DNS （域名服务器）

稳定性: 2 - 稳定的

dns 模块包含两类函数：

1) 第一类函数，使用底层操作系统工具进行域名解析，且无需进行网络通信。 这类函数只有一个：dns.lookup()。例子，查找 iana.org：

const dns = require('dns');

dns.lookup('iana.org', (err, address, family) => {
  console.log('IP 地址: %j 地址族: IPv%s', address, family);
});
// IP 地址: "192.0.43.8" 地址族: IPv4

2) 第二类函数，连接到一个真实的 DNS 服务器进行域名解析，且始终使用网络进行 DNS 查询。 这类函数包含了 dns 模块中除 dns.lookup() 以外的所有函数。 这些函数使用与 dns.lookup() 不同的配置文件（例如 /etc/hosts）。 这类函数适合于那些不想使用底层操作系统工具进行域名解析、而是想使用网络进行 DNS 查询的开发者。

例子，解析 'archive.org' 然后逆向解析返回的 IP 地址：

const dns = require('dns');

dns.resolve4('archive.org', (err, addresses) => {
  if (err) throw err;

  console.log(`IP 地址: ${JSON.stringify(addresses)}`);

  addresses.forEach((a) => {
    dns.reverse(a, (err, hostnames) => {
      if (err) {
        throw err;
      }
      console.log(`IP 地址 ${a} 逆向解析到域名: ${JSON.stringify(hostnames)}`);
    });
  });
});

两类函数有微妙的差别，详见 实现上的注意事项。

dns.getServers()

新增于: v0.11.3
返回一组用于域名解析的IP地址(类型为字符串)数组。

dns.lookup(hostname[, options], callback)

版本历史

• hostname <string>
• options <integer> | <Object>
  • family <integer> The record family. Must be 4 or 6. IPv4 and IPv6 addresses are both returned by default.
  • hints <number> One or more supported getaddrinfo flags. Multiple flags may be passed by bitwise ORing their values.
  • all <boolean> When true, the callback returns all resolved addresses in an array. Otherwise, returns a single address. Defaults to false.
• callback <Function>
  • err <Error>
  • address <string> A string representation of an IPv4 or IPv6 address.
  • family <integer> 4 or 6, denoting the family of address.

解析hostname(例如：'nodejs.org')为第一个找到的A（IPv4）或AAAA（IPv6）记录。options可以是对象或者整数。如果options没有被提供，那么IPv4 和 IPv6都是有效的。如果options是整数，只能是4或6。

另外，options可以是一个含有以下属性的对象：

• family <Number> - T地址族。如果提供，必须为整数4或6。如果没有提供，只接受IPv4和IPv6地址。
• hints: <Number> - 如果提供，它必须是一个或多个支持的getaddrinfo标识。如果没有提供，那么没有标识被传递给getaddrinfo。多个标识可以通过在逻辑上ORing它们的值，来传递给hints。支持的getaddrinfo标识请参阅下文。有关支持的标志的更多信息请查询supported getaddrinfo flags章节。
• all: <Boolean> - 值为true时， 回调函数返回一个包含所有解析后地址的数组，否则只返回一个地址。默认值为false。

所有的参数都是可选的。

回调函数包含(err, address, family)参数。address是IPv4或IPv6地址字符串。family、是整数4或6，表示地址族（不一定是最初传递给查找的值）。

当all属性被设置为true时，回调函数参数变为(err, addresses)，addresses则变成一个由address 和 family 属性组成的对象数组。

发生错误时，err是一个Error对象，err.code是错误码。不仅在主机名不存在时，在如没有可用的文件描述符等情况下查找失败，err.code也会被设置为'ENOENT'。

dns.lookup() 不需要与DNS协议有任何关系。它仅仅是一个连接名字和地址的操作系统功能。在任何的node.js程序中，它的实现对表现有一些微妙但是重要的影响。在使用dns.lookup()之前请花些时间查询Implementation considerations section章节。

使用例子：

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)

新增于: v0.11.14

• address <string>
• port <number>
• callback <Function>
  • err <Error>
  • hostname <string> e.g. example.com
  • service <string> e.g. http

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)

新增于: v0.1.27

• hostname <string> Hostname to resolve.
• rrtype <string> Resource record type. Default: 'A'.
• callback <Function>
  • err <Error>
  • records <string[]> | <Object[]> | <string[][]> | <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:

On error, err is an Error object, where err.code is one of the DNS error codes.

dns.resolve4(hostname[, options], callback)

版本历史

• hostname <string> Hostname to resolve.
• options <Object>
  • ttl <boolean> Retrieve the Time-To-Live value (TTL) of each record. When true, 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)

版本历史

• hostname <string> Hostname to resolve.
• options <Object>
  • ttl <boolean> Retrieve the Time-To-Live value (TTL) of each record. When true, 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)

新增于: v0.3.2

• 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)

新增于: v0.1.27

• 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)

新增于: v0.9.12

• 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

For example:

{
  flags: 's',
  service: 'SIP+D2U',
  regexp: '',
  replacement: '_sip._udp.example.com',
  order: 30,
  preference: 100
}

dns.resolveNs(hostname, callback)

新增于: v0.1.90

• 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)

新增于: v6.0.0

• 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)

新增于: v0.11.10

• hostname <string>
• callback <Function>
  • err <Error>
  • address <Object>

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)

新增于: v0.1.27

• 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)

新增于: v0.1.27

• hostname <string>
• callback <Function>
  • err <Error>
  • addresses <string[][]>

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-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.reverse(ip, callback)

新增于: v0.1.16

• 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)

新增于: v0.11.3

• servers <string[]>

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 is 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.

错误码

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.

实现上的注意事项

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.