return control of the sockets default outgoing interface to the system for future multicast packets.

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.

This method throws EBADF if called on an unbound socket.

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 ttl argument may be between 0 and 255. The default on most systems is 1.

This method throws EBADF if called on an unbound socket.

socket.setRecvBufferSize(size)#

• size <integer>

Sets the SO_RCVBUF socket option. Sets the maximum socket receive buffer in bytes.

This method throws ERR_SOCKET_BUFFER_SIZE if called on an unbound socket.

socket.setSendBufferSize(size)#

• size <integer>

Sets the SO_SNDBUF socket option. Sets the maximum socket send buffer in bytes.

This method throws ERR_SOCKET_BUFFER_SIZE if called on an unbound socket.

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 ttl argument may be between between 1 and 255. The default on most systems is 64.

This method throws EBADF if called on an unbound socket.

socket.unref()#

• Returns: <dgram.Socket>

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.

dgram module functions#

dgram.createSocket(options[, callback])#

• options <Object> Available options are:
  • type <string> The family of socket. Must be either 'udp4' or 'udp6'. Required.
  • reuseAddr <boolean> When true socket.bind() will reuse the address, even if another process has already bound a socket on it. Default: false.
  • ipv6Only <boolean> Setting ipv6Only to true will disable dual-stack support, i.e., binding to address :: won't make 0.0.0.0 be bound. Default: false.
  • recvBufferSize <number> Sets the SO_RCVBUF socket value.
  • sendBufferSize <number> Sets the SO_SNDBUF socket value.
  • lookup <Function> Custom lookup function. Default: dns.lookup().
• callback <Function> Attached as a listener for 'message' events. Optional.
• Returns: <dgram.Socket>

Creates a dgram.Socket object. 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.
• Returns: <dgram.Socket>

Creates a dgram.Socket object of the specified type.

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.