DNS | Node.js v7.10.1 Documentation (original) (raw)

DNS#

Stability: 2 - Stable

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.

dns.getServers()#

Added in: v0.11.3

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. 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}]

Supported getaddrinfo flags#

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

dns.lookupService(address, port, callback)#

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
});

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()
'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 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.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:

For example:

{
  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 addresses argument passed to the callback function is 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 addresses of the servers to be used when resolving. The serversargument is an array of IPv4 or IPv6 addresses.

If a port is specified on the address, it will be removed.

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:

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. 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 than4 (its current default value). For more information on libuv's threadpool, seethe official libuv documentation.

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.