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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 `iana.org`.

    const dns = require('dns');

    dns.lookup('iana.org', (err, address, family) => {
      console.log('address: %j family: IPv%s', address, family);
    });
    // address: "192.0.43.8" family: IPv4

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 `'archive.org'` then reverse resolves the IP addresses that are returned.

    const dns = require('dns');

    dns.resolve4('archive.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.

## Class dns.Resolver#

Added in: v8.3.0

An independent resolver for DNS requests.

Note that creating a new resolver uses the default server settings. Setting the servers used for a resolver using `resolver.setServers()` does not affect other resolver:

    const { Resolver } = require('dns');
    const resolver = new Resolver();
    resolver.setServers(['4.4.4.4']);

    // This request will use the server at 4.4.4.4, independent of global settings.
    resolver.resolve4('example.org', (err, addresses) => {
      // ...
    });

The following methods from the `dns` module are available:

- `resolver.getServers()`
- `resolver.setServers()`
- `resolver.resolve()`
- `resolver.resolve4()`
- `resolver.resolve6()`
- `resolver.resolveAny()`
- `resolver.resolveCname()`
- `resolver.resolveMx()`
- `resolver.resolveNaptr()`
- `resolver.resolveNs()`
- `resolver.resolvePtr()`
- `resolver.resolveSoa()`
- `resolver.resolveSrv()`
- `resolver.resolveTxt()`
- `resolver.reverse()`

### resolver.cancel()#

Added in: v8.3.0

Cancel all outstanding DNS queries made by this resolver. The corresponding callbacks will be called with an error with code `ECANCELLED`.

## dns.getServers()#

Added in: v0.11.3

Returns an array of IP address strings, formatted according to rfc5952, that are currently configured for DNS resolution. A string will include a port section if a custom port is used.

For example:

    [
      '4.4.4.4',
      '2001:4860:4860::8888',
      '4.4.4.4:1053',
      '[2001:4860:4860::8888]:1053'
    ]

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

History

Version | Changes
--- | ---
v1.2.0 | The `all` option is supported now.
v0.1.90 | Added in: v0.1.90

- `hostname` <string>
- `options` <integer> | <Object>
  - `family` <integer> The record family. Must be `4` or `6`. IPv4 and IPv6 addresses are both returned by default.
  - `hints` <number> One or more supported `getaddrinfo` flags. Multiple flags may be passed by bitwise `OR`ing their values.
  - `all` <boolean> When `true`, the callback returns all resolved addresses in an array. Otherwise, returns a single address. Default: `false`
  - `verbatim` <boolean> When `true`, the callback receives IPv4 and IPv6 addresses in the order the DNS resolver returned them. When `false`, IPv4 addresses are placed before IPv6 addresses. Default: currently `false` (addresses are reordered) but this is expected to change in the not too distant future. New code should use `{ verbatim: true }`.
- `callback` <Function>
  - `err` <Error>
  - `address` <string> A string representation of an IPv4 or IPv6 address.
  - `family` <integer> `4` or `6`, denoting the family of `address`.

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

With the `all` option set to `true`, the arguments for `callback` 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()`.

Example usage:

    const dns = require('dns');
    const options = {
      family: 6,
      hints: dns.ADDRCONFIG | dns.V4MAPPED,
    };
    dns.lookup('example.com', options, (err, address, family) =>
      console.log('address: %j family: IPv%s', address, family));
    // address: "2606:2800:220:1:248:1893:25c8:1946" family: IPv6

    // When options.all is true, the result will be an Array.
    options.all = true;
    dns.lookup('example.com', options, (err, addresses) =>
      console.log('addresses: %j', addresses));
    // addresses: [{"address":"2606:2800:220:1:248:1893:25c8:1946","family":6}]

If this method is invoked as its `util.promisify()`ed version, and `all` is not set to `true`, it returns a Promise for an object with `address` and `family` properties.

### 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)#

Added in: v0.11.14

- `address` <string>
- `port` <number>
- `callback` <Function>
  - `err` <Error>
  - `hostname` <string> e.g. `example.com`
  - `service` <string> e.g. `http`

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

If `address` is not a valid IP address, a `TypeError` will be thrown. The `port` will be coerced to a number. If it is not a legal port, a `TypeError` will be thrown.

On an 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
    });

If this method is invoked as its `util.promisify()`ed version, it returns a Promise for an object with `hostname` and `service` properties.

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

Added in: v0.1.27

- `hostname` <string> Hostname to resolve.
- `rrtype` <string> Resource record type. Default: `'A'`
- `callback` <Function>
  - `err` <Error>
  - `records` <string[]> | <Object[]> | <Object>

Uses the DNS protocol to resolve a hostname (e.g. `'nodejs.org'`) into an array of the resource records. The `callback` function has arguments `(err, records)`. When successful, `records` will be an array of resource records. The type and structure of individual results varies based on `rrtype`:

`rrtype` | `records` contains | Result type | Shorthand method
--- | --- | --- | ---
`'A'` | IPv4 addresses (default) | <string> | `dns.resolve4()`
`'AAAA'` | IPv6 addresses | <string> | `dns.resolve6()`
`'CNAME'` | canonical name records | <string> | `dns.resolveCname()`
`'MX'` | mail exchange records | <Object> | `dns.resolveMx()`
`'NAPTR'` | name authority pointer records | <Object> | `dns.resolveNaptr()`
`'NS'` | name server records | <string> | `dns.resolveNs()`
`'PTR'` | pointer records | <string> | `dns.resolvePtr()`
`'SOA'` | start of authority records | <Object> | `dns.resolveSoa()`
`'SRV'` | service records | <Object> | `dns.resolveSrv()`
`'TXT'` | text records | <string[]> | `dns.resolveTxt()`
`'ANY'` | any records | <Object> | `dns.resolveAny()`

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

## dns.resolve4(hostname[, options], callback)#

History

Version | Changes
--- | ---
v7.2.0 | This method now supports passing `options`, specifically `options.ttl`.
v0.1.16 | Added in: v0.1.16

- `hostname` <string> Hostname to resolve.
- `options` <Object>
  - `ttl` <boolean> Retrieve the Time-To-Live value (TTL) of each record. When `true`, the callback receives an array of `{ address: '1.2.3.4', ttl: 60 }` objects rather than an array of strings, with the TTL expressed in seconds.
- `callback` <Function>
  - `err` <Error>
  - `addresses` <string[]> | <Object[]>

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[, options], callback)#

History

Version | Changes
--- | ---
v7.2.0 | This method now supports passing `options`, specifically `options.ttl`.
v0.1.16 | Added in: v0.1.16

- `hostname` <string> Hostname to resolve.
- `options` <Object>
  - `ttl` <boolean> Retrieve the Time-To-Live value (TTL) of each record. When `true`, the callback receives an array of `{ address: '0:1:2:3:4:5:6:7', ttl: 60 }` objects rather than an array of strings, with the TTL expressed in seconds.
- `callback` <Function>
  - `err` <Error>
  - `addresses` <string[]> | <Object[]>

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)#

Added in: v0.3.2

- `hostname` <string>
- `callback` <Function>
  - `err` <Error>
  - `addresses` <string[]>

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

## dns.resolveMx(hostname, callback)#

Added in: v0.1.27

- `hostname` <string>
- `callback` <Function>
  - `err` <Error>
  - `addresses` <Object[]>

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.resolveNaptr(hostname, callback)#

Added in: v0.9.12

- `hostname` <string>
- `callback` <Function>
  - `err` <Error>
  - `addresses` <Object[]>

Uses the DNS protocol to resolve regular expression based records (`NAPTR` records) for the `hostname`. The `addresses` argument passed to the `callback` function will contain an array of objects with the following properties:

- `flags`
- `service`
- `regexp`
- `replacement`
- `order`
- `preference`

For example:

    {
      flags: 's',
      service: 'SIP+D2U',
      regexp: '',
      replacement: '_sip._udp.example.com',
      order: 30,
      preference: 100
    }

## dns.resolveNs(hostname, callback)#

Added in: v0.1.90

- `hostname` <string>
- `callback` <Function>
  - `err` <Error>
  - `addresses` <string[]>

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.resolvePtr(hostname, callback)#

Added in: v6.0.0

- `hostname` <string>
- `callback` <Function>
  - `err` <Error>
  - `addresses` <string[]>

Uses the DNS protocol to resolve pointer records (`PTR` records) for the `hostname`. The `addresses` argument passed to the `callback` function will be an array of strings containing the reply records.

## dns.resolveSoa(hostname, callback)#

Added in: v0.11.10

- `hostname` <string>
- `callback` <Function>
  - `err` <Error>
  - `address` <Object>

Uses the DNS protocol to resolve a start of authority record (`SOA` record) for the `hostname`. The `address` 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)#

Added in: v0.1.27

- `hostname` <string>
- `callback` <Function>
  - `err` <Error>
  - `addresses` <Object[]>

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)#

Added in: v0.1.27

- `hostname` <string>
- `callback` <Function>
  - `err` <Error>
  - `records` <string[][]>

Uses the DNS protocol to resolve text queries (`TXT` records) for the `hostname`. The `records` argument passed to the `callback` function is a two-dimensional 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.resolveAny(hostname, callback)#

- `hostname` <string>
- `callback` <Function>
  - `err` <Error>
  - `ret` <Object[]>

Uses the DNS protocol to resolve all records (also known as `ANY` or `*` query). The `ret` argument passed to the `callback` function will be an array containing various types of records. Each object has a property `type` that indicates the type of the current record. And depending on the `type`, additional properties will be present on the object:

Type | Properties
--- | ---
`"A"` | `address` / `ttl`
`"AAAA"` | `address` / `ttl`
`"CNAME"` | `value`
`"MX"` | Refer to `dns.resolveMx()`
`"NAPTR"` | Refer to `dns.resolveNaptr()`
`"NS"` | `value`
`"PTR"` | `value`
`"SOA"` | Refer to `dns.resolveSoa()`
`"SRV"` | Refer to `dns.resolveSrv()`
`"TXT"` | This type of record contains an array property called `entries` which refers to `dns.resolveTxt()`, eg. `{ entries: ['...'], type: 'TXT' }`

Here is an example of the `ret` object passed to the callback:

    [ { type: 'A', address: '127.0.0.1', ttl: 299 },
      { type: 'CNAME', value: 'example.com' },
      { type: 'MX', exchange: 'alt4.aspmx.l.example.com', priority: 50 },
      { type: 'NS', value: 'ns1.example.com' },
      { type: 'TXT', entries: [ 'v=spf1 include:_spf.example.com ~all' ] },
      { type: 'SOA',
        nsname: 'ns1.example.com',
        hostmaster: 'admin.example.com',
        serial: 156696742,
        refresh: 900,
        retry: 900,
        expire: 1800,
        minttl: 60 } ]

## dns.reverse(ip, callback)#

Added in: v0.1.16

- `ip` <string>
- `callback` <Function>
  - `err` <Error>
  - `hostnames` <string[]>

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

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

## dns.setServers(servers)#

Added in: v0.11.3

- `servers` <string[]> array of rfc5952 formatted addresses

Sets the IP address and port of servers to be used when performing DNS resolution. The `servers` argument is an array of rfc5952 formatted addresses. If the port is the IANA default DNS port (53) it can be omitted.

For example:

    dns.setServers([
      '4.4.4.4',
      '[2001:4860:4860::8888]',
      '4.4.4.4:1053',
      '[2001:4860:4860::8888]:1053'
    ]);

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. This can have surprising negative performance implications for some applications, see the `UV_THREADPOOL_SIZE` documentation for more information.

Note that various networking APIs will call `dns.lookup()` internally to resolve host names. If that is an issue, consider resolving the hostname to and address using `dns.resolve()` and using the address instead of a host name. Also, some networking APIs (such as `socket.connect()` and `dgram.createSocket()`) allow the default resolver, `dns.lookup()`, to be replaced.

### `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`.