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doc: improvements to dns.markdown copy 

General improvements to dns.markdown copy and examples

PR-URL: https://github.com/nodejs/node/pull/4449
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
v4.x
James M Snell 9 years ago
committed by Myles Borins
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      doc/api/dns.markdown

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@ -2,39 +2,36 @@
Stability: 2 - Stable Stability: 2 - Stable
Use `require('dns')` to access this module. The `dns` module contains functions belonging to two different categories:
This module contains functions that belong to two different categories:
1) Functions that use the underlying operating system facilities to perform 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 This category contains only one function: [`dns.lookup()`][]. __Developers
looking to perform name resolution in the same way that other applications on looking to perform name resolution in the same way that other applications on
the same operating system behave should use [`dns.lookup()`][].__ 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'); 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); console.log('addresses:', addresses);
}); });
2) Functions that connect to an actual DNS server to perform name resolution, 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 and that _always_ use the network to perform DNS queries. This category
contains all functions in the `dns` module but [`dns.lookup()`][]. These contains all functions in the `dns` module _except_ [`dns.lookup()`][]. These
functions do not use the same set of configuration files than what functions do not use the same set of configuration files used by
[`dns.lookup()`][] uses. For instance, _they do not use the configuration from [`dns.lookup()`][] (e.g. `/etc/hosts`). These functions should be used by
`/etc/hosts`_. These functions should be used by developers who do not want to developers who do not want to use the underlying operating system's facilities
use the underlying operating system's facilities for name resolution, and for name resolution, and instead want to _always_ perform DNS queries.
instead want to _always_ perform DNS queries.
Here is an example which resolves `'www.google.com'` then reverse Below is an example that resolves `'nodejs.org'` then reverse resolves the IP
resolves the IP addresses which are returned. addresses that are returned.
const dns = require('dns'); const dns = require('dns');
dns.resolve4('www.google.com', (err, addresses) => { dns.resolve4('nodejs.org', (err, addresses) => {
if (err) throw err; if (err) throw err;
console.log(`addresses: ${JSON.stringify(addresses)}`); 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 There are subtle consequences in choosing one over the other, please consult
[Implementation considerations section][] for more information. the [Implementation considerations section][] for more information.
## dns.getServers() ## dns.getServers()
Returns an array of IP addresses as strings that are currently being used for Returns an array of IP address strings that are being used for name
resolution resolution.
## dns.lookup(hostname[, options], callback) ## 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 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 IPv6 addresses are both valid. If `options` is
an integer, then it must be `4` or `6`. an integer, then it must be `4` or `6`.
Alternatively, `options` can be an object containing these properties: Alternatively, `options` can be an object containing these properties:
@ -79,110 +76,134 @@ Alternatively, `options` can be an object containing these properties:
All properties are optional. An example usage of options is shown below. All properties are optional. An example usage of options is shown below.
```
{ {
family: 4, family: 4,
hints: dns.ADDRCONFIG | dns.V4MAPPED, hints: dns.ADDRCONFIG | dns.V4MAPPED,
all: false all: false
} }
```
The callback has arguments `(err, address, family)`. `address` is a string The `callback` function has arguments `(err, address, family)`. `address` is a
representation of an IP v4 or v6 address. `family` is either the integer 4 or 6 string representation of an IPv4 or IPv6 address. `family` is either the
and denotes the family of `address` (not necessarily the value initially passed integer `4` or `6` and denotes the family of `address` (not necessarily the
to `lookup`). value initially passed to `lookup`).
With the `all` option set, the arguments change to `(err, addresses)`, with With the `all` option set to `true`, the arguments change to
`addresses` being an array of objects with the properties `address` and `(err, addresses)`, with `addresses` being an array of objects with the
`family`. properties `address` and `family`.
On error, `err` is an [`Error`][] object, where `err.code` is the error code. 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 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 the hostname does not exist but also when the lookup fails in other ways
such as no available file descriptors. such as no available file descriptors.
`dns.lookup()` doesn't necessarily have anything to do with the DNS protocol. `dns.lookup()` does not necessarily have anything to do with the DNS protocol.
It's only an operating system facility that can associate name with addresses, The implementation uses an operating system facility that can associate names
and vice versa. 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()`][].
Its implementation can have subtle but important consequences on the behavior - `dns.ADDRCONFIG`: Returned address types are determined by the types
of any Node.js program. Please take some time to consult the [Implementation of addresses supported by the current system. For example, IPv4 addresses
considerations section][] before using it. 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) ## dns.lookupService(address, port, callback)
Resolves the given address and port into a hostname and service using Resolves the given `address` and `port` into a hostname and service using
`getnameinfo`. the operating system's underlying `getnameinfo` implementation.
The callback has arguments `(err, hostname, service)`. The `hostname` and The callback has arguments `(err, hostname, service)`. The `hostname` and
`service` arguments are strings (e.g. `'localhost'` and `'http'` respectively). `service` arguments are strings (e.g. `'localhost'` and `'http'` respectively).
On error, `err` is an [`Error`][] object, where `err.code` is the error code. 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) ## dns.resolve(hostname[, rrtype], callback)
Resolves a hostname (e.g. `'google.com'`) into an array of the record types Uses the DNS protocol to resolve a hostname (e.g. `'nodejs.org'`) into an
specified by rrtype. array of the record types specified by `rrtype`.
Valid rrtypes are: Valid values for `rrtype` are:
* `'A'` (IPV4 addresses, default) * `'A'` - IPV4 addresses, default
* `'AAAA'` (IPV6 addresses) * `'AAAA'` - IPV6 addresses
* `'MX'` (mail exchange records) * `'MX'` - mail exchange records
* `'TXT'` (text records) * `'TXT'` - text records
* `'SRV'` (SRV records) * `'SRV'` - SRV records
* `'PTR'` (used for reverse IP lookups) * `'PTR'` - used for reverse IP lookups
* `'NS'` (name server records) * `'NS'` - name server records
* `'CNAME'` (canonical name records) * `'CNAME'` - canonical name records
* `'SOA'` (start of authority record) * `'SOA'` - start of authority record
The callback has arguments `(err, addresses)`. The type of each item The `callback` function has arguments `(err, addresses)`. When successful,
in `addresses` is determined by the record type, and described in the `addresses` will be an array. The type of each item in `addresses` is
documentation for the corresponding lookup methods below. 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 On error, `err` is an [`Error`][] object, where `err.code` is
one of the error codes listed below. one of the error codes listed below.
## dns.resolve4(hostname, callback) ## dns.resolve4(hostname, callback)
The same as [`dns.resolve()`][], but only for IPv4 queries (`A` records). Uses the DNS protocol to resolve a IPv4 addresses (`A` records) for the
`addresses` is an array of IPv4 addresses (e.g. `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']`). `['74.125.79.104', '74.125.79.105', '74.125.79.106']`).
## dns.resolve6(hostname, callback) ## 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) ## dns.resolveCname(hostname, callback)
The same as [`dns.resolve()`][], but only for canonical name records (`CNAME` Uses the DNS protocol to resolve `CNAME` records for the `hostname`. The
records). `addresses` is an array of the canonical name records available for `addresses` argument passed to the `callback` function
`hostname` (e.g., `['bar.example.com']`). will contain an of canonical name records available for the `hostname`
(e.g. `['bar.example.com']`).
## dns.resolveMx(hostname, callback) ## dns.resolveMx(hostname, callback)
The same as [`dns.resolve()`][], but only for mail exchange queries Uses the DNS protocol to resolve mail exchange records (`MX` records) for the
(`MX` records). `hostname`. The `addresses` argument passed to the `callback` function will
contain an array of objects containing both a `priority` and `exchange`
`addresses` is an array of MX records, each with a priority and an exchange property (e.g. `[{priority: 10, exchange: 'mx.example.com'}, ...]`).
attribute (e.g. `[{'priority': 10, 'exchange': 'mx.example.com'},...]`).
## dns.resolveNs(hostname, callback) ## dns.resolveNs(hostname, callback)
The same as [`dns.resolve()`][], but only for name server records Uses the DNS protocol to resolve name server records (`NS` records) for the
(`NS` records). `addresses` is an array of the name server records available `hostname`. The `addresses` argument passed to the `callback` function will
for `hostname` (e.g., `['ns1.example.com', 'ns2.example.com']`). contain an array of name server records available for `hostname`
(e.g., `['ns1.example.com', 'ns2.example.com']`).
## dns.resolveSoa(hostname, callback) ## dns.resolveSoa(hostname, callback)
The same as [`dns.resolve()`][], but only for start of authority record queries Uses the DNS protocol to resolve a start of authority record (`SOA` record) for
(`SOA` record). the `hostname`. The `addresses` argument passed to the `callback` function will
be an object with the following properties:
`addresses` is an object with the following structure: * `nsname`
* `hostmaster`
* `serial`
* `refresh`
* `retry`
* `expire`
* `minttl`
```
{ {
nsname: 'ns.example.com', nsname: 'ns.example.com',
hostmaster: 'root.example.com', hostmaster: 'root.example.com',
@ -192,41 +213,56 @@ The same as [`dns.resolve()`][], but only for start of authority record queries
expire: 604800, expire: 604800,
minttl: 3600 minttl: 3600
} }
```
## dns.resolveSrv(hostname, callback) ## dns.resolveSrv(hostname, callback)
The same as [`dns.resolve()`][], but only for service records (`SRV` records). Uses the DNS protocol to resolve service records (`SRV` records) for the
`addresses` is an array of the SRV records available for `hostname`. Properties `hostname`. The `addresses` argument passed to the `callback` function will
of SRV records are priority, weight, port, and name (e.g., be an array of objects with the following properties:
`[{'priority': 10, 'weight': 5, 'port': 21223, 'name': 'service.example.com'}, ...]`).
* `priority`
* `weight`
* `port`
* `name`
{
priority: 10,
weight: 5,
port: 21223,
name: 'service.example.com'
}
## dns.resolveTxt(hostname, callback) ## dns.resolveTxt(hostname, callback)
The same as [`dns.resolve()`][], but only for text queries (`TXT` records). Uses the DNS protocol to resolve text queries (`TXT` records) for the
`addresses` is a 2-d array of the text records available for `hostname` (e.g., `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 `[ ['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. treated separately.
## dns.reverse(ip, callback) ## 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 On error, `err` is an [`Error`][] object, where `err.code` is
one of the error codes listed below. one of the error codes listed below.
## dns.setServers(servers) ## dns.setServers(servers)
Given an array of IP addresses as strings, set them as the servers to use for Sets the IP addresses of the servers to be used when resolving. The `servers`
resolving argument is an array of IPv4 or IPv6 addresses.
If a port specified on the address it will be removed.
If you specify a port with the address it will be stripped, as the underlying An error will be thrown if an invalid address is provided.
library doesn't support that.
This will throw if you pass invalid input. The `dns.setServers()` method must not be called while a DNS query is in
progress.
## Error codes ## 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.ADDRGETNETWORKPARAMS`: Could not find GetNetworkParams function.
- `dns.CANCELLED`: DNS query cancelled. - `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 ## Implementation considerations
Although [`dns.lookup()`][] and `dns.resolve*()/dns.reverse()` functions have Although [`dns.lookup()`][] and the various `dns.resolve*()/dns.reverse()`
the same goal of associating a network name with a network address (or vice functions have the same goal of associating a network name with a network
versa), their behavior is quite different. These differences can have subtle but address (or vice versa), their behavior is quite different. These differences
significant consequences on the behavior of Node.js programs. 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 Under the hood, [`dns.lookup()`][] uses the same operating system facilities
as most other programs. For instance, [`dns.lookup()`][] will almost always 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 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 operating systems, the behavior of the [`dns.lookup()`][] function can be
tweaked by changing settings in `nsswitch.conf(5)` and/or `resolv.conf(5)`, but modified by changing settings in `nsswitch.conf(5)` and/or `resolv.conf(5)`,
be careful that changing these files will change the behavior of all other but note that changing these files will change the behavior of _all other
programs running on the same operating system. programs running on the same operating system_.
Though the call will be asynchronous from JavaScript's perspective, it is Though the call to `dns.lookup()` will be asynchronous from JavaScript's
implemented as a synchronous call to `getaddrinfo(3)` that runs on libuv's perspective, it is implemented as a synchronous call to `getaddrinfo(3)` that
threadpool. Because libuv's threadpool has a fixed size, it means that if for runs on libuv's threadpool. Because libuv's threadpool has a fixed size, it
whatever reason the call to `getaddrinfo(3)` takes a long time, other means that if for whatever reason the call to `getaddrinfo(3)` takes a long
operations that could run on libuv's threadpool (such as filesystem time, other operations that could run on libuv's threadpool (such as filesystem
operations) will experience degraded performance. In order to mitigate this operations) will experience degraded performance. In order to mitigate this
issue, one potential solution is to increase the size of libuv's threadpool by 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 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 `4` (its current default value). For more information on libuv's threadpool, see
[the official libuv documentation][]. [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 These functions are implemented quite differently than [`dns.lookup()`][]. They
do not use `getaddrinfo(3)` and they _always_ perform a DNS query on the do not use `getaddrinfo(3)` and they _always_ perform a DNS query on the

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