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# DNS#

    Stability: 2 - Stable

The `dns` module contains functions belonging to two different categories:

1) Functions that use the underlying operating system facilities to perform name resolution, and that do not necessarily perform any network communication. This category contains only one function: `dns.lookup()`. Developers looking to perform name resolution in the same way that other applications on the same operating system behave should use `dns.lookup()`.

For example, looking up `nodejs.org`.

    const dns = require('dns');

    dns.lookup('nodejs.org', (err, addresses, family) => {
      console.log('addresses:', addresses);
    });

2) Functions that connect to an actual DNS server to perform name resolution, and that always use the network to perform DNS queries. This category contains all functions in the `dns` module except `dns.lookup()`. These functions do not use the same set of configuration files used by `dns.lookup()` (e.g. `/etc/hosts`). These functions should be used by developers who do not want to use the underlying operating system's facilities for name resolution, and instead want to always perform DNS queries.

Below is an example that resolves `'nodejs.org'` then reverse resolves the IP addresses that are returned.

    const dns = require('dns');

    dns.resolve4('nodejs.org', (err, addresses) => {
      if (err) throw err;

      console.log(`addresses: ${JSON.stringify(addresses)}`);

      addresses.forEach((a) => {
        dns.reverse(a, (err, hostnames) => {
          if (err) {
            throw err;
          }
          console.log(`reverse for ${a}: ${JSON.stringify(hostnames)}`);
        });
      });
    });

There are subtle consequences in choosing one over the other, please consult the Implementation considerations section for more information.

## dns.getServers()#

Returns an array of IP address strings that are being used for name resolution.

## dns.lookup(hostname[, options], callback)#

Resolves a hostname (e.g. `'nodejs.org'`) into the first found A (IPv4) or AAAA (IPv6) record. `options` can be an object or integer. If `options` is not provided, then IPv4 and IPv6 addresses are both valid. If `options` is an integer, then it must be `4` or `6`.

Alternatively, `options` can be an object containing these properties:

- `family` {Number} - The record family. If present, must be the integer `4` or `6`. If not provided, both IP v4 and v6 addresses are accepted.
- `hints`: {Number} - If present, it should be one or more of the supported `getaddrinfo` flags. If `hints` is not provided, then no flags are passed to `getaddrinfo`. Multiple flags can be passed through `hints` by logically `OR`ing their values. See supported `getaddrinfo` flags below for more information on supported flags.
- `all`: {Boolean} - When `true`, the callback returns all resolved addresses in an array, otherwise returns a single address. Defaults to `false`.

All properties are optional. An example usage of options is shown below.

    {
      family: 4,
      hints: dns.ADDRCONFIG | dns.V4MAPPED,
      all: false
    }

The `callback` function has arguments `(err, address, family)`. `address` is a string representation of an IPv4 or IPv6 address. `family` is either the integer `4` or `6` and denotes the family of `address` (not necessarily the value initially passed to `lookup`).

With the `all` option set to `true`, the arguments change to `(err, addresses)`, with `addresses` being an array of objects with the properties `address` and `family`.

On error, `err` is an `Error` object, where `err.code` is the error code. Keep in mind that `err.code` will be set to `'ENOENT'` not only when the hostname does not exist but also when the lookup fails in other ways such as no available file descriptors.

`dns.lookup()` does not necessarily have anything to do with the DNS protocol. The implementation uses an operating system facility that can associate names with addresses, and vice versa. This implementation can have subtle but important consequences on the behavior of any Node.js program. Please take some time to consult the Implementation considerations section before using `dns.lookup()`.

### Supported getaddrinfo flags#

The following flags can be passed as hints to `dns.lookup()`.

- `dns.ADDRCONFIG`: Returned address types are determined by the types of addresses supported by the current system. For example, IPv4 addresses are only returned if the current system has at least one IPv4 address configured. Loopback addresses are not considered.
- `dns.V4MAPPED`: If the IPv6 family was specified, but no IPv6 addresses were found, then return IPv4 mapped IPv6 addresses. Note that it is not supported on some operating systems (e.g FreeBSD 10.1).

## dns.lookupService(address, port, callback)#

Resolves the given `address` and `port` into a hostname and service using the operating system's underlying `getnameinfo` implementation.

The callback has arguments `(err, hostname, service)`. The `hostname` and `service` arguments are strings (e.g. `'localhost'` and `'http'` respectively).

On error, `err` is an `Error` object, where `err.code` is the error code.

    const dns = require('dns');
    dns.lookupService('127.0.0.1', 22, (err, hostname, service) => {
      console.log(hostname, service);
        // Prints: localhost ssh
    });

## dns.resolve(hostname[, rrtype], callback)#

Uses the DNS protocol to resolve a hostname (e.g. `'nodejs.org'`) into an array of the record types specified by `rrtype`.

Valid values for `rrtype` are:

- `'A'` - IPV4 addresses, default
- `'AAAA'` - IPV6 addresses
- `'MX'` - mail exchange records
- `'TXT'` - text records
- `'SRV'` - SRV records
- `'PTR'` - used for reverse IP lookups
- `'NS'` - name server records
- `'CNAME'` - canonical name records
- `'SOA'` - start of authority record

The `callback` function has arguments `(err, addresses)`. When successful, `addresses` will be an array. The type of each item in `addresses` is determined by the record type, and described in the documentation for the corresponding lookup methods below.

On error, `err` is an `Error` object, where `err.code` is one of the error codes listed below.

## dns.resolve4(hostname, callback)#

Uses the DNS protocol to resolve a IPv4 addresses (`A` records) for the `hostname`. The `addresses` argument passed to the `callback` function will contain an array of IPv4 addresses (e.g. `['74.125.79.104', '74.125.79.105', '74.125.79.106']`).

## dns.resolve6(hostname, callback)#

Uses the DNS protocol to resolve a IPv6 addresses (`AAAA` records) for the `hostname`. The `addresses` argument passed to the `callback` function will contain an array of IPv6 addresses.

## dns.resolveCname(hostname, callback)#

Uses the DNS protocol to resolve `CNAME` records for the `hostname`. The `addresses` argument passed to the `callback` function will contain an of canonical name records available for the `hostname` (e.g. `['bar.example.com']`).

## dns.resolveMx(hostname, callback)#

Uses the DNS protocol to resolve mail exchange records (`MX` records) for the `hostname`. The `addresses` argument passed to the `callback` function will contain an array of objects containing both a `priority` and `exchange` property (e.g. `[{priority: 10, exchange: 'mx.example.com'}, ...]`).

## dns.resolveNs(hostname, callback)#

Uses the DNS protocol to resolve name server records (`NS` records) for the `hostname`. The `addresses` argument passed to the `callback` function will contain an array of name server records available for `hostname` (e.g., `['ns1.example.com', 'ns2.example.com']`).

## dns.resolveSoa(hostname, callback)#

Uses the DNS protocol to resolve a start of authority record (`SOA` record) for the `hostname`. The `addresses` argument passed to the `callback` function will be an object with the following properties:

- `nsname`
- `hostmaster`
- `serial`
- `refresh`
- `retry`
- `expire`

- `minttl`

  {

      nsname: 'ns.example.com',
      hostmaster: 'root.example.com',
      serial: 2013101809,
      refresh: 10000,
      retry: 2400,
      expire: 604800,
      minttl: 3600

  }

## dns.resolveSrv(hostname, callback)#

Uses the DNS protocol to resolve service records (`SRV` records) for the `hostname`. The `addresses` argument passed to the `callback` function will be an array of objects with the following properties:

- `priority`
- `weight`
- `port`

- `name`

  {

      priority: 10,
      weight: 5,
      port: 21223,
      name: 'service.example.com'

  }

## dns.resolveTxt(hostname, callback)#

Uses the DNS protocol to resolve text queries (`TXT` records) for the `hostname`. The `addresses` argument passed to the `callback` function is is a two-dimentional array of the text records available for `hostname` (e.g., `[ ['v=spf1 ip4:0.0.0.0 ', '~all' ] ]`). Each sub-array contains TXT chunks of one record. Depending on the use case, these could be either joined together or treated separately.

## dns.reverse(ip, callback)#

Performs a reverse DNS query that resolves an IPv4 or IPv6 address to an array of hostnames.

The `callback` function has arguments `(err, hostnames)`, where `hostnames` is an array of resolved hostnames for the given `ip`.

On error, `err` is an `Error` object, where `err.code` is one of the error codes listed below.

## dns.setServers(servers)#

Sets the IP addresses of the servers to be used when resolving. The `servers` argument is an array of IPv4 or IPv6 addresses.

If a port specified on the address it will be removed.

An error will be thrown if an invalid address is provided.

The `dns.setServers()` method must not be called while a DNS query is in progress.

## Error codes#

Each DNS query can return one of the following error codes:

- `dns.NODATA`: DNS server returned answer with no data.
- `dns.FORMERR`: DNS server claims query was misformatted.
- `dns.SERVFAIL`: DNS server returned general failure.
- `dns.NOTFOUND`: Domain name not found.
- `dns.NOTIMP`: DNS server does not implement requested operation.
- `dns.REFUSED`: DNS server refused query.
- `dns.BADQUERY`: Misformatted DNS query.
- `dns.BADNAME`: Misformatted hostname.
- `dns.BADFAMILY`: Unsupported address family.
- `dns.BADRESP`: Misformatted DNS reply.
- `dns.CONNREFUSED`: Could not contact DNS servers.
- `dns.TIMEOUT`: Timeout while contacting DNS servers.
- `dns.EOF`: End of file.
- `dns.FILE`: Error reading file.
- `dns.NOMEM`: Out of memory.
- `dns.DESTRUCTION`: Channel is being destroyed.
- `dns.BADSTR`: Misformatted string.
- `dns.BADFLAGS`: Illegal flags specified.
- `dns.NONAME`: Given hostname is not numeric.
- `dns.BADHINTS`: Illegal hints flags specified.
- `dns.NOTINITIALIZED`: c-ares library initialization not yet performed.
- `dns.LOADIPHLPAPI`: Error loading iphlpapi.dll.
- `dns.ADDRGETNETWORKPARAMS`: Could not find GetNetworkParams function.
- `dns.CANCELLED`: DNS query cancelled.

## Implementation considerations#

Although `dns.lookup()` and the various `dns.resolve*()/dns.reverse()` functions have the same goal of associating a network name with a network address (or vice versa), their behavior is quite different. These differences can have subtle but significant consequences on the behavior of Node.js programs.

### `dns.lookup()`#

Under the hood, `dns.lookup()` uses the same operating system facilities as most other programs. For instance, `dns.lookup()` will almost always resolve a given name the same way as the `ping` command. On most POSIX-like operating systems, the behavior of the `dns.lookup()` function can be modified by changing settings in `nsswitch.conf(5)` and/or `resolv.conf(5)`, but note that changing these files will change the behavior of all other programs running on the same operating system.

Though the call to `dns.lookup()` will be asynchronous from JavaScript's perspective, it is implemented as a synchronous call to `getaddrinfo(3)` that runs on libuv's threadpool. Because libuv's threadpool has a fixed size, it means that if for whatever reason the call to `getaddrinfo(3)` takes a long time, other operations that could run on libuv's threadpool (such as filesystem operations) will experience degraded performance. In order to mitigate this issue, one potential solution is to increase the size of libuv's threadpool by setting the `'UV_THREADPOOL_SIZE'` environment variable to a value greater than `4` (its current default value). For more information on libuv's threadpool, see the official libuv documentation.

### `dns.resolve()`, `dns.resolve*()` and `dns.reverse()`#

These functions are implemented quite differently than `dns.lookup()`. They do not use `getaddrinfo(3)` and they always perform a DNS query on the network. This network communication is always done asynchronously, and does not use libuv's threadpool.

As a result, these functions cannot have the same negative impact on other processing that happens on libuv's threadpool that `dns.lookup()` can have.

They do not use the same set of configuration files than what `dns.lookup()` uses. For instance, they do not use the configuration from `/etc/hosts`.