The dns module contains functions belonging to two different categories:
1) Functions that use the underlying operating system facilities to perform\nname resolution, and that do not necessarily perform any network communication.\nThis category contains only one function: dns.lookup(). Developers\nlooking to perform name resolution in the same way that other applications on\nthe same operating system behave should use dns.lookup().
For example, looking up iana.org.
const dns = require('dns');\n\ndns.lookup('iana.org', (err, address, family) => {\n console.log('address: %j family: IPv%s', address, family);\n});\n// address: "192.0.43.8" family: IPv4\n2) Functions that connect to an actual DNS server to perform name resolution,\nand that always use the network to perform DNS queries. This category\ncontains all functions in the dns module except dns.lookup(). These\nfunctions do not use the same set of configuration files used by\ndns.lookup() (e.g. /etc/hosts). These functions should be used by\ndevelopers who do not want to use the underlying operating system's facilities\nfor name resolution, and instead want to always perform DNS queries.
Below is an example that resolves 'archive.org' then reverse resolves the IP\naddresses that are returned.
const dns = require('dns');\n\ndns.resolve4('archive.org', (err, addresses) => {\n if (err) throw err;\n\n console.log(`addresses: ${JSON.stringify(addresses)}`);\n\n addresses.forEach((a) => {\n dns.reverse(a, (err, hostnames) => {\n if (err) {\n throw err;\n }\n console.log(`reverse for ${a}: ${JSON.stringify(hostnames)}`);\n });\n });\n});\nThere are subtle consequences in choosing one over the other, please consult\nthe Implementation considerations section for more information.
\n", "methods": [ { "textRaw": "dns.getServers()", "type": "method", "name": "getServers", "meta": { "added": [ "v0.11.3" ], "changes": [] }, "desc": "Returns an array of IP address strings that are being used for name\nresolution.
\n", "signatures": [ { "params": [] } ] }, { "textRaw": "dns.lookup(hostname[, options], callback)", "type": "method", "name": "lookup", "meta": { "added": [ "v0.1.90" ], "changes": [ { "version": "v1.2.0", "pr-url": "https://github.com/nodejs/node/pull/744", "description": "The `all` option is supported now." } ] }, "signatures": [ { "params": [ { "textRaw": "`hostname` {string} ", "name": "hostname", "type": "string" }, { "textRaw": "`options` {integer | Object} ", "options": [ { "textRaw": "`family` {integer} The record family. Must be `4` or `6`. IPv4 and IPv6 addresses are both returned by default. ", "name": "family", "type": "integer", "desc": "The record family. Must be `4` or `6`. IPv4 and IPv6 addresses are both returned by default." }, { "textRaw": "`hints` {number} One or more [supported `getaddrinfo` flags][]. Multiple flags may be passed by bitwise `OR`ing their values. ", "name": "hints", "type": "number", "desc": "One or more [supported `getaddrinfo` flags][]. Multiple flags may be passed by bitwise `OR`ing their values." }, { "textRaw": "`all` {boolean} When `true`, the callback returns all resolved addresses in an array. Otherwise, returns a single address. Defaults to `false`. ", "name": "all", "type": "boolean", "desc": "When `true`, the callback returns all resolved addresses in an array. Otherwise, returns a single address. Defaults to `false`." } ], "name": "options", "type": "integer | Object", "optional": true }, { "textRaw": "`callback` {Function} ", "options": [ { "textRaw": "`err` {Error} ", "name": "err", "type": "Error" }, { "textRaw": "`address` {string} A string representation of an IPv4 or IPv6 address. ", "name": "address", "type": "string", "desc": "A string representation of an IPv4 or IPv6 address." }, { "textRaw": "`family` {integer} `4` or `6`, denoting the family of `address`. ", "name": "family", "type": "integer", "desc": "`4` or `6`, denoting the family of `address`." } ], "name": "callback", "type": "Function" } ] }, { "params": [ { "name": "hostname" }, { "name": "options", "optional": true }, { "name": "callback" } ] } ], "desc": "Resolves a hostname (e.g. 'nodejs.org') into the first found A (IPv4) or\nAAAA (IPv6) record. All option properties are optional. If options is an\ninteger, then it must be 4 or 6 – if options is not provided, then IPv4\nand IPv6 addresses are both returned if found.
With the all option set to true, the arguments for callback change to\n(err, addresses), with addresses being an array of objects with the\nproperties address and family.
On error, err is an Error object, where err.code is the error code.\nKeep in mind that err.code will be set to 'ENOENT' not only when\nthe hostname does not exist but also when the lookup fails in other ways\nsuch as no available file descriptors.
dns.lookup() does not necessarily have anything to do with the DNS protocol.\nThe implementation uses an operating system facility that can associate names\nwith addresses, and vice versa. This implementation can have subtle but\nimportant consequences on the behavior of any Node.js program. Please take some\ntime to consult the Implementation considerations section before using\ndns.lookup().
Example usage:
\nconst dns = require('dns');\nconst options = {\n family: 6,\n hints: dns.ADDRCONFIG | dns.V4MAPPED,\n};\ndns.lookup('example.com', options, (err, address, family) =>\n console.log('address: %j family: IPv%s', address, family));\n// address: "2606:2800:220:1:248:1893:25c8:1946" family: IPv6\n\n// When options.all is true, the result will be an Array.\noptions.all = true;\ndns.lookup('example.com', options, (err, addresses) =>\n console.log('addresses: %j', addresses));\n// addresses: [{"address":"2606:2800:220:1:248:1893:25c8:1946","family":6}]\nIf this method is invoked as its util.promisify()ed version, and all\nis not set to true, it returns a Promise for an object with address and\nfamily properties.
The following flags can be passed as hints to dns.lookup().
dns.ADDRCONFIG: Returned address types are determined by the types\nof addresses supported by the current system. For example, IPv4 addresses\nare only returned if the current system has at least one IPv4 address\nconfigured. Loopback addresses are not considered.dns.V4MAPPED: If the IPv6 family was specified, but no IPv6 addresses were\nfound, then return IPv4 mapped IPv6 addresses. Note that it is not supported\non some operating systems (e.g FreeBSD 10.1).Resolves the given address and port into a hostname and service using\nthe operating system's underlying getnameinfo implementation.
If address is not a valid IP address, a TypeError will be thrown.\nThe port will be coerced to a number. If it is not a legal port, a TypeError\nwill be thrown.
On an error, err is an Error object, where err.code is the error code.
const dns = require('dns');\ndns.lookupService('127.0.0.1', 22, (err, hostname, service) => {\n console.log(hostname, service);\n // Prints: localhost ssh\n});\nIf this method is invoked as its util.promisify()ed version, it returns a\nPromise for an object with hostname and service properties.
Uses the DNS protocol to resolve a hostname (e.g. 'nodejs.org') into an array\nof the resource records. The callback function has arguments\n(err, records). When successful, records will be an array of resource\nrecords. The type and structure of individual results varies based on rrtype:
rrtype | \nrecords contains | \nResult type | \nShorthand method | \n
|---|---|---|---|
'A' | \nIPv4 addresses (default) | \n{string} | \ndns.resolve4() | \n
'AAAA' | \nIPv6 addresses | \n{string} | \ndns.resolve6() | \n
'CNAME' | \ncanonical name records | \n{string} | \ndns.resolveCname() | \n
'MX' | \nmail exchange records | \n{Object} | \ndns.resolveMx() | \n
'NAPTR' | \nname authority pointer records | \n{Object} | \ndns.resolveNaptr() | \n
'NS' | \nname server records | \n{string} | \ndns.resolveNs() | \n
'PTR' | \npointer records | \n{string} | \ndns.resolvePtr() | \n
'SOA' | \nstart of authority records | \n{Object} | \ndns.resolveSoa() | \n
'SRV' | \nservice records | \n{Object} | \ndns.resolveSrv() | \n
'TXT' | \ntext records | \n{string} | \ndns.resolveTxt() | \n
On error, err is an Error object, where err.code is one of the\nDNS error codes.
Uses the DNS protocol to resolve a IPv4 addresses (A records) for the\nhostname. The addresses argument passed to the callback function\nwill contain an array of IPv4 addresses (e.g.\n['74.125.79.104', '74.125.79.105', '74.125.79.106']).
Uses the DNS protocol to resolve a IPv6 addresses (AAAA records) for the\nhostname. The addresses argument passed to the callback function\nwill contain an array of IPv6 addresses.
Uses the DNS protocol to resolve CNAME records for the hostname. The\naddresses argument passed to the callback function\nwill contain an array of canonical name records available for the hostname\n(e.g. ['bar.example.com']).
Uses the DNS protocol to resolve mail exchange records (MX records) for the\nhostname. The addresses argument passed to the callback function will\ncontain an array of objects containing both a priority and exchange\nproperty (e.g. [{priority: 10, exchange: 'mx.example.com'}, ...]).
Uses the DNS protocol to resolve regular expression based records (NAPTR\nrecords) for the hostname. The addresses argument passed to the callback\nfunction will contain an array of objects with the following properties:
flagsserviceregexpreplacementorderpreferenceFor example:
\n\n{\n flags: 's',\n service: 'SIP+D2U',\n regexp: '',\n replacement: '_sip._udp.example.com',\n order: 30,\n preference: 100\n}\nUses the DNS protocol to resolve name server records (NS records) for the\nhostname. The addresses argument passed to the callback function will\ncontain an array of name server records available for hostname\n(e.g. ['ns1.example.com', 'ns2.example.com']).
Uses the DNS protocol to resolve pointer records (PTR records) for the\nhostname. The addresses argument passed to the callback function will\nbe an array of strings containing the reply records.
Uses the DNS protocol to resolve a start of authority record (SOA record) for\nthe hostname. The address argument passed to the callback function will\nbe an object with the following properties:
nsnamehostmasterserialrefreshretryexpireminttl{\n nsname: 'ns.example.com',\n hostmaster: 'root.example.com',\n serial: 2013101809,\n refresh: 10000,\n retry: 2400,\n expire: 604800,\n minttl: 3600\n}\nUses the DNS protocol to resolve service records (SRV records) for the\nhostname. The addresses argument passed to the callback function will\nbe an array of objects with the following properties:
priorityweightportname{\n priority: 10,\n weight: 5,\n port: 21223,\n name: 'service.example.com'\n}\nUses the DNS protocol to resolve text queries (TXT records) for the\nhostname. The addresses argument passed to the callback function is\nis a two-dimensional array of the text records available for hostname (e.g.,\n[ ['v=spf1 ip4:0.0.0.0 ', '~all' ] ]). Each sub-array contains TXT chunks of\none record. Depending on the use case, these could be either joined together or\ntreated separately.
Performs a reverse DNS query that resolves an IPv4 or IPv6 address to an\narray of hostnames.
\nOn error, err is an Error object, where err.code is\none of the DNS error codes.
Sets the IP addresses of the servers to be used when resolving. The servers\nargument is an array of IPv4 or IPv6 addresses.
If a port is specified on the address, it will be removed.
\nAn error will be thrown if an invalid address is provided.
\nThe dns.setServers() method must not be called while a DNS query is in\nprogress.
Each DNS query can return one of the following error codes:
\ndns.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.Although dns.lookup() and the various dns.resolve*()/dns.reverse()\nfunctions have the same goal of associating a network name with a network\naddress (or vice versa), their behavior is quite different. These differences\ncan have subtle but significant consequences on the behavior of Node.js\nprograms.
Under the hood, dns.lookup() uses the same operating system facilities\nas most other programs. For instance, dns.lookup() will almost always\nresolve a given name the same way as the ping command. On most POSIX-like\noperating systems, the behavior of the dns.lookup() function can be\nmodified by changing settings in nsswitch.conf(5) and/or resolv.conf(5),\nbut note that changing these files will change the behavior of all other\nprograms running on the same operating system.
Though the call to dns.lookup() will be asynchronous from JavaScript's\nperspective, it is implemented as a synchronous call to getaddrinfo(3) that\nruns on libuv's threadpool. Because libuv's threadpool has a fixed size, it\nmeans that if for whatever reason the call to getaddrinfo(3) takes a long\ntime, other operations that could run on libuv's threadpool (such as filesystem\noperations) will experience degraded performance. In order to mitigate this\nissue, one potential solution is to increase the size of libuv's threadpool by\nsetting the 'UV_THREADPOOL_SIZE' environment variable to a value greater than\n4 (its current default value). For more information on libuv's threadpool, see\nthe official libuv documentation.
These functions are implemented quite differently than dns.lookup(). They\ndo not use getaddrinfo(3) and they always perform a DNS query on the\nnetwork. This network communication is always done asynchronously, and does not\nuse libuv's threadpool.
As a result, these functions cannot have the same negative impact on other\nprocessing 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()\nuses. For instance, they do not use the configuration from /etc/hosts.