@ -2,39 +2,36 @@
Stability: 2 - Stable
Use `require('dns')` to access this module.
This module contains functions that belong to two different categories:
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 do any network communication.
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()`][].__
Here is an example that does a lookup of `www.google.com ` .
For example, looking up `nodejs.org ` .
const dns = require('dns');
dns.lookup('www.google.com', function onLookup(err, addresses, family) {
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 but [`dns.lookup()`][]. These
functions 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` _. 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.
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.
Here is an example which resolves `'www.google.com'` then reverse
resolves the IP addresses which are returned.
Below is an example that resolves `'nodejs.org'` then reverse resolves the IP
addresses that are returned.
const dns = require('dns');
dns.resolve4('www.google.com ', (err, addresses) => {
dns.resolve4('nodejs.org ', (err, addresses) => {
if (err) throw err;
console.log(`addresses: ${JSON.stringify(addresses)}`);
@ -49,19 +46,19 @@ resolves the IP addresses which are returned.
});
});
There are subtle consequences in choosing one or another, please consult the
[Implementation considerations section][] for more information.
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 addresses as strings that are currently being used for
resolution
Returns an array of IP address strings that are being used for name
resolution.
## dns.lookup(hostname[, options], callback)
Resolves a hostname (e.g. `'google.com '` ) into the first found A (IPv4) or
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 IP v4 and v6 addresses are both valid. If `options` is
not provided, then IPv4 and IP v6 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:
@ -79,154 +76,193 @@ Alternatively, `options` can be an object containing these properties:
All properties are optional. An example usage of options is shown below.
```
{
family: 4,
hints: dns.ADDRCONFIG | dns.V4MAPPED,
all: false
}
```
{
family: 4,
hints: dns.ADDRCONFIG | dns.V4MAPPED,
all: false
}
The callback has arguments `(err, address, family)` . `address` is a string
representation of an IP v4 or v6 address. `family` is either the integer 4 or 6
and denotes the family of `address` (not necessarily the value initially passed
to `lookup` ).
The `callback` function has arguments `(err, address, family)` . `address` is a
string representation of an IPv4 or IP v6 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, the arguments change to `(err, addresses)` , with
`addresses` being an array of objects with the properties `address` and
`family` .
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()` doesn't necessarily have anything to do with the DNS protocol.
It's only an operating system facility that can associate name with addresses,
and vice versa.
`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
Its 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 it.
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
`getnameinfo` .
Resolves the given ` address` ` port`
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)
Resolves a hostname (e.g. `'google.com'` ) into an array of the record types
specified by rrtype.
Uses the DNS protocol to resolve a hostname (e.g. `'nodejs.org'` ) into an
array of the record types specified by ` rrtype`
Valid rrtypes are:
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)
* `'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 has arguments `(err, addresses)` . The type of each item
in `addresses` is determined by the record type, and described in the
documentation for the corresponding lookup methods below.
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)
The same as [`dns.resolve()`][], but only for IPv4 queries (`A` records).
`addresses` is an array of IPv4 addresses (e.g.
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)
The same as [`dns.resolve4()`][] except for IPv6 queries (an `AAAA` query).
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)
The same as [`dns.resolve()`][], but only for canonical name records (`CNAME`
records). `addresses` is an array of the canonical name records available for
`hostname` (e.g., `['bar.example.com']` ).
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)
The same as [`dns.resolve()`][], but only for mail exchange queries
(`MX` records).
`addresses` is an array of MX records, each with a priority and an exchange
attribute (e.g. `[{'priority': 10, 'exchange': 'mx.example.com'},...]` ).
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)
The same as [`dns.resolve()`][], but only for name server records
(`NS` records). `addresses` is an array of the name server records available
for `hostname` (e.g., `['ns1.example.com', 'ns2.example.com']` ).
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)
The same as [`dns.resolve()`][], but only for start of authority record queries
(`SOA` record).
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
}
`addresses` is an object with the following structure:
## dns.resolveSrv(hostname, callback)
```
{
nsname: 'ns.example.com',
hostmaster: 'root.example.com',
serial: 2013101809,
refresh: 10000,
retry: 2400,
expire: 604800,
minttl: 3600
}
```
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:
## dns.resolveSrv(hostname, callback)
* `priority`
* `weight`
* `port`
* `name`
The same as [`dns.resolve()`][], but only for service records (`SRV` records).
`addresses` is an array of the SRV records available for `hostname` . Properties
of SRV records are priority, weight, port, and name (e.g.,
`[{'priority': 10, 'weight': 5, 'port': 21223, 'name': 'service.example.com'}, ...]` ).
{
priority: 10,
weight: 5,
port: 21223,
name: 'service.example.com'
}
## dns.resolveTxt(hostname, callback)
The same as [`dns.resolve()`][], but only for text queries (`TXT` records).
`addresses` is a 2-d array of the text records available for `hostname` (e.g.,
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, the could be either joined together or
one record. Depending on the use case, these could be either joined together or
treated separately.
## dns.reverse(ip, callback)
Reverse resolves an ip address to an array of hostnames.
Performs a reverse DNS query that resolves an IPv4 or IPv6 address to an
array of hostnames.
The callback has arguments `(err, 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)
Given an array of IP addresses as strings, set them as the servers to use for
resolving
Sets the IP addresses of the servers to be used when resolving. The `servers`
argument is an array of IPv4 or IPv6 addresses.
If you specify a port with the address it will be stripped, as the underlying
library doesn't support that.
If a port specified on the address it will be removed.
This will throw if you pass invalid input.
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
@ -257,47 +293,36 @@ Each DNS query can return one of the following error codes:
- `dns.ADDRGETNETWORKPARAMS` : Could not find GetNetworkParams function.
- `dns.CANCELLED` : DNS query cancelled.
## 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).
## Implementation considerations
Although [`dns.lookup()`][] and `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.
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
### `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
tweak ed by changing settings in `nsswitch.conf(5)` and/or `resolv.conf(5)` , but
be careful that changing these files will change the behavior of all other
programs running on the same operating system.
Though the call 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
modifi ed 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
setting the ` 'UV_THREADPOOL_SIZE'`
` 4`
[the official libuv documentation][].
### dns.resolve, functions starting with dns.resolve and dns.reverse
### `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
James M Snell
9 years ago
Jeremiah Senkpiel