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); // server listening 0.0.0.0:41234
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.
The 'close'
event is emitted after a socket is closed with close()
. Once triggered, no new 'message'
events will be emitted on this socket.
exception
<Error>
The 'error'
event is emitted whenever any error occurs. The event handler function is passed a single Error
object.
The 'listening'
event is emitted whenever a socket begins listening for datagram messages. This occurs as soon as UDP sockets are created.
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
.
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.
Returns an object containing the address information for a socket. For UDP sockets, this object will contain address
, family
and port
properties.
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.
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', () => { const address = server.address(); console.log(`server listening ${address.address}:${address.port}`); }); server.bind(41234); // server listening 0.0.0.0:41234
options
<Object> Required. Supports the following properties:
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.
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 });
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.
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.
SO_RCVBUF
socket receive buffer size in bytes.SO_SNDBUF
socket send buffer size in bytes.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.
msg
<Buffer> | <Uint8Array> | <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 hostname or IP address.callback
<Function> Called when the message has been sent.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
or Uint8Array
, 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. 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 both must be set if either are used. They are supported only when the first argument is a Buffer
or Uint8Array
.
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. It is important to run benchmarks to determine the optimal strategy on a case-by-case basis. Generally speaking, however, sending multiple buffers 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.
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.
multicastInterface
<string>
Note: 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.
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'); });
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'); });
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.
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.
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 socket.setMulticastTTL()
is a number of hops between 0 and 255. The default on most systems is 1
but can vary.
size
<integer>
Sets the SO_RCVBUF
socket option. Sets the maximum socket receive buffer in bytes.
size
<integer>
Sets the SO_SNDBUF
socket option. Sets the maximum socket send buffer in bytes.
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.
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.
As of Node.js v0.10, dgram.Socket#bind()
changed to an asynchronous execution model. Legacy code would use synchronous behavior:
const s = dgram.createSocket('udp4'); s.bind(1234); s.addMembership('224.0.0.114');
Such legacy code would need to 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'); });
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
.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.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
.
type
<string> - Either 'udp4'
or 'udp6'
.callback
<Function> - Attached as a listener to 'message'
events.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
.
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