nodejs/doc/api/dns.markdown

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[, options], callback)

Resolves a hostname (e.g. 'google.com') into the first found A (IPv4) or AAAA (IPv6) record. options can be an object or integer. If options is not provided, then IP v4 and v6 addresses are both valid. If options is an integer, then it must be 4 or 6.

Alternatively, options can be an object containing two properties, family and hints. Both properties are optional. If family is provided, it must be the integer 4 or 6. If family is not provided then IP v4 and v6 addresses are accepted. The hints field, if present, 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 ORing their values. An example usage of options is shown below.

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

See supported getaddrinfo flags below for more information on supported flags.

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.lookupService(address, port, callback)

Resolves the given address and port into a hostname and service using getnameinfo.

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.

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 2-d 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, the could be either joined together or treated separately.

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.

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.