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Net

Stability: 2 - Stable

The node: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:

import net from 'node:net';
const net = require('node:net');

IPC support

The node: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 file system pathname. It will throw an error when the length of pathname is greater than the length of sizeof(sockaddr_un.sun_path). 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 file system and will persist until unlinked. On Linux, You can use Unix abstract socket by adding \0 to the beginning of the path, such as \0abstract. The path to the Unix abstract socket is not visible in the file system and it will disappear automatically when all open references to the socket are closed.

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.BlockList

The BlockList object can be used with some network APIs to specify rules for disabling inbound or outbound access to specific IP addresses, IP ranges, or IP subnets.

blockList.addAddress(address[, type])

Adds a rule to block the given IP address.

blockList.addAddresses(addresses[, type])

Adds multiple address rules to the block list in a single operation. This is more efficient than calling blockList.addAddress() repeatedly when adding a large number of individual addresses, as the addresses are inserted under a single internal lock acquisition.

blockList.addCIDR(cidr)

Adds a subnet rule using CIDR notation. The address family is automatically detected from the address (IPv6 if the address contains ':', IPv4 otherwise). This is equivalent to calling blockList.addSubnet() with the parsed network address, prefix length, and family.

blockList.addCIDRs(cidrs)

Adds multiple subnet rules using CIDR notation in a single call. The address family for each entry is automatically detected. This is equivalent to calling blockList.addCIDR() for each element of the array.

blockList.addRange(start, end[, type])

Adds a rule to block a range of IP addresses from start (inclusive) to end (inclusive).

blockList.addSubnet(net, prefix[, type])

Adds a rule to block a range of IP addresses specified as a subnet mask.

blockList.check(address[, type])

Returns true if the given IP address matches any of the rules added to the BlockList.

const blockList = new net.BlockList();
blockList.addAddress('123.123.123.123');
blockList.addRange('10.0.0.1', '10.0.0.10');
blockList.addSubnet('8592:757c:efae:4e45::', 64, 'ipv6');

console.log(blockList.check('123.123.123.123'));  // Prints: true
console.log(blockList.check('10.0.0.3'));  // Prints: true
console.log(blockList.check('222.111.111.222'));  // Prints: false

// IPv6 notation for IPv4 addresses works:
console.log(blockList.check('::ffff:7b7b:7b7b', 'ipv6')); // Prints: true
console.log(blockList.check('::ffff:123.123.123.123', 'ipv6')); // Prints: true

blockList.clear()

Clears all rules from the BlockList.

blockList.fromJSON(value)

Stability: 1.2 - Release candidate

const blockList = new net.BlockList();
const data = [
  'Subnet: IPv4 192.168.1.0/24',
  'Address: IPv4 10.0.0.5',
  'Range: IPv4 192.168.2.1-192.168.2.10',
  'Range: IPv4 10.0.0.1-10.0.0.10',
];
blockList.fromJSON(data);
blockList.fromJSON(JSON.stringify(data));

BlockList.isBlockList(value)

BlockList.PRIVATE_RANGES

A frozen array of CIDR strings representing private, loopback, and link-local IP address ranges. This can be passed to blockList.addCIDRs() to quickly populate a blocklist with all non-routable address ranges.

The included ranges are:

const blockList = new net.BlockList();
blockList.addCIDRs(net.BlockList.PRIVATE_RANGES);

console.log(blockList.check('10.0.0.1'));      // Prints: true
console.log(blockList.check('127.0.0.1'));     // Prints: true
console.log(blockList.check('8.8.8.8'));       // Prints: false

blockList.removeAddress(address[, type])

Removes a rule that was previously added with blockList.addAddress(). The address must match exactly the value used when the rule was added. If the specified address does not exist, this is a no-op.

blockList.removeCIDR(cidr)

Removes a subnet rule using CIDR notation. The address family is automatically detected from the address. This is equivalent to calling blockList.removeSubnet() with the parsed network address, prefix length, and family. If the specified subnet does not exist, this is a no-op.

blockList.removeRange(start, end[, type])

Removes a rule that was previously added with blockList.addRange(). The start and end addresses must match exactly the values used when the rule was added. If the specified range does not exist, this is a no-op.

blockList.removeSubnet(net, prefix[, type])

Removes a rule that was previously added with blockList.addSubnet(). The network address and prefix must match exactly the values used when the rule was added. If the specified subnet does not exist, this is a no-op.

blockList.rules

The list of rules added to the blocklist.

blockList.size

The number of rules in the blocklist. This is equivalent to blockList.rules.length but does not allocate the rules array.

blockList.toJSON()

Stability: 1.2 - Release candidate

Class: net.SocketAddress

new net.SocketAddress([options])

socketaddress.address

socketaddress.family

socketaddress.flowlabel

socketaddress.port

SocketAddress.parse(input)

Class: net.Server

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

A listening TCP net.Server can be transferred to a worker thread by listing it in the transferList of a worker_threads postMessage() call. This moves the underlying listening socket to the receiving thread, where it resumes accepting connections. See Transferring TCP handles to other threads.

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

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'

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

Event: '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().

Event: 'drop'

When the number of connections reaches the threshold of server.maxConnections, the server will drop new connections and emit 'drop' event instead. If it is a TCP server, the argument is as follows, otherwise the argument is undefined.

server.address()

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])

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[Symbol.asyncDispose]()

Calls server.close() and returns a promise that fulfills when the server has closed.

server[Symbol.asyncIterator]()

Stability: 1 - Experimental

Returns an async iterator over the server’s incoming connections, allowing them to be consumed with for await...of as an alternative to the 'connection' event. Iteration ends when the server emits 'close', and rejects if the server emits 'error'.

The loop only advances to the next connection once the current iteration’s body has finished awaiting, so connection handling should be dispatched to a separate async task rather than awaited inline. Otherwise connections are serialized: each one waits for the previous to be fully handled.

import { createServer } from 'node:net';

const server = createServer().listen(8124);

async function handleConnection(socket) {
  // ...handle the connection, awaiting as needed.
  socket.end('hello world!');
}

for await (const socket of server) {
  // Dispatch handling to a separate task so the loop keeps accepting
  // connections instead of serializing them.
  handleConnection(socket);
}

The server does not stop accepting connections while the loop body runs, so a consumer slower than the connection rate can buffer them without bound. Use server.maxConnections to bound concurrency.

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 an IPC server depending on what it listens to.

Possible signatures:

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.error('Address in use, retrying...');
    setTimeout(() => {
      server.close();
      server.listen(PORT, HOST);
    }, 1000);
  }
});

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

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), a BoundSocket, or an object with an fd member that is a valid file descriptor.

When handle is a BoundSocket, the server adopts the already-bound socket and starts listening on it. Adoption consumes the bound socket (see ownership transfer).

Listening on a file descriptor is not supported on Windows.

server.listen(options[, callback])

If handle is specified, the server adopts that pre-bound socket. Otherwise, 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.

Using the signal option to destroy a long-lived server as a resource cleanup mechanism is deprecated. The signal option remains appropriate for cancellation, externally propagated aborts, and timeouts. See DEP0209.

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,
});

When exclusive is true and the underlying handle is shared, it is possible that several workers query a handle with different backlogs. In this case, the first backlog passed to the master process will be used.

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.

If the signal option is enabled, calling .abort() on the corresponding AbortController is similar to calling .close() on the server:

const controller = new AbortController();
server.listen({
  host: 'localhost',
  port: 80,
  signal: controller.signal,
});
// Later, when you want to close the server.
controller.abort();

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

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

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

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

server.maxConnections

When the number of connections reaches the server.maxConnections threshold:

  1. If the process is not running in cluster mode, Node.js will close the connection.

  2. If the process is running in cluster mode, Node.js will, by default, route the connection to another worker process. To close the connection instead, set server.dropMaxConnection to true.

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

server.dropMaxConnection

Set this property to true to begin closing connections once the number of connections reaches the server.maxConnections threshold. This setting is only effective in cluster mode.

server.ref()

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()

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

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.

Transferring TCP handles to other threads

A connected TCP net.Socket can be moved to another thread by listing it in the transferList of a worker_threads postMessage() call. After the transfer, the source socket is destroyed on the sending thread (further use fails with ERR_STREAM_DESTROYED rather than silently dropping data), and the socket continues to work on the receiving thread. This makes it possible to accept connections on one thread and distribute them across a pool of worker threads, for example to build a node:cluster-like model on top of worker threads.

The socket must be a freshly accepted or created TCP connection: it must still be attached to a live handle, must not be connecting or destroyed, and must not have started reading or have buffered data. Otherwise postMessage() throws ERR_WORKER_HANDLE_NOT_TRANSFERABLE. Only TCP sockets are supported.

const net = require('node:net');
const { Worker } = require('node:worker_threads');

// worker.js receives `{ socket }` messages and handles each connection.
const worker = new Worker('./worker.js');

const server = net.createServer((socket) => {
  // Hand the freshly accepted connection off to the worker thread.
  worker.postMessage({ socket }, [socket]);
});
server.listen(8000);

A listening net.Server can be transferred the same way, which moves the listening socket itself (and its pending accept queue) to the receiving thread.

An un-adopted TCP BoundSocket can also be transferred, which moves the bound (but not yet listening or connected) socket. This allows a port to be reserved synchronously on one thread and adopted by a server or outgoing connection on another. Pipe binds are not transferable. After the transfer, the source BoundSocket behaves as if it had been adopted: address(), fd() and close() throw ERR_SOCKET_HANDLE_ADOPTED.

new net.Socket([options])

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'

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: 'connectionAttempt'

Emitted when a new connection attempt is started. This may be emitted multiple times if the family autoselection algorithm is enabled in socket.connect(options).

Event: 'connectionAttemptFailed'

Emitted when a connection attempt failed. This may be emitted multiple times if the family autoselection algorithm is enabled in socket.connect(options).

Event: 'connectionAttemptTimeout'

Emitted when a connection attempt is still pending after the configured autoSelectFamilyAttemptTimeout. If another attempt is about to start, the pending attempt remains active and may still establish the connection, unless localPort requires sequential attempts. This is only emitted if the family autoselection algorithm is enabled in socket.connect(options).

Event: 'data'

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 signals the end of transmission, thus ending the readable side of the socket.

By default (allowHalfOpen is false) the socket will send an end of transmission 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'

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.

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 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.autoSelectFamilyAttemptedAddresses

This property is only present if the family autoselection algorithm is enabled in socket.connect(options) and it is an array of the addresses that have been attempted.

Each address is a string in the form of $IP:$PORT. If the connection was successful, then the last address is the one that the socket is currently connected to.

socket.bufferSize

Stability: 0 - Deprecated: Use writable.writableLength instead.

This property shows the number of characters buffered for writing. The buffer may contain strings whose length after encoding is not yet known. So this number is only an approximation of the number of bytes in the buffer.

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.

The consequence of this internal buffering is that memory may grow. 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()

Initiate a connection on a given socket.

Possible signatures:

This function is asynchronous. When the connection is established, the 'connect' event will be emitted. If there is a problem connecting, instead of a 'connect' event, an 'error' event will be emitted with the error passed to the 'error' listener. The last parameter connectListener, if supplied, will be added as a listener for the 'connect' event once.

This function should only be used for reconnecting a socket after 'close' has been emitted or otherwise it may lead to undefined behavior.

socket.connect(options[, connectListener])

Initiate a connection on a given socket. Normally this method is not needed, the socket should be created and opened with net.createConnection(). Use this only when implementing a custom Socket.

For TCP connections, available options are:

For IPC connections, available options are:

socket.connect(path[, connectListener])

Initiate an IPC connection on the given socket.

Alias to socket.connect(options[, connectListener]) called with { path: path } as options.

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

Initiate a TCP connection on the given socket.

Alias to socket.connect(options[, connectListener]) called with {port: port, host: host} as options.

socket.connecting

If true, socket.connect(options[, connectListener]) was called and has not yet finished. It will stay true until the socket becomes connected, then it is set to false and the 'connect' event is emitted. Note that the socket.connect(options[, connectListener]) callback is a listener for the 'connect' event.

socket.destroy([error])

Ensures that no more I/O activity happens on the current connection. Destroys the stream and closes the connection.

See writable.destroy() for further details.

socket.destroyed

See writable.destroyed for further details.

socket.destroySoon()

Destroys the socket after all data is written. If the 'finish' event was already emitted the socket is destroyed immediately. If the socket is still writable it implicitly calls socket.end().

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

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

See writable.end() for further details.

socket.localAddress

The string representation of the local IP address the remote client is connecting on. For example, in a server listening on '0.0.0.0', if a client connects on '192.168.1.1', the value of socket.localAddress would be '192.168.1.1'.

socket.localPort

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

socket.localFamily

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

socket.pause()

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

socket.pending

This is true if the socket is not connected yet, either because .connect() has not yet been called or because it is still in the process of connecting (see socket.connecting).

socket.ref()

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

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'. Value may be undefined if the socket is destroyed (for example, if the client disconnected).

socket.remotePort

The numeric representation of the remote port. For example, 80 or 21. Value may be undefined if the socket is destroyed (for example, if the client disconnected).

socket.server

Reference to the server that accepted the socket. This is null for sockets that were not accepted by a server.

socket.resetAndDestroy()

Close the TCP connection by sending an RST packet and destroy the stream. If this TCP socket is in connecting status, it will send an RST packet and destroy this TCP socket once it is connected. Otherwise, it will call socket.destroy with an ERR_SOCKET_CLOSED Error. If this is not a TCP socket (for example, a pipe), calling this method will immediately throw an ERR_INVALID_HANDLE_TYPE Error.

socket.resume()

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

socket.setEncoding([encoding])

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

socket.setKeepAlive()

Enable/disable keep-alive functionality, and optionally configure the keepalive probe timing. Returns the socket itself.

Possible signatures:

Enabling keep-alive sets 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.

Set interval (in milliseconds) to set the delay between successive keepalive probes once they begin (TCP_KEEPINTVL). Set count to the number of unacknowledged probes sent before the connection is dropped (TCP_KEEPCNT). Both are only applied when keep-alive is enabled. Omitting interval or count uses the defaults of 1000 ms and 10. As with initialDelay, a non-positive interval or count leaves the corresponding system default unchanged.

initialDelay and interval are specified in milliseconds but the underlying socket options are configured in whole seconds; the values are divided by 1000 and rounded down before being applied. For example, setting initialDelay to 400 will result in a TCP_KEEPIDLE of 0 seconds (since 400 / 1000 rounds down to 0).

Enabling the keep-alive functionality will set the following socket options:

On Windows versions older than build 1709, keep-alive is configured through SIO_KEEPALIVE_VALS, which has no probe-count field, so count is ignored on those platforms.

socket.setKeepAlive([options])

Configure keep-alive using an options object. See socket.setKeepAlive() for a description of each property.

socket.setKeepAlive({ enable: true, initialDelay: 1000, interval: 1000, count: 10 });

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

Configure keep-alive using positional arguments. See socket.setKeepAlive() for a description of each argument.

socket.setNoDelay([noDelay])

Enable/disable the use of Nagle’s algorithm.

When a TCP connection is created, it will have Nagle’s algorithm enabled.

Nagle’s algorithm delays data before it is sent via the network. It attempts to optimize throughput at the expense of latency.

Passing true for noDelay or not passing an argument will disable Nagle’s algorithm for the socket. Passing false for noDelay will enable Nagle’s algorithm.

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.

socket.getTypeOfService()

Returns the current Type of Service (TOS) field for IPv4 packets or Traffic Class for IPv6 packets for this socket.

setTypeOfService() may be called before the socket is connected; the value will be cached and applied when the socket establishes a connection. getTypeOfService() will return the currently set value even before connection.

On some platforms (e.g., Linux), certain TOS/ECN bits may be masked or ignored, and behavior can differ between IPv4 and IPv6 or dual-stack sockets. Callers should verify platform-specific semantics.

socket.setTypeOfService(tos)

Sets the Type of Service (TOS) field for IPv4 packets or Traffic Class for IPv6 Packets sent from this socket. This can be used to prioritize network traffic.

setTypeOfService() may be called before the socket is connected; the value will be cached and applied when the socket establishes a connection. getTypeOfService() will return the currently set value even before connection.

On some platforms (e.g., Linux), certain TOS/ECN bits may be masked or ignored, and behavior can differ between IPv4 and IPv6 or dual-stack sockets. Callers should verify platform-specific semantics.

socket.timeout

The socket timeout in milliseconds as set by socket.setTimeout(). It is undefined if a timeout has not been set.

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 unrefed calling unref() again will have no effect.

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, which may not be immediately.

See Writable stream write() method for more information.

socket.readyState

This property represents the state of the connection as a string.

Class: net.BoundSocket

Allows for the synchronous creation of a pre-bound socket, that can be passed to listen() or new net.Socket() later on. For listen() this enables synchronous port reservation, while for new net.Socket(), it allows control over the local egress port/IP, via bind(2) semantics.

A BoundSocket binds either a TCP endpoint (host or port) or a Unix domain/named-pipe endpoint (path); the two are mutually exclusive. For a path, the file system entry is reserved in the constructor, so conflicts such as EADDRINUSE throw synchronously exactly as a TCP bind does. On Linux a leading '\0' in path selects the abstract namespace (no file system entry); an abstract path on any other platform throws ERR_INVALID_ARG_VALUE.

Adoption transfers ownership of the socket; afterwards address() and close() throw ERR_SOCKET_HANDLE_ADOPTED. A handle that is never adopted must be closed to avoid leaking the socket. Closing a pipe BoundSocket removes its file system entry; abstract and TCP binds have none to remove.

When a pipe BoundSocket bound to a source path is adopted as a client, that path is reported as the socket’s localAddress once it connects.

An un-adopted TCP BoundSocket can be moved to another thread by listing it in the transferList of a worker_threads postMessage() call, see Transferring TCP handles to other threads. It can likewise be sent to a child process as the sendHandle argument of subprocess.send(). In both cases the source is left in the adopted state. Pipe binds cannot be moved either way.

When an adopted BoundSocket connects to a numeric IP literal, connect(2) is issued synchronously, so socket.localAddress is resolved once socket.connect() returns. Connection failures are still reported via a deferred 'error' event.

import net from 'node:net';

const bound = new net.BoundSocket();
const { port } = bound.address();
console.log(`Reserved port ${port} for server`);

const server = net.createServer();
server.listen(bound); // Adopt as a server, or pass to new net.Socket() instead.

new net.BoundSocket([options])

boundSocket.address()

Returns the bound local address. When bound with port: 0, port is the OS-assigned ephemeral port.

boundSocket.isPipe

true when the socket was bound with a path (a Unix domain socket or Windows named pipe), false for a TCP bind. The getter’s presence on net.BoundSocket.prototype also serves as a capability probe for path support.

boundSocket.fd()

Returns the file descriptor of the bound socket. Ownership remains with the BoundSocket, so the descriptor must not be closed by the caller. The descriptor is only available before the handle is adopted; afterwards it belongs to the adopting net.Server or net.Socket and fd() throws ERR_SOCKET_HANDLE_ADOPTED.

boundSocket.close()

Releases the bound socket. Only needed when the handle is never adopted.

boundSocket[Symbol.dispose]()

Closes the handle if it has not been adopted or closed; otherwise a no-op.

net.connect()

Aliases to net.createConnection().

Possible signatures:

net.connect(options[, connectListener])

Alias to net.createConnection(options[, connectListener]).

net.connect(path[, connectListener])

Alias to net.createConnection(path[, connectListener]).

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

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

net.createConnection()

A factory function, which creates a new net.Socket, immediately initiates connection with socket.connect(), then returns the net.Socket that starts the connection.

When the connection is established, a 'connect' event will be emitted on the returned socket. The last parameter connectListener, if supplied, will be added as a listener for the 'connect' event once.

Possible signatures:

The net.connect() function is an alias to this function.

net.createConnection(options[, connectListener])

For available options, see new net.Socket([options]) and socket.connect(options[, connectListener]).

Additional options:

Following is an example of a client of the echo server described in the net.createServer() section:

import net from 'node: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');
});
const net = require('node: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:

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

Following is an example of a client using the port and onread option. In this case, the onread option will be only used to call new net.Socket([options]) and the port option will be used to call socket.connect(options[, connectListener]).

import net from 'node:net';
import { Buffer } from 'node:buffer';
net.createConnection({
  port: 8124,
  onread: {
    // Reuses a 4KiB Buffer for every read from the socket.
    buffer: Buffer.alloc(4 * 1024),
    callback: function(nread, buf) {
      // Received data is available in `buf` from 0 to `nread`.
      console.log(buf.toString('utf8', 0, nread));
    },
  },
});
const net = require('node:net');
net.createConnection({
  port: 8124,
  onread: {
    // Reuses a 4KiB Buffer for every read from the socket.
    buffer: Buffer.alloc(4 * 1024),
    callback: function(nread, buf) {
      // Received data is available in `buf` from 0 to `nread`.
      console.log(buf.toString('utf8', 0, nread));
    },
  },
});

net.createConnection(path[, connectListener])

Initiates an IPC connection.

This function creates a new net.Socket with all options set to default, immediately initiates connection with socket.connect(path[, connectListener]), then returns the net.Socket that starts the connection.

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

Initiates a TCP connection.

This function creates a new net.Socket with all options set to default, immediately initiates connection with socket.connect(port[, host][, connectListener]), then returns the net.Socket that starts the connection.

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

Creates a new TCP or IPC server.

If allowHalfOpen is set to true, when the other end of the socket signals the end of transmission, the server will only send back the end of transmission when socket.end() is explicitly called. For example, in the context of TCP, when a FIN packet is received, a FIN packet is sent back only when socket.end() is explicitly called. Until then the connection is half-closed (non-readable but still writable). See 'end' event and RFC 1122 (section 4.2.2.13) 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 an IPC server, depending on what it listen() to.

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

import net from 'node: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');
});
const net = require('node: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:

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.getDefaultAutoSelectFamily()

Gets the current default value of the autoSelectFamily option of socket.connect(options). The initial default value is true, unless the command line option --no-network-family-autoselection is provided.

net.setDefaultAutoSelectFamily(value)

Sets the default value of the autoSelectFamily option of socket.connect(options).

net.getDefaultAutoSelectFamilyAttemptTimeout()

Gets the current default value of the autoSelectFamilyAttemptTimeout option of socket.connect(options). The initial default value is 500 or the value specified via the command line option --network-family-autoselection-attempt-timeout.

net.setDefaultAutoSelectFamilyAttemptTimeout(value)

Sets the default value of the autoSelectFamilyAttemptTimeout option of socket.connect(options).

net.isIP(input)

Returns 6 if input is an IPv6 address, including an IPv4-mapped IPv6 address. Returns 4 if input is an IPv4 address in dot-decimal notation with no leading zeroes. Otherwise, returns 0.

net.isIP('::1'); // returns 6
net.isIP('::ffff:127.0.0.1'); // returns 6
net.isIP('127.0.0.1'); // returns 4
net.isIP('127.000.000.001'); // returns 0
net.isIP('127.0.0.1/24'); // returns 0
net.isIP('fhqwhgads'); // returns 0

net.isIPv4(input)

Returns true if input is an IPv4 address in dot-decimal notation with no leading zeroes. Otherwise, returns false.

net.isIPv4('127.0.0.1'); // returns true
net.isIPv4('127.000.000.001'); // returns false
net.isIPv4('127.0.0.1/24'); // returns false
net.isIPv4('fhqwhgads'); // returns false

net.isIPv6(input)

Returns true if input is an IPv6 address, including an IPv4-mapped IPv6 address. Otherwise, returns false.

net.isIPv6('::1'); // returns true
net.isIPv6('::ffff:127.0.0.1'); // returns true
net.isIPv6('fhqwhgads'); // returns false

net/promises API

Stability: 1 - Experimental

The net/promises API provides a set of net functions that return Promise objects rather than relying on events. The API is accessible via require('node:net').promises or require('node:net/promises').

netPromises.connect(options)

netPromises.connect(path)

netPromises.connect(port[, host])

A promise-based alternative to net.connect(). The returned promise is fulfilled with the socket once its 'connect' event fires, and is rejected if the connection fails or the signal is aborted. When the promise rejects, the underlying socket is destroyed.

This API is named for the action it performs and awaits — connecting — to parallel netPromises.listen(). It is not named createConnection(), because that name belongs to the socket-factory taxonomy of the callback API, which has no counterpart here.

import { connect } from 'node:net/promises';

const socket = await connect({ port: 8124 });
socket.write('hello world!');
socket.end();

netPromises.listen([options])

Creates a net.Server and begins listening. The returned promise is fulfilled with the server once its 'listening' event fires, and is rejected if the server fails to bind or the signal is aborted before it is listening. When the promise rejects, the server is closed.

The resolved server is async iterable, so incoming connections can be consumed with for await...of (see server[Symbol.asyncIterator]()).

import { listen } from 'node:net/promises';

const server = await listen({ port: 8124 });
console.log('listening on', server.address().port);