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Node.js v9.9.0 Documentation
Table of Contents
- DNS
- Class dns.Resolver
- dns.getServers()
- dns.lookup(hostname[, options], callback)
- 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#
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.
[
'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 be4
or6
. IPv4 and IPv6 addresses are both returned by default.hints
<number> One or more supportedgetaddrinfo
flags. Multiple flags may be passed by bitwiseOR
ing their values.all
<boolean> Whentrue
, the callback returns all resolved addresses in an array. Otherwise, returns a single address. Default:false
verbatim
<boolean> Whentrue
, the callback receives IPv4 and IPv6 addresses in the order the DNS resolver returned them. Whenfalse
, IPv4 addresses are placed before IPv6 addresses. Default: currentlyfalse
(addresses are reordered) but this is expected to change in the not too distant future. New code should use{ verbatim: true }
.
callback
<Function>
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>
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. Whentrue
, 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. Whentrue
, 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
{
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>
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)#
hostname
<string>callback
<Function>err
<Error>ret
<Object[]>
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.
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
.