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, callback)
  • dns.resolve6(hostname, 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.resolveTxt(hostname, callback)
  • dns.reverse(ip, callback)
  • dns.setServers(servers)
  • Error codes
  • Implementation considerations
    • dns.lookup()
    • dns.resolve(), dns.resolve*() and dns.reverse()

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 nodejs.org.

const dns = require('dns');

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

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

const dns = require('dns');

dns.resolve4('nodejs.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 logically 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. An example usage of options is shown below.

{
  family: 4,
  hints: dns.ADDRCONFIG | dns.V4MAPPED,
  all: false
}

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().

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.

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)#

Uses the DNS protocol to resolve a hostname (e.g. 'nodejs.org') into an array of the record types specified by rrtype.

Valid values for rrtype 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
• 'NAPTR' - name authority pointer record

The callback function has arguments (err, addresses). When successful, addresses will be an array, except when resolving an SOA record which returns an object structured in the same manner as one returned by the dns.resolveSoa() method. The type of each item in addresses is determined by the record type, and described in the documentation for the corresponding lookup methods.

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

dns.resolve4(hostname, callback)#

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, callback)#

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)#

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)#

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)#

Uses the DNS protocol to resolve regular expression based records (NAPTR records) for the hostname. The callback function has arguments (err, addresses). 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)#

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.resolveSoa(hostname, callback)#

Uses the DNS protocol to resolve a start of authority record (SOA record) for the hostname. The addresses 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)#

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)#

Uses the DNS protocol to resolve text queries (TXT records) for the hostname. The addresses argument passed to the callback function is is a two-dimentional 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)#

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

The callback function has arguments (err, hostnames), where hostnames is an array of resolved hostnames for the given ip.

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

dns.setServers(servers)#

Sets the IP addresses of the servers to be used when resolving. The servers argument is an array of IPv4 or IPv6 addresses.

If a port 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:

• 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. 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(), 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.