# 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`.

```js
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.

```js
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][] 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.

```js
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.

On error, `err` is an [`Error`][] object, where `err.code` is
one of the error codes listed [here](#dns_error_codes).

## 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 array 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 [DNS error codes][].

## 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`_.

[DNS error codes]: #dns_error_codes
[`dns.lookup()`]: #dns_dns_lookup_hostname_options_callback
[`dns.resolve()`]: #dns_dns_resolve_hostname_rrtype_callback
[`dns.resolve4()`]: #dns_dns_resolve4_hostname_callback
[`Error`]: errors.html#errors_class_error
[Implementation considerations section]: #dns_implementation_considerations
[supported `getaddrinfo` flags]: #dns_supported_getaddrinfo_flags
[the official libuv documentation]: http://docs.libuv.org/en/latest/threadpool.html
