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@nodejs

# 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 constant 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 look ups 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, domains) {
          if (err) {
            console.log('reverse for ' + a + ' failed: ' +
              err.message);
          } else {
            console.log('reverse for ' + a + ': ' +
              JSON.stringify(domains));
          }
        });
      });
    });

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

Resolves a domain (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`).

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

Resolves a domain (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) and `'CNAME'` (canonical name records).

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` would be an instanceof `Error` object, where `err.errno` is one of the error codes listed below and `err.message` is a string describing the error in English.

## dns.resolve4(domain, 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(domain, callback)#

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

## dns.resolveMx(domain, 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(domain, callback)#

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

## dns.resolveSrv(domain, callback)#

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

## dns.reverse(ip, callback)#

Reverse resolves an ip address to an array of domain names.

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

## dns.resolveNs(domain, 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 `domain` (e.g., `['ns1.example.com', 'ns2.example.com']`).

## dns.resolveCname(domain, 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 `domain` (e.g., `['bar.example.com']`).

If there an an error, `err` will be non-null and an instanceof the Error object.

Each DNS query can return an error code.

- `dns.TEMPFAIL`: timeout, SERVFAIL or similar.
- `dns.PROTOCOL`: got garbled reply.
- `dns.NXDOMAIN`: domain does not exists.
- `dns.NODATA`: domain exists but no data of reqd type.
- `dns.NOMEM`: out of memory while processing.
- `dns.BADQUERY`: the query is malformed.