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DNS

Stability: 2 - Stable

Use require('dns') to access this module.

This module contains functions that belong to two different categories:

  1. Functions that use the underlying operating system facilities to perform name resolution, and that do not necessarily do 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.

Here is an example that does a lookup of www.google.com.

var dns = require('dns');

dns.lookup('www.google.com', function onLookup(err, addresses, family) {
  console.log('addresses:', addresses);
});
  1. 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 but dns.lookup. These functions 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. 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.

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));
    });
  });
});

There are subtle consequences in choosing one or another, please consult the Implementation considerations section for more information.

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 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 ORing their values. See supported getaddrinfo flags below 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 has arguments (err, address, family). address is a string representation of a IP v4 or v6 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, 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 doesn't necessarily have anything to do with the DNS protocol. It's only an operating system facility that can associate name with addresses, and vice versa.

Its implementation can have subtle but important consequences on the behavior of any io.js program. Please take some time to consult the Implementation considerations section before using it.

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.

Implementation considerations

Although dns.lookup and 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 io.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 tweaked by changing settings in nsswitch.conf(5) and/or resolv.conf(5), but be careful that changing these files will change the behavior of all other programs running on the same operating system.

Though the call 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, functions starting with 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.