DNS | Node.js v11.15.0 Documentation (original) (raw)

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
  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 except dns.lookup(). These functions do not use the same set of configuration files used bydns.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 <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>a</mi><mo>:</mo></mrow><annotation encoding="application/x-tex">{a}: </annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.4306em;"></span><span class="mord"><span class="mord mathnormal">a</span></span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">:</span></span></span></span>{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#

Added in: v8.3.0

An independent resolver for DNS requests.

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

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.cancel()#

Added in: v8.3.0

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

dns.getServers()#

Added in: v0.11.3

Returns an array of IP address strings, formatted according to RFC 5952, that are currently configured for DNS resolution. A string will include a port section if a custom port is used.

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

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

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 usingdns.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 allis not set to true, it returns a Promise for an Object with address andfamily properties.

Supported getaddrinfo flags#

The following flags can be passed as hints to dns.lookup().

dns.lookupService(address, port, callback)[src]#

Added in: v0.11.14

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 TypeErrorwill 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 aPromise for an Object with hostname and service properties.

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

Added in: v0.1.27

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

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

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

Uses the DNS protocol to resolve a IPv4 addresses (A records) for thehostname. 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)#

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

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(), e.g. { 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 server operators may choose not to respond to ANYqueries. It may be better to call individual methods like dns.resolve4(),dns.resolveMx(), and so on. For more details, see RFC 8482.

dns.resolveCname(hostname, callback)#

Added in: v0.3.2

Uses the DNS protocol to resolve CNAME records for the hostname. Theaddresses 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)#

Added in: v0.1.27

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

dns.resolveNaptr(hostname, callback)#

Added in: v0.9.12

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

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

dns.resolveNs(hostname, callback)#

Added in: v0.1.90

Uses the DNS protocol to resolve name server records (NS records) for thehostname. 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)#

Added in: v6.0.0

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

dns.resolveSoa(hostname, callback)#

Added in: v0.11.10

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

dns.resolveSrv(hostname, callback)#

Added in: v0.1.27

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

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

dns.resolveTxt(hostname, callback)#

Added in: v0.1.27

Uses the DNS protocol to resolve text queries (TXT records) for thehostname. 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.reverse(ip, callback)#

Added in: v0.1.16

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)#

Added in: v0.11.3

Sets the IP address and port of servers to be used when performing DNS resolution. The servers argument is an array of RFC 5952 formatted addresses. If the port is the IANA default DNS port (53) it can be omitted.

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.

The dns.setServers() method affects only dns.resolve(), [dns.resolve*()][] and dns.reverse() (and specifically not dns.lookup()).

Note that this method works much likeresolve.conf. That is, if attempting to resolve with the first server provided results in aNOTFOUND error, the resolve() method will not attempt to resolve with subsequent servers provided. Fallback DNS servers will only be used if the earlier ones time out or result in some other error.

DNS Promises API#

The dns.promises API provides an alternative set of asynchronous DNS methods that return Promise objects rather than using callbacks. The API is accessible via require('dns').promises.

Class: dnsPromises.Resolver#

Added in: v10.6.0

An independent resolver for DNS requests.

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

const { Resolver } = require('dns').promises;
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').then((addresses) => {
  // ...
});

// Alternatively, the same code can be written using async-await style.
(async function() {
  const addresses = await resolver.resolve4('example.org');
})();

The following methods from the dnsPromises API are available:

dnsPromises.getServers()#

Added in: v10.6.0

Returns an array of IP address strings, formatted according to RFC 5952, that are currently configured for DNS resolution. A string will include a port section if a custom port is used.

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

dnsPromises.lookup(hostname[, options])#

Added in: v10.6.0

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 Promise is resolved with addressesbeing an array of objects with the properties address and family.

On error, the Promise is rejected with an Error object, where err.codeis 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.

dnsPromises.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 dnsPromises.lookup().

Example usage:

const dns = require('dns');
const dnsPromises = dns.promises;
const options = {
  family: 6,
  hints: dns.ADDRCONFIG | dns.V4MAPPED,
};

dnsPromises.lookup('example.com', options).then((result) => {
  console.log('address: %j family: IPv%s', result.address, result.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;
dnsPromises.lookup('example.com', options).then((result) => {
  console.log('addresses: %j', result);
  // addresses: [{"address":"2606:2800:220:1:248:1893:25c8:1946","family":6}]
});

dnsPromises.lookupService(address, port)#

Added in: v10.6.0

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 TypeErrorwill be thrown.

On error, the Promise is rejected with an Error object, where err.codeis the error code.

const dnsPromises = require('dns').promises;
dnsPromises.lookupService('127.0.0.1', 22).then((result) => {
  console.log(result.hostname, result.service);
  // Prints: localhost ssh
});

dnsPromises.resolve(hostname[, rrtype])#

Added in: v10.6.0

Uses the DNS protocol to resolve a hostname (e.g. 'nodejs.org') into an array of the resource records. When successful, the Promise is resolved with an array of resource records. The type and structure of individual results vary based on rrtype:

rrtype records contains Result type Shorthand method
'A' IPv4 addresses (default) dnsPromises.resolve4()
'AAAA' IPv6 addresses dnsPromises.resolve6()
'ANY' any records dnsPromises.resolveAny()
'CNAME' canonical name records dnsPromises.resolveCname()
'MX' mail exchange records dnsPromises.resolveMx()
'NAPTR' name authority pointer records dnsPromises.resolveNaptr()
'NS' name server records dnsPromises.resolveNs()
'PTR' pointer records dnsPromises.resolvePtr()
'SOA' start of authority records dnsPromises.resolveSoa()
'SRV' service records dnsPromises.resolveSrv()
'TXT' text records <string[]> dnsPromises.resolveTxt()

On error, the Promise is rejected with an Error object, where err.codeis one of the DNS error codes.

dnsPromises.resolve4(hostname[, options])#

Added in: v10.6.0

Uses the DNS protocol to resolve IPv4 addresses (A records) for thehostname. On success, the Promise is resolved with an array of IPv4 addresses (e.g. ['74.125.79.104', '74.125.79.105', '74.125.79.106']).

dnsPromises.resolve6(hostname[, options])#

Added in: v10.6.0

Uses the DNS protocol to resolve IPv6 addresses (AAAA records) for thehostname. On success, the Promise is resolved with an array of IPv6 addresses.

dnsPromises.resolveAny(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve all records (also known as ANY or * query). On success, the Promise is resolved with 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 dnsPromises.resolveMx()
'NAPTR' Refer to dnsPromises.resolveNaptr()
'NS' value
'PTR' value
'SOA' Refer to dnsPromises.resolveSoa()
'SRV' Refer to dnsPromises.resolveSrv()
'TXT' This type of record contains an array property called entries which refers to dnsPromises.resolveTxt(), e.g. { entries: ['...'], type: 'TXT' }

Here is an example of the result object:

[ { 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 } ]

dnsPromises.resolveCname(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve CNAME records for the hostname. On success, the Promise is resolved with an array of canonical name records available for the hostname (e.g. ['bar.example.com']).

dnsPromises.resolveMx(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve mail exchange records (MX records) for thehostname. On success, the Promise is resolved with an array of objects containing both a priority and exchange property (e.g.[{priority: 10, exchange: 'mx.example.com'}, ...]).

dnsPromises.resolveNaptr(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve regular expression based records (NAPTRrecords) for the hostname. On success, the Promise is resolved with an array of objects with the following properties:

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

dnsPromises.resolveNs(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve name server records (NS records) for thehostname. On success, the Promise is resolved with an array of name server records available for hostname (e.g.['ns1.example.com', 'ns2.example.com']).

dnsPromises.resolvePtr(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve pointer records (PTR records) for thehostname. On success, the Promise is resolved with an array of strings containing the reply records.

dnsPromises.resolveSoa(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve a start of authority record (SOA record) for the hostname. On success, the Promise is resolved with an object with the following properties:

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

dnsPromises.resolveSrv(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve service records (SRV records) for thehostname. On success, the Promise is resolved with an array of objects with the following properties:

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

dnsPromises.resolveTxt(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve text queries (TXT records) for thehostname. On success, the Promise is resolved with 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.

dnsPromises.reverse(ip)#

Added in: v10.6.0

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

On error, the Promise is rejected with an Error object, where err.codeis one of the DNS error codes.

dnsPromises.setServers(servers)#

Added in: v10.6.0

Sets the IP address and port of servers to be used when performing DNS resolution. The servers argument is an array of RFC 5952 formatted addresses. If the port is the IANA default DNS port (53) it can be omitted.

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

Note that this method works much likeresolve.conf. That is, if attempting to resolve with the first server provided results in aNOTFOUND error, the resolve() method will not attempt to resolve with subsequent servers provided. Fallback DNS servers will only be used if the earlier ones time out or result in some other error.

Error codes#

Each DNS query can return one of the following error codes:

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_SIZEdocumentation 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 an 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.