# DNS

    Stability: 3 - Stable

Use `require('dns')` to access this module. All methods in the dns module
use C-Ares except for `dns.lookup` which uses `getaddrinfo(3)` in a thread
pool. C-Ares is much faster than `getaddrinfo` but the system resolver is
more consistent with how other programs operate. When a user does
`net.connect(80, 'google.com')` or `http.get({ host: 'google.com' })` the
`dns.lookup` method is used. Users who need to do a large number of lookups
quickly should use the methods that go through C-Ares.

Here is an example which resolves `'www.google.com'` then reverse
resolves the IP addresses which are returned.

    var dns = require('dns');

    dns.resolve4('www.google.com', function (err, addresses) {
      if (err) throw err;

      console.log('addresses: ' + JSON.stringify(addresses));

      addresses.forEach(function (a) {
        dns.reverse(a, function (err, hostnames) {
          if (err) {
            throw err;
          }

          console.log('reverse for ' + a + ': ' + JSON.stringify(hostnames));
        });
      });
    });

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

Resolves a hostname (e.g. `'google.com'`) into the first found A (IPv4) or
AAAA (IPv6) record.
The `family` can be the integer `4` or `6`. Defaults to `null` that indicates
both Ip v4 and v6 address family.

The callback has arguments `(err, address, family)`.  The `address` argument
is a string representation of a IP v4 or v6 address. The `family` argument
is either the integer 4 or 6 and denotes the family of `address` (not
necessarily the value initially passed to `lookup`).

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.resolve(hostname, [rrtype], callback)

Resolves a hostname (e.g. `'google.com'`) into an array of the record types
specified by rrtype.

Valid rrtypes 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 has arguments `(err, addresses)`.  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)

The same as `dns.resolve()`, but only for IPv4 queries (`A` records).
`addresses` is an array of IPv4 addresses (e.g.
`['74.125.79.104', '74.125.79.105', '74.125.79.106']`).

## dns.resolve6(hostname, callback)

The same as `dns.resolve4()` except for IPv6 queries (an `AAAA` query).


## dns.resolveMx(hostname, callback)

The same as `dns.resolve()`, but only for mail exchange queries (`MX` records).

`addresses` is an array of MX records, each with a priority and an exchange
attribute (e.g. `[{'priority': 10, 'exchange': 'mx.example.com'},...]`).

## dns.resolveTxt(hostname, callback)

The same as `dns.resolve()`, but only for text queries (`TXT` records).
`addresses` is an array of the text records available for `hostname` (e.g.,
`['v=spf1 ip4:0.0.0.0 ~all']`).

## dns.resolveSrv(hostname, callback)

The same as `dns.resolve()`, but only for service records (`SRV` records).
`addresses` is an array of the SRV records available for `hostname`. Properties
of SRV records are priority, weight, port, and name (e.g.,
`[{'priority': 10, {'weight': 5, 'port': 21223, 'name': 'service.example.com'}, ...]`).

## dns.resolveSoa(hostname, callback)

The same as `dns.resolve()`, but only for start of authority record queries 
(`SOA` record).

`addresses` is an object with the following structure:

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

## dns.resolveNs(hostname, callback)

The same as `dns.resolve()`, but only for name server records (`NS` records).
`addresses` is an array of the name server records available for `hostname`
(e.g., `['ns1.example.com', 'ns2.example.com']`).

## dns.resolveCname(hostname, callback)

The same as `dns.resolve()`, but only for canonical name records (`CNAME`
records). `addresses` is an array of the canonical name records available for
`hostname` (e.g., `['bar.example.com']`).

## dns.reverse(ip, callback)

Reverse resolves an ip address to an array of hostnames.

The callback has arguments `(err, hostnames)`.

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

## dns.getServers()

Returns an array of IP addresses as strings that are currently being used for
resolution

## dns.setServers(servers)

Given an array of IP addresses as strings, set them as the servers to use for
resolving

If you specify a port with the address it will be stripped, as the underlying
library doesn't support that.

This will throw if you pass invalid input.

## 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.
