TCP or [IPC][] servers ([`net.createServer()`][]) and clients

([`net.createConnection()`][]).

It can be accessed using:

```js

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

on file creation. If the UNIX domain socket (that is visible as a file system

path) is created and used in conjunction with one of Node.js' API abstractions

such as [`net.createServer()`][], it will be unlinked as part of

[`server.close()`][]. On the other hand, if it is created and used outside of

these abstractions, the user will need to manually remove it. The same applies

when the path was created by a Node.js API but the program crashes abruptly.

In short, a UNIX domain socket once successfully created 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:

```js

net.createServer().listen(

```

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* `options` {Object} See

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

* `connectionListener` {Function} Automatically set as a listener for the

[`'connection'`][] event.

* Returns: {net.Server}

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Emitted when the server closes. Note that if connections exist, this

event is not emitted until all connections are ended.

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

`net.Socket`.

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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

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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.

}).on('error', (err) => {

// handle errors here

throw err;

});

});

```

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* Returns: {net.Server}

connections. This function is asynchronous, the server is finally closed

when all connections are ended and the server emits a [`'close'`][] event.

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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

Possible signatures:

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

* [`server.listen(options[, callback])`][`server.listen(options)`]

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

* <a href="#net_server_listen_port_host_backlog_callback">

<code>server.listen([port[, host[, backlog]]][, callback])</code></a>

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

after a certain amount of time:

```js

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])

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* `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.

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* `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`

<a href="#net_server_listen_port_host_backlog_callback">

<code>server.listen([port[, host[, backlog]]][, callback])</code></a>.

Otherwise, if `path` is specified, it behaves the same as

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

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.

```js

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.

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* `backlog` {number} Common parameter of [`server.listen()`][] functions.

* `callback` {Function} Common parameter of [`server.listen()`][] functions.

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* `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.

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`).

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

high.

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* Returns: {net.Server}

Opposite of `unref()`, calling `ref()` on a previously `unref`ed server will

*not* let the program exit if it's the only server left (the default behavior).

If the server is `ref`ed calling `ref()` again will have no effect.

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* 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 `unref`ed calling

`unref()` again will have no effect.

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This class is an abstraction of a TCP socket or a streaming [IPC][] endpoint

(uses named pipes on Windows, and UNIX domain sockets otherwise). A

`net.Socket` is also a [duplex stream][], so it can be both readable and

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.

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.

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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