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>

1 files changed, 156 insertions, 131 deletions

diff --git a/doc/api/dns.markdown b/doc/api/dns.markdown
index 1b01f0ee1c..86bee21648 100644
--- a/doc/api/dns.markdown
+++ b/doc/api/dns.markdown
@@ -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 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:
@@ -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 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, 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` 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)
 
-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
-tweaked 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
+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
+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, 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