Node.js v4.3.1-rc.3 Documentation
Table of Contents
- DNS
- dns.getServers()
- dns.lookup(hostname[, options], callback)
- 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.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#
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 integer4
or6
. If not provided, both IP v4 and v6 addresses are accepted.hints
: {Number} - If present, it should be one or more of the supportedgetaddrinfo
flags. Ifhints
is not provided, then no flags are passed togetaddrinfo
. Multiple flags can be passed throughhints
by logicallyOR
ing their values. See supportedgetaddrinfo
flags below for more information on supported flags.all
: {Boolean} - Whentrue
, the callback returns all resolved addresses in an array, otherwise returns a single address. Defaults tofalse
.
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
The callback
function has arguments (err, addresses)
. When successful,
addresses
will be an array. The type of each item in addresses
is
determined by the record type, and described in the documentation for the
corresponding lookup methods below.
On error, err
is an Error
object, where err.code
is
one of the error codes listed below.
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 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.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 error codes listed below.
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
.