node/doc/api/net.md

Net

Stability: 2 - Stable

The net module provides an asynchronous network API for creating stream-based servers (net.createServer()) and clients (net.createConnection()) that implement TCP or local communications (domain sockets on UNIX, named pipes on Windows). It can be accessed using:

const net = require('net');

Class: net.Server

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

new net.Server([options][, connectionListener])

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

net.Server is an EventEmitter with the following events:

Event: 'close'

Emitted when the server closes. Note that if connections exist, this event is not emitted until all connections are ended.

Event: 'connection'

• {net.Socket} The connection object

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

Event: 'error'

• {Error}

Emitted when an error occurs. Unlike net.Socket, the 'close' event will not be emitted directly following this event unless server.close() is manually called. See the example in discussion of server.listen().

Event: 'listening'

Emitted when the server has been bound after calling server.listen().

server.address()

Returns the bound address, the address family name, and port of the server as reported by the operating system. Useful to find which port was assigned when getting an OS-assigned address. Returns an object with port, family, and address properties: { port: 12346, family: 'IPv4', address: '127.0.0.1' }

Example:

var server = net.createServer((socket) => {
  socket.end('goodbye\n');
}).on('error', (err) => {
  // handle errors here
  throw err;
});

// grab an arbitrary unused port.
server.listen(() => {
  console.log('opened server on', server.address());
});

Don't call server.address() until the 'listening' event has been emitted.

server.close([callback])

Stops the server from accepting new connections and keeps existing connections. This function is asynchronous, the server is finally closed when all connections are ended and the server emits a 'close' event. The optional callback will be called once the 'close' event occurs. Unlike that event, it will be called with an Error as its only argument if the server was not open when it was closed.

server.connections

Stability: 0 - Deprecated: Use server.getConnections() instead.

The number of concurrent connections on the server.

This becomes null when sending a socket to a child with child_process.fork(). To poll forks and get current number of active connections use asynchronous server.getConnections instead.

server.getConnections(callback)

Asynchronously get the number of concurrent connections on the server. Works when sockets were sent to forks.

Callback should take two arguments err and count.

server.listen()

Start a server listening for connections. A net.Server can be a TCP or local (domain sockets on UNIX, named pipes on Windows) server depending on what it listens to.

Note: Unix named pipes (FIFOs) are not supported.

Possible signatures:

• server.listen(handle[, backlog][, callback])
• server.listen(options[, callback])
• server.listen(path[, backlog][, callback]) for local servers
• server.listen([port][, host][, backlog][, callback]) for TCP servers

This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callback will be added as a listener for the 'listening' event.

All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).

Note:

• All net.Socket are set to SO_REUSEADDR (See socket(7) for details).
• The server.listen() method may be called multiple times. Each subsequent call will re-open the server using the provided options.

One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requested port / path / handle. One way to handle this would be to retry after a certain amount of time:

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

server.listen(handle[, backlog][, callback])

• handle {Object}
• backlog {number} Common parameter of server.listen() functions
• callback {Function} Common parameter of server.listen() functions

Start a server listening for connections on a given handle that has already been bound to a port, a UNIX domain socket, or a Windows named pipe.

The handle object can be either a server, a socket (anything with an underlying _handle member), or an object with a fd member that is a valid file descriptor.

Note: Listening on a file descriptor is not supported on Windows.

server.listen(options[, callback])

• options {Object} Required. Supports the following properties:
  • port {number} Optional.
  • host {string} Optional.
  • path {string} Optional. Will be ignored if port is specified.
  • backlog {number} Optional. Common parameter of server.listen() functions
  • exclusive {boolean} Optional. Default to false
• callback {Function} Optional. Common parameter of server.listen() functions

If port is specified, it behaves the same as server.listen([port][, hostname][, backlog][, callback]). Otherwise, if path is specified, it behaves the same as server.listen(path[, backlog][, callback]). If none of them is specified, an error will be thrown.

If exclusive is false (default), then cluster workers will use the same underlying handle, allowing connection handling duties to be shared. When exclusive is true, the handle is not shared, and attempted port sharing results in an error. An example which listens on an exclusive port is shown below.

server.listen({
  host: 'localhost',
  port: 80,
  exclusive: true
});

server.listen(path[, backlog][, callback])

• path {string}
• backlog {number} Common parameter of server.listen() functions
• callback {Function} Common parameter of server.listen() functions

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

On UNIX, the local domain is usually known as the UNIX domain. The path is a filesystem path name. It gets truncated to sizeof(sockaddr_un.sun_path) bytes, decreased by 1. It varies on different operating system between 91 and 107 bytes. The typical values are 107 on Linux and 103 on OS X. The path is subject to the same naming conventions and permissions checks as would be done on file creation, will be visible in the filesystem, and will persist until unlinked.

On Windows, the local domain is implemented using a named pipe. The path must refer to an entry in \\?\pipe\ or \\.\pipe\. Any characters are permitted, but the latter may do some processing of pipe names, such as resolving .. sequences. Despite appearances, the pipe name space is flat. Pipes will not persist, they are removed when the last reference to them is closed. Do not forget JavaScript string escaping requires paths to be specified with double-backslashes, such as:

net.createServer().listen(
    path.join('\\\\?\\pipe', process.cwd(), 'myctl'))

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

• port {number}
• host {string}
• backlog {number} Common parameter of server.listen() functions
• callback {Function} Common parameter of server.listen() functions

Start a TCP server listening for connections on the given port and host.

If port is omitted or is 0, the operating system will assign an arbitrary unused port, which can be retrieved by using server.address().port after the 'listening' event has been emitted.

If host is omitted, the server will accept connections on the unspecified IPv6 address (::) when IPv6 is available, or the unspecified IPv4 address (0.0.0.0) otherwise.

Note: in most operating systems, listening to the unspecified IPv6 address (::) may cause the net.Server to also listen on the unspecified IPv4 address (0.0.0.0).

server.listening

A Boolean indicating whether or not the server is listening for connections.

server.maxConnections

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

It is not recommended to use this option once a socket has been sent to a child with child_process.fork().

server.ref()

Opposite of unref, calling ref on a previously unrefd server will not let the program exit if it's the only server left (the default behavior). If the server is refd calling ref again will have no effect.

Returns server.

server.unref()

Calling unref on a server will allow the program to exit if this is the only active server in the event system. If the server is already unrefd calling unref again will have no effect.

Returns server.

Class: net.Socket

This object is an abstraction of a TCP or local socket. net.Socket instances 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.js and passed to the user through the 'connection' event of a server.

new net.Socket([options])

Construct a new socket object.

options is an object with the following defaults:

{
  fd: null,
  allowHalfOpen: false,
  readable: false,
  writable: false
}

fd allows you to specify the existing file descriptor of socket. Set readable and/or writable to true to allow reads and/or writes on this socket (NOTE: Works only when fd is passed). About allowHalfOpen, refer to net.createServer() and 'end' event.

net.Socket instances are EventEmitter with the following events:

Event: 'close'

• had_error {boolean} true if the socket had a transmission 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.

Event: 'connect'

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

Event: 'data'

• {Buffer}

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

Note that the data will be lost if there is no listener when a Socket emits a 'data' event.

Event: 'drain'

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

See also: the return values of socket.write()

Event: 'end'

Emitted when the other end of the socket sends a FIN packet, thus ending the readable side of the socket.

By default (allowHalfOpen is false) the socket will send a FIN packet back and destroy its file descriptor once it has written out its pending write queue. However, if allowHalfOpen is set to true, the socket will not automatically end() its writable side, allowing the user to write arbitrary amounts of data. The user must call end() explicitly to close the connection (i.e. sending a FIN packet back).

Event: 'error'

• {Error}

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

Event: 'lookup'

Emitted after resolving the hostname but before connecting. Not applicable to UNIX sockets.

• err {Error|null} The error object. See dns.lookup().
• address {string} The IP address.
• family {string|null} The address type. See dns.lookup().
• host {string} The hostname.

Event: 'timeout'

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()

socket.address()

Returns the bound address, the address family name and port of the socket as reported by the operating system. Returns an object with three properties, e.g. { port: 12346, family: 'IPv4', address: '127.0.0.1' }

socket.bufferSize

net.Socket has the property that socket.write() always works. This is to help users get up and running quickly. The computer cannot always keep up with the amount of data that is written to a socket - the network connection simply might be too slow. Node.js 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 socket.pause() and socket.resume().

socket.bytesRead

The amount of received bytes.

socket.bytesWritten

The amount of bytes sent.

socket.connect(options[, connectListener])

Opens the connection for a given socket.

For TCP sockets, options argument should be an object which specifies:

• port: Port the client should connect to (Required).

• host: Host the client should connect to. Defaults to 'localhost'.

• localAddress: Local interface to bind to for network connections.

• localPort: Local port to bind to for network connections.

• family : Version of IP stack. Defaults to 4.

• hints: dns.lookup() hints. Defaults to 0.

• lookup : Custom lookup function. Defaults to dns.lookup.

For local domain sockets, options argument should be an object which specifies:

• path: Path the client should connect to (Required).

Normally this method is not needed, as net.createConnection opens the socket. Use this only if you are implementing a custom Socket.

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 connectListener parameter will be added as a listener for the 'connect' event.

socket.connect(path[, connectListener])

socket.connect(port[, host][, connectListener])

As socket.connect(options[, connectListener]), with options as either {port: port, host: host} or {path: path}.

socket.connecting

If true - socket.connect(options[, connectListener]) was called and haven't yet finished. Will be set to false before emitting connect event and/or calling socket.connect(options[, connectListener])'s callback.

socket.destroy([exception])

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

If exception is specified, an 'error' event will be emitted and any listeners for that event will receive exception as an argument.

socket.destroyed

A Boolean value that indicates if the connection is destroyed or not. Once a connection is destroyed no further data can be transferred using it.

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

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().

socket.localAddress

The string representation of the local IP address the remote client is connecting on. For example, if you are listening on '0.0.0.0' and the client connects on '192.168.1.1', the value would be '192.168.1.1'.

socket.localPort

The numeric representation of the local port. For example, 80 or 21.

socket.pause()

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

socket.ref()

Opposite of unref, calling ref on a previously unrefd socket will not let the program exit if it's the only socket left (the default behavior). If the socket is refd calling ref again will have no effect.

Returns socket.

socket.remoteAddress

The string representation of the remote IP address. For example, '74.125.127.100' or '2001:4860:a005::68'. Value may be undefined if the socket is destroyed (for example, if the client disconnected).

socket.remoteFamily

The string representation of the remote IP family. 'IPv4' or 'IPv6'.

socket.remotePort

The numeric representation of the remote port. For example, 80 or 21.

socket.resume()

Resumes reading after a call to socket.pause().

socket.setEncoding([encoding])

Set the encoding for the socket as a Readable Stream. See stream.setEncoding() for more information.

socket.setKeepAlive([enable][, initialDelay])

Enable/disable keep-alive functionality, and optionally set the initial delay before the first keepalive probe is sent on an idle socket. enable defaults to false.

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. Defaults to 0.

Returns socket.

socket.setNoDelay([noDelay])

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

Returns socket.

socket.setTimeout(timeout[, callback])

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 call socket.end() or socket.destroy() to end the connection.

socket.setTimeout(3000);
socket.on('timeout', () => {
  console.log('socket timeout');
  socket.end();
})

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.

Returns socket.

socket.unref()

Calling unref on a socket will allow the program to exit if this is the only active socket in the event system. If the socket is already unrefd calling unref again will have no effect.

Returns socket.

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

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.

net.connect(options[, connectListener])

A factory function, which returns a new net.Socket and automatically connects with the supplied options.

The options are passed to both the net.Socket constructor and the socket.connect method.

The connectListener parameter will be added as a listener for the 'connect' event once.

Here is an example of a client of the previously described echo server:

const net = require('net');
const client = net.connect({port: 8124}, () => {
  // 'connect' listener
  console.log('connected to server!');
  client.write('world!\r\n');
});
client.on('data', (data) => {
  console.log(data.toString());
  client.end();
});
client.on('end', () => {
  console.log('disconnected from server');
});

To connect on the socket /tmp/echo.sock the second line would just be changed to

const client = net.connect({path: '/tmp/echo.sock'});

net.connect(path[, connectListener])

A factory function, which returns a new unix net.Socket and automatically connects to the supplied path.

The connectListener parameter will be added as a listener for the 'connect' event once.

net.connect(port[, host][, connectListener])

A factory function, which returns a new net.Socket and automatically connects to the supplied port and host.

If host is omitted, 'localhost' will be assumed.

The connectListener parameter will be added as a listener for the 'connect' event once.

net.createConnection(options[, connectListener])

A factory function, which returns a new net.Socket and automatically connects with the supplied options.

The options are passed to both the net.Socket constructor and the socket.connect method.

Passing timeout as an option will call socket.setTimeout() after the socket is created, but before it is connecting.

The connectListener parameter will be added as a listener for the 'connect' event once.

Here is an example of a client of the previously described echo server:

const net = require('net');
const client = net.createConnection({port: 8124}, () => {
  //'connect' listener
  console.log('connected to server!');
  client.write('world!\r\n');
});
client.on('data', (data) => {
  console.log(data.toString());
  client.end();
});
client.on('end', () => {
  console.log('disconnected from server');
});

To connect on the socket /tmp/echo.sock the second line would just be changed to

const client = net.connect({path: '/tmp/echo.sock'});

net.createConnection(path[, connectListener])

A factory function, which returns a new unix net.Socket and automatically connects to the supplied path.

The connectListener parameter will be added as a listener for the 'connect' event once.

net.createConnection(port[, host][, connectListener])

A factory function, which returns a new net.Socket and automatically connects to the supplied port and host.

If host is omitted, 'localhost' will be assumed.

The connectListener parameter will be added as a listener for the 'connect' event once.

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

Creates a new TCP or local server.

• options {Object}
  • allowHalfOpen {boolean} Default to false. Indicates whether half-opened TCP connections are allowed.
  • pauseOnConnect {boolean} Default to false. Indicates whether the socket should be paused on incoming connections.
• connectionListener {Function} Automatically set as a listener for the 'connection' event

If allowHalfOpen is set to true, when the other end of the socket sends a FIN packet, the server will only send a FIN packet back when socket.end() is explicitly called, until then the connection is half-closed (non-readable but still writable). See 'end' event and RFC 1122 for more information.

If pauseOnConnect is set to true, then the socket associated with each incoming connection will be paused, and no data will be read from its handle. This allows connections to be passed between processes without any data being read by the original process. To begin reading data from a paused socket, call socket.resume().

The server can be a TCP server or a local server, depending on what it listen() to.

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

const net = require('net');
const server = net.createServer((c) => {
  // 'connection' listener
  console.log('client connected');
  c.on('end', () => {
    console.log('client disconnected');
  });
  c.write('hello\r\n');
  c.pipe(c);
});
server.on('error', (err) => {
  throw err;
});
server.listen(8124, () => {
  console.log('server bound');
});

Test this by using telnet:

telnet localhost 8124

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

server.listen('/tmp/echo.sock', () => {
  console.log('server bound');
});

Use nc to connect to a UNIX domain socket server:

nc -U /tmp/echo.sock

net.isIP(input)

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.

net.isIPv4(input)

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

net.isIPv6(input)

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