Table of Contents

• DNS
  • dns.getServers()
  • dns.lookup(hostname[, options], callback)
    • Supported getaddrinfo flags
  • dns.lookupService(address, port, callback)
  • dns.resolve(hostname[, rrtype], callback)
  • dns.resolve4(hostname[, options], callback)
  • dns.resolve6(hostname[, options], callback)
  • dns.resolveCname(hostname, callback)
  • dns.resolveMx(hostname, callback)
  • dns.resolveNaptr(hostname, callback)
  • dns.resolveNs(hostname, callback)
  • dns.resolveSoa(hostname, callback)
  • dns.resolveSrv(hostname, callback)
  • dns.resolvePtr(hostname, callback)
  • dns.resolveTxt(hostname, callback)
  • dns.reverse(ip, callback)
  • dns.setServers(servers)
  • Error codes
  • Implementation considerations
    • dns.lookup()
    • dns.resolve(), dns.resolve*() and dns.reverse()

DNS#

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 iana.org.

const dns = require('dns');

dns.lookup('nodejs.org', (err, addresses, family) => {
  console.log('addresses:', addresses);
});
// address: "192.0.43.8" family: IPv4

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 'archive.org' then reverse resolves the IP addresses that are returned.

const dns = require('dns');

dns.resolve4('archive.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 bitwise ORing 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.

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().

Example usage:

const dns = require('dns');
const options = {
  family: 6,
  hints: dns.ADDRCONFIG | dns.V4MAPPED,
};
dns.lookup('example.com', options, (err, address, family) =>
  console.log('address: %j family: IPv%s', address, family));
// address: "2606:2800:220:1:248:1893:25c8:1946" family: IPv6

// When options.all is true, the result will be an Array.
options.all = true;
dns.lookup('example.com', options, (err, addresses) =>
  console.log('addresses: %j', addresses));
// addresses: [{"address":"2606:2800:220:1:248:1893:25c8:1946","family":6}]

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.

If address is not a valid IP address, a TypeError will be thrown. The port will be coerced to a number. If it is not a legal port, a TypeError will be thrown.

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.

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)#

• hostname <string> Hostname to resolve.
• rrtype <string> Resource record type. Default: 'A'.
• callback <Function>
  • err <Error>
  • records <string[]> | <Object[]> | <Object>

Uses the DNS protocol to resolve a hostname (e.g. 'nodejs.org') into an array of the resource records. The callback function has arguments (err, records). When successful, records will be an array of resource records. The type and structure of individual results varies based on rrtype:

rrtype	records contains	Result type	Shorthand method
'A'	IPv4 addresses (default)	<string>