Table of Contents

• DNS
  • Class dns.Resolver
    • resolver.cancel()
  • 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.resolvePtr(hostname, callback)
  • dns.resolveSoa(hostname, callback)
  • dns.resolveSrv(hostname, callback)
  • dns.resolveTxt(hostname, callback)
  • dns.resolveAny(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('iana.org', (err, address, family) => {
  console.log('address: %j family: IPv%s', address, family);
});
// 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.

Class dns.Resolver#

An independent resolver for DNS requests.

Note that creating a new resolver uses the default server settings. Setting the servers used for a resolver using resolver.setServers() does not affect other resolver:

const { Resolver } = require('dns');
const resolver = new Resolver();
resolver.setServers(['4.4.4.4']);

// This request will use the server at 4.4.4.4, independent of global settings.
resolver.resolve4('example.org', (err, addresses) => {
  // ...
});

The following methods from the dns module are available:

• resolver.getServers()
• resolver.setServers()
• resolver.resolve()
• resolver.resolve4()
• resolver.resolve6()
• resolver.resolveAny()
• resolver.resolveCname()
• resolver.resolveMx()
• resolver.resolveNaptr()
• resolver.resolveNs()
• resolver.resolvePtr()
• resolver.resolveSoa()
• resolver.resolveSrv()
• resolver.resolveTxt()
• resolver.reverse()

resolver.cancel()#

Cancel all outstanding DNS queries made by this resolver. The corresponding callbacks will be called with an error with code ECANCELLED.

dns.getServers()#

Returns an array of IP address strings, formatted according to rfc5952, that are currently configured for DNS resolution. A string will include a port section if a custom port is used.

For example:

[
  '4.4.4.4',
  '2001:4860:4860::8888',
  '4.4.4.4:1053',
  '[2001:4860:4860::8888]:1053'
]

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

• hostname <string>
• options <integer> | <Object>
  • family <integer> The record family. Must be 4 or 6. IPv4 and IPv6 addresses are both returned by default.
  • hints <number> One or more supported getaddrinfo flags. Multiple flags may be passed by bitwise ORing their values.
  • all <boolean> When true, the callback returns all resolved addresses in an array. Otherwise, returns a single address. Default: false
  • verbatim <boolean> When true, the callback receives IPv4 and IPv6 addresses in the order the DNS resolver returned them. When false, IPv4 addresses are placed before IPv6 addresses. Default: currently false (addresses are reordered) but this is expected to change in the not too distant future. New code should use { verbatim: true }.
• callback <Function>
  • err <Error>
  • address <string> A string representation of an IPv4 or IPv6 address.
  • family <integer> 4 or 6, denoting the family of address.

Resolves a hostname (e.g. 'nodejs.org') into the first found A (IPv4) or AAAA (IPv6) record. All option properties are optional. If options is an integer, then it must be 4 or 6 – if options is not provided, then IPv4 and IPv6 addresses are both returned if found.

With the all option set to true, the arguments for callback 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}]

If this method is invoked as its util.promisify()ed version, and all is not set to true, it returns a Promise for an object with address and family properties.

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)#

• address <string>
• port <number>
• callback <Function>
  • err <Error>
  • hostname <string> e.g. example.com
  • service <string> e.g. http

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.

On an 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
});

If this method is invoked as its util.promisify()ed version, it returns a Promise for an object with hostname and service properties.

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>	dns.resolve4()
'AAAA'	IPv6 addresses	<string>	dns.resolve6()
'CNAME'	canonical name records	<string>	dns.resolveCname()
'MX'	mail exchange records	<Object>	dns.resolveMx()
'NAPTR'	name authority pointer records	<Object>	dns.resolveNaptr()
'NS'	name server records	<string>	dns.resolveNs()
'PTR'	pointer records	<string>	dns.resolvePtr()
'SOA'	start of authority records	<Object>	dns.resolveSoa()
'SRV'	service records	<Object>	dns.resolveSrv()
'TXT'	text records	<string[]>	dns.resolveTxt()
'ANY'	any records	<Object>	dns.resolveAny()

On error, err is an Error object, where err.code is one of the DNS error codes.

dns.resolve4(hostname[, options], callback)#

• hostname <string> Hostname to resolve.
• options <Object>
  • ttl <boolean> Retrieve the Time-To-Live value (TTL) of each record. When true, the callback receives an array of { address: '1.2.3.4', ttl: 60 } objects rather than an array of strings, with the TTL expressed in seconds.
• callback <Function>
  • err <Error>
  • addresses <string[]> | <Object[]>

Uses the DNS protocol to resolve a IPv4 addresses (A records) for the hostname. The addresses argument passed to the callback function will contain an array of IPv4 addresses (e.g. ['74.125.79.104', '74.125.79.105', '74.125.79.106']).

dns.resolve6(hostname[, options], callback)#

• hostname <string> Hostname to resolve.
• options <Object>
  • ttl <boolean> Retrieve the Time-To-Live value (TTL) of each record. When true, the callback receives an array of { address: '0:1:2:3:4:5:6:7', ttl: 60 } objects rather than an array of strings, with the TTL expressed in seconds.
• callback <Function>
  • err <Error>
  • addresses <string[]> | <Object[]>

Uses the DNS protocol to resolve a IPv6 addresses (AAAA records) for the hostname. The addresses argument passed to the callback function will contain an array of IPv6 addresses.

dns.resolveCname(hostname, callback)#

• hostname <string>
• callback <Function>
  • err <Error>
  • addresses <string[]>

Uses the DNS protocol to resolve CNAME records for the hostname. The addresses argument passed to the callback function will contain an array of canonical name records available for the hostname (e.g. ['bar.example.com']).

dns.resolveMx(hostname, callback)#

• hostname <string>
• callback <Function>
  • err <Error>
  • addresses <Object[]>

Uses the DNS protocol to resolve mail exchange records (MX records) for the hostname. The addresses argument passed to the callback function will contain an array of objects containing both a priority and exchange property (e.g. [{priority: 10, exchange: 'mx.example.com'}, ...]).

dns.resolveNaptr(hostname, callback)#

• hostname <string>
• callback <Function>
  • err <Error>
  • addresses <Object[]>

Uses the DNS protocol to resolve regular expression based records (NAPTR records) for the hostname. The addresses argument passed to the callback function will contain an array of objects with the following properties:

• flags
• service
• regexp
• replacement
• order
• preference

For example:

{
  flags: 's',
  service: 'SIP+D2U',
  regexp: '',
  replacement: '_sip._udp.example.com',
  order: 30,
  preference: 100
}

dns.resolveNs(hostname, callback)#

• hostname <string>
• callback <Function>
  • err <Error>
  • addresses <string[]>

Uses the DNS protocol to resolve name server records (NS records) for the hostname. The addresses argument passed to the callback function will contain an array of name server records available for hostname (e.g. ['ns1.example.com', 'ns2.example.com']).

dns.resolvePtr(hostname, callback)#

• hostname <string>
• callback <Function>
  • err <Error>
  • addresses <string[]>

Uses the DNS protocol to resolve pointer records (PTR records) for the hostname. The addresses argument passed to the callback function will be an array of strings containing the reply records.

dns.resolveSoa(hostname, callback)#

• hostname <string>
• callback <Function>
  • err <Error>
  • address <Object>

Uses the DNS protocol to resolve a start of authority record (SOA record) for the hostname. The address argument passed to the callback function will be an object with the following properties:

• nsname
• hostmaster
• serial
• refresh
• retry
• expire
• minttl

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

dns.resolveSrv(hostname, callback)#

• hostname <string>
• callback <Function>
  • err <Error>
  • addresses <Object[]>

Uses the DNS protocol to resolve service records (SRV records) for the hostname. The addresses argument passed to the callback function will be an array of objects with the following properties:

• priority
• weight
• port
• name

{
  priority: 10,
  weight: 5,
  port: 21223,
  name: 'service.example.com'
}

dns.resolveTxt(hostname, callback)#

• hostname <string>
• callback <Function>
  • err <Error>
  • records <string[][]>

Uses the DNS protocol to resolve text queries (TXT records) for the hostname. The records argument passed to the callback function is a two-dimensional 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, these could be either joined together or treated separately.

dns.resolveAny(hostname, callback)#

Uses the DNS protocol to resolve all records (also known as ANY or * query). The ret argument passed to the callback function will be an array containing various types of records. Each object has a property type that indicates the type of the current record. And depending on the type, additional properties will be present on the object:

Type	Properties
"A"	address / ttl
"AAAA"	address / ttl
"CNAME"	value
"MX"	Refer to dns.resolveMx()
"NAPTR"	Refer to dns.resolveNaptr()
"NS"	value
"PTR"	value
"SOA"	Refer to dns.resolveSoa()
"SRV"	Refer to dns.resolveSrv()
"TXT"	This type of record contains an array property called entries which refers to dns.resolveTxt(), eg. { entries: ['...'], type: 'TXT' }

Here is an example of the ret object passed to the callback:

[ { type: 'A', address: '127.0.0.1', ttl: 299 },
  { type: 'CNAME', value: 'example.com' },
  { type: 'MX', exchange: 'alt4.aspmx.l.example.com', priority: 50 },
  { type: 'NS', value: 'ns1.example.com' },
  { type: 'TXT', entries: [ 'v=spf1 include:_spf.example.com ~all' ] },
  { type: 'SOA',
    nsname: 'ns1.example.com',
    hostmaster: 'admin.example.com',
    serial: 156696742,
    refresh: 900,
    retry: 900,
    expire: 1800,
    minttl: 60 } ]

dns.reverse(ip, callback)#

• ip <string>
• callback <Function>
  • err <Error>
  • hostnames <string[]>

Performs a reverse DNS query that resolves an IPv4 or IPv6 address to an array of hostnames.

On error, err is an Error object, where err.code is one of the DNS error codes.

dns.setServers(servers)#

• servers <string[]> array of rfc5952 formatted addresses

Sets the IP address and port of servers to be used when performing DNS resolution. The servers argument is an array of rfc5952 formatted addresses. If the port is the IANA default DNS port (53) it can be omitted.

For example:

dns.setServers([
  '4.4.4.4',
  '[2001:4860:4860::8888]',
  '4.4.4.4:1053',
  '[2001:4860:4860::8888]:1053'
]);

An error will be thrown if an invalid address is provided.

The dns.setServers() method must not be called while a DNS query is in progress.

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.

Implementation considerations#

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

Though the call to dns.lookup() will be asynchronous from JavaScript's perspective, it is implemented as a synchronous call to getaddrinfo(3) that runs on libuv's threadpool. This can have surprising negative performance implications for some applications, see the UV_THREADPOOL_SIZE documentation for more information.

Note that various networking APIs will call dns.lookup() internally to resolve host names. If that is an issue, consider resolving the hostname to and address using dns.resolve() and using the address instead of a host name. Also, some networking APIs (such as socket.connect() and dgram.createSocket()) allow the default resolver, dns.lookup(), to be replaced.

dns.resolve(), 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.