Table of Contents

• Net

  • IPC Support

    • Identifying paths for IPC connections

  • Class: net.Server

    • new net.Server([options][, connectionListener])
    • Event: 'close'
    • Event: 'connection'
    • Event: 'error'
    • Event: 'listening'
    • server.address()
    • server.close([callback])
    • server.connections
    • server.getConnections(callback)

    • server.listen()

      • server.listen(handle[, backlog][, callback])
      • server.listen(options[, callback])
      • server.listen(path[, backlog][, callback])
      • server.listen([port[, host[, backlog]]][, callback])
    • server.listening
    • server.maxConnections
    • server.ref()
    • server.unref()

  • Class: net.Socket

    • new net.Socket([options])
    • Event: 'close'
    • Event: 'connect'
    • Event: 'data'
    • Event: 'drain'
    • Event: 'end'
    • Event: 'error'
    • Event: 'lookup'
    • Event: 'ready'
    • Event: 'timeout'
    • socket.address()
    • socket.bufferSize
    • socket.bytesRead
    • socket.bytesWritten

    • socket.connect()

      • socket.connect(options[, connectListener])
      • socket.connect(path[, connectListener])
      • socket.connect(port[, host][, connectListener])
    • socket.connecting
    • socket.destroy([error])
    • socket.destroyed
    • socket.end([data[, encoding]][, callback])
    • socket.localAddress
    • socket.localPort
    • socket.pause()
    • socket.pending
    • socket.ref()
    • socket.remoteAddress
    • socket.remoteFamily
    • socket.remotePort
    • socket.resume()
    • socket.setEncoding([encoding])
    • socket.setKeepAlive([enable][, initialDelay])
    • socket.setNoDelay([noDelay])
    • socket.setTimeout(timeout[, callback])
    • socket.unref()
    • socket.write(data[, encoding][, callback])

  • net.connect()

    • net.connect(options[, connectListener])
    • net.connect(path[, connectListener])
    • net.connect(port[, host][, connectListener])

  • net.createConnection()

    • net.createConnection(options[, connectListener])
    • net.createConnection(path[, connectListener])
    • net.createConnection(port[, host][, connectListener])
  • net.createServer([options][, connectionListener])
  • net.isIP(input)
  • net.isIPv4(input)
  • net.isIPv6(input)

Net#

The net module provides an asynchronous network API for creating stream-based TCP or IPC servers (net.createServer()) and clients (net.createConnection()).

It can be accessed using:

const net = require('net');

IPC Support#

The net module supports IPC with named pipes on Windows, and Unix domain sockets on other operating systems.

Identifying paths for IPC connections#

net.connect(), net.createConnection(), server.listen() and socket.connect() take a path parameter to identify IPC endpoints.

On Unix, the local domain is also known as the Unix domain. The path is a filesystem pathname. It gets truncated to an OS-dependent length of sizeof(sockaddr_un.sun_path) - 1. Typical values are 107 bytes on Linux and 103 bytes on macOS. If a Node.js API abstraction creates the Unix domain socket, it will unlink the Unix domain socket as well. For example, net.createServer() may create a Unix domain socket and server.close() will unlink it. But if a user creates the Unix domain socket outside of these abstractions, the user will need to remove it. The same applies when a Node.js API creates a Unix domain socket but the program then crashes. In short, a Unix domain socket 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 how it might look, the pipe namespace is flat. Pipes will not persist. They are removed when the last reference to them is closed. Unlike Unix domain sockets, Windows will close and remove the pipe when the owning process exits.

JavaScript string escaping requires paths to be specified with extra backslash escaping such as:

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

Class: net.Server#

• Extends: <EventEmitter>

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

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

• options <Object> See net.createServer([options][, connectionListener]).
• connectionListener <Function> Automatically set as a listener for the 'connection' event.
• Returns: <net.Server>

net.Server is an EventEmitter with the following events:

Event: 'close'#

Emitted when the server closes. 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: <Object> | <string> | <null>

Returns the bound address, the address family name, and port of the server as reported by the operating system if listening on an IP socket (useful to find which port was assigned when getting an OS-assigned address): { port: 12346, family: 'IPv4', address: '127.0.0.1' }.

For a server listening on a pipe or Unix domain socket, the name is returned as a string.

const 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());
});

server.address() returns null before the 'listening' event has been emitted or after calling server.close().

server.close([callback])#

• callback <Function> Called when the server is closed.
• Returns: <net.Server>

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#

• <integer> | <null>

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)#

• callback <Function>
• Returns: <net.Server>

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 an IPC server depending on what it listens to.

Possible signatures:

• server.listen(handle[, backlog][, callback])
• server.listen(options[, callback])
• server.listen(path[, backlog][, callback]) for IPC 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).

All net.Socket are set to SO_REUSEADDR (see socket(7) for details).

The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.

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>
• Returns: <net.Server>

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 an fd member that is a valid file descriptor.

Listening on a file descriptor is not supported on Windows.

server.listen(options[, callback])#

• options <Object> Required. Supports the following properties:

  • port <number>
  • host <string>
  • path <string> Will be ignored if port is specified. See Identifying paths for IPC connections.
  • backlog <number> Common parameter of server.listen() functions.
  • exclusive <boolean> Default: false
  • readableAll <boolean> For IPC servers makes the pipe readable for all users. Default: false.
  • writableAll <boolean> For IPC servers makes the pipe writable for all users. Default: false.
  • ipv6Only <boolean> For TCP servers, setting ipv6Only to true will disable dual-stack support, i.e., binding to host :: won't make 0.0.0.0 be bound. Default: false.
• callback <Function> functions.
• Returns: <net.Server>

If port is specified, it behaves the same as server.listen([port[, host[, 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
});

Starting an IPC server as root may cause the server path to be inaccessible for unprivileged users. Using readableAll and writableAll will make the server accessible for all users.

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

• path <string> Path the server should listen to. See Identifying paths for IPC connections.
• backlog <number> Common parameter of server.listen() functions.
• callback <Function>.
• Returns: <net.Server>

Start an IPC server listening for connections on the given path.

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

• port <number>
• host <string>
• backlog <number> Common parameter of server.listen() functions.
• callback <Function>.
• Returns: <net.Server>

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.

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#

• <boolean> Indicates whether or not the server is listening for connections.

server.maxConnections#

• <integer>

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()#

• Returns: <net.Server>

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

server.unref()#

• Returns: <net.Server>

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 unrefed calling unref() again will have no effect.

Class: net.Socket#

• Extends: <stream.Duplex>

This class is an abstraction of a TCP socket or a streaming IPC endpoint (uses named pipes on Windows, and Unix domain sockets otherwise). It is also an EventEmitter.

A net.Socket can be created by the user and used directly to interact with a server. For example, it is returned by net.createConnection(), so the user can use it to talk to the server.

It can also be created by Node.js and passed to the user when a connection is received. For example, it is passed to the listeners of a 'connection' event emitted on a net.Server, so the user can use it to interact with the client.

new net.Socket([options])#

• options <Object> Available options are:

  • fd <number> If specified, wrap around an existing socket with the given file descriptor, otherwise a new socket will be created.
  • allowHalfOpen <boolean> Indicates whether half-opened TCP connections are allowed. See net.createServer() and the 'end' event for details. Default: false.
  • readable <boolean> Allow reads on the socket when an fd is passed, otherwise ignored. Default: false.
  • writable <boolean> Allow writes on the socket when an fd is passed, otherwise ignored. Default: false.
• Returns: <net.Socket>

Creates a new socket object.

The newly created socket can be either a TCP socket or a streaming IPC endpoint, depending on what it connect() to.

Event: 'close'#

• hadError <boolean> true if the socket had a transmission error.

Emitted once the socket is fully closed. The argument hadError 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 net.createConnection().

Event: 'data'#

• <Buffer> | <string>

Emitted when data is received. The argument data will be a Buffer or String. Encoding of data is set by socket.setEncoding().

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 host name 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 host name.

Event: 'ready'#

Emitted when a socket is ready to be used.

Triggered immediately after 'connect'.

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: <Object>

Returns the bound address, the address family name and port of the socket as reported by the operating system: { port: 12346, family: 'IPv4', address: '127.0.0.1' }

socket.bufferSize#