The net module provides you with an asynchronous network wrapper. It contains methods for creating both servers and clients (called streams). You can include this module with require("net");

net.createServer([options], [connectionListener])Edit

Creates a new TCP server. The connectionListener argument is automatically set as a listener for the 'connection' event.

options is an object with the following defaults:

{ allowHalfOpen: false

If allowHalfOpen is true, then the socket won't automatically send FIN packet when the other end of the socket sends a FIN packet. The socket becomes non-readable, but still writable. You should call the end() method explicitly. See 'end' event for more information.


Construct a new socket object and opens a socket to the given location. When the socket is established the 'connect' event will be emitted.

The arguments for this method change the type of connection:

  • net.createConnection(port, [host])

Creates a TCP connection to port on host. If host is omitted,localhost will be assumed.

  • net.createConnection(path)

Creates unix socket connection to path


This class is used to create a TCP or UNIX server.

Here is an example of a echo server which listens for connections on port 8124:

var net = require('net');
var server = net.createServer(function (c) {
server.listen(8124, 'localhost');

Test this by using telnet:

telnet localhost 8124

To listen on the socket /tmp/echo.sock the last line would just be changed to


Use nc to connect to a UNIX domain socket server:

nc -U /tmp/echo.sock

net.Server is an EventEmitter with the following events:

server.listen(port, [host], [callback])Edit

Begin accepting connections on the specified port and host. If the host is omitted, the server will accept connections directed to any IPv4 address (INADDR_ANY).

This function is asynchronous. The last parameter callback will be called when the server has been bound.

One issue some users run into is getting EADDRINUSE errors. Meaning another server is already running on the requested port. One way of handling this would be to wait a second and the try again. This can be done with

server.on('error', function (e) {
  if (e.code == 'EADDRINUSE') {
    console.log('Address in use, retrying...');
    setTimeout(function () {
      server.listen(PORT, HOST);
    }, 1000);

(Note: All sockets in Node are set SO_REUSEADDR already)

server.listen(path, [callback])Edit

Start a UNIX socket server listening for connections on the given path.

This function is asynchronous. The last parameter callback will be called when the server has been bound.


Start a server listening for connections on the given file descriptor.

This file descriptor must have already had the bind(2) and listen(2)system calls invoked on it.


Stops the server from accepting new connections. This function is asynchronous, the server is finally closed when the server emits a 'close'event.


Returns the bound address of the server as seen by the operating system. Useful to find which port was assigned when giving getting an OS-assigned address


var server = net.createServer(function (socket) {

// grab a random port.
server.listen(function() {
  address = server.address();
  console.log("opened server on %j", address);


Set this property to reject connections when the server's connection count gets high.


The number of concurrent connections on the server.

Event: 'connection'Edit

function (socket) {}

Emitted when a new connection is made. socket is an instance ofnet.Socket.

Event: 'close'Edit

function () {}

Emitted when the server closes.


This object is an abstraction of of a TCP or UNIX socket. net.Socketinstances implement a duplex Stream interface. They can be created by the user and used as a client (with connect()) or they can be created by Node and passed to the user through the 'connection' event of a server.

net.Socket instances are EventEmitters with the following events:

new net.Socket([options])Edit

Construct a new socket object.

options is an object with the following defaults:

{ fd: null
  type: null
  allowHalfOpen: false

fd allows you to specify the existing file descriptor of socket. type specified underlying protocol. It can be 'tcp4', 'tcp6', or 'unix'. AboutallowHalfOpen, refer to createServer() and 'end' event.

socket.connect(port, [host], [callback])Edit

socket.connect(path, [callback])Edit

Opens the connection for a given socket. If port and host are given, then the socket will be opened as a TCP socket, if host is omitted, localhost will be assumed. If a path is given, the socket will be opened as a unix socket to that path.

Normally this method is not needed, as net.createConnection opens the socket. Use this only if you are implementing a custom Socket or if a Socket is closed and you want to reuse it to connect to another server.

This function is asynchronous. When the 'connect' event is emitted the socket is established. If there is a problem connecting, the 'connect' event will not be emitted, the 'error' event will be emitted with the exception.

The callback parameter will be added as an listener for the 'connect' event.


net.Socket has the property that socket.write() always works. This is to help users get up an running quickly. The computer cannot necessarily keep up with the amount of data that is written to a socket - the network connection simply might be too slow. Node will internally queue up the data written to a socket and send it out over the wire when it is possible. (Internally it is polling on the socket's file descriptor for being writable).

The consequence of this internal buffering is that memory may grow. This property shows the number of characters currently buffered to be written. (Number of characters is approximately equal to the number of bytes to be written, but the buffer may contain strings, and the strings are lazily encoded, so the exact number of bytes is not known.)

Users who experience large or growing bufferSize should attempt to "throttle" the data flows in their program with pause() and resume()`.


Sets the encoding (either 'ascii', 'utf8', or 'base64') for data that is received.


This function has been removed in v0.3. It used to upgrade the connection to SSL/TLS. See the TLS section for the new API.

socket.write(data, [encoding], [callback])Edit

Sends data on the socket. The second parameter specifies the encoding in the case of a string--it defaults to UTF8 encoding.

Returns true if the entire data was flushed successfully to the kernel buffer. Returns false if all or part of the data was queued in user memory. 'drain'will be emitted when the buffer is again free.

The optional callback parameter will be executed when the data is finally written out - this may not be immediately.

socket.write(data, [encoding], [fileDescriptor], [callback])Edit

For UNIX sockets, it is possible to send a file descriptor through the socket. Simply add the fileDescriptor argument and listen for the 'fd' event on the other end.

socket.end([data], [encoding])Edit

Half-closes the socket. I.E., it sends a FIN packet. It is possible the server will still send some data.

If data is specified, it is equivalent to calling socket.write(data, encoding) followed by socket.end().


Ensures that no more I/O activity happens on this socket. Only necessary in case of errors (parse error or so).


Pauses the reading of data. That is, 'data' events will not be emitted. Useful to throttle back an upload.


Resumes reading after a call to pause().

socket.setTimeout(timeout, [callback])Edit

Sets the socket to timeout after timeout milliseconds of inactivity on the socket. By default net.Socket do not have a timeout.

When an idle timeout is triggered the socket will receive a 'timeout' event but the connection will not be severed. The user must manually end() ordestroy() the socket.

If timeout is 0, then the existing idle timeout is disabled.

The optional callback parameter will be added as a one time listener for the'timeout' event.


Disables the Nagle algorithm. By default TCP connections use the Nagle algorithm, they buffer data before sending it off. Setting noDelay will immediately fire off data each time socket.write() is called.

socket.setKeepAlive(enable=false, [initialDelay])Edit

Enable/disable keep-alive functionality, and optionally set the initial delay before the first keepalive probe is sent on an idle socket. Set initialDelay (in milliseconds) to set the delay between the last data packet received and the first keepalive probe. Setting 0 for initialDelay will leave the value unchanged from the default (or previous) setting.


The string representation of the remote IP address. For example,'' or '2001:4860:a005::68'.

This member is only present in server-side connections.

Event: 'connect'Edit

function () { }

Emitted when a socket connection successfully is established. See connect().

Event: 'data'Edit

function (data) { }

Emitted when data is received. The argument data will be a Buffer orString. Encoding of data is set by socket.setEncoding(). (See theReadable Stream section for more information.)

Event: 'end'Edit

function () { }

Emitted when the other end of the socket sends a FIN packet.

By default (allowHalfOpen == false) the socket will destroy its file descriptor once it has written out its pending write queue. However, by settingallowHalfOpen == true the socket will not automatically end() its side allowing the user to write arbitrary amounts of data, with the caveat that the user is required to end() their side now.

Event: 'timeout'Edit

function () { }

Emitted if the socket times out from inactivity. This is only to notify that the socket has been idle. The user must manually close the connection.

See also: socket.setTimeout()

Event: 'drain'Edit

function () { }

Emitted when the write buffer becomes empty. Can be used to throttle uploads.

Event: 'error'Edit

function (exception) { }

Emitted when an error occurs. The 'close' event will be called directly following this event.

Event: 'close'Edit

function (had_error) { }

Emitted once the socket is fully closed. The argument had_error is a boolean which says if the socket was closed due to a transmission error.



Tests if input is an IP address. Returns 0 for invalid strings, returns 4 for IP version 4 addresses, and returns 6 for IP version 6 addresses.


Returns true if input is a version 4 IP address, otherwise returns false.


Returns true if input is a version 6 IP address, otherwise returns false.

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