# DNS Stability: 3 - Stable Use `require('dns')` to access this module. All methods in the dns module use C-Ares except for `dns.lookup` which uses `getaddrinfo(3)` in a thread pool. C-Ares is much faster than `getaddrinfo` but the system resolver is more constant with how other programs operate. When a user does `net.connect(80, 'google.com')` or `http.get({ host: 'google.com' })` the `dns.lookup` method is used. Users who need to do a large number of look ups quickly should use the methods that go through C-Ares. Here is an example which resolves `'www.google.com'` then reverse resolves the IP addresses which are returned. var dns = require('dns'); dns.resolve4('www.google.com', function (err, addresses) { if (err) throw err; console.log('addresses: ' + JSON.stringify(addresses)); addresses.forEach(function (a) { dns.reverse(a, function (err, domains) { if (err) { throw err; } console.log('reverse for ' + a + ': ' + JSON.stringify(domains)); }); }); }); ## dns.lookup(domain, [family], callback) Resolves a domain (e.g. `'google.com'`) into the first found A (IPv4) or AAAA (IPv6) record. The `family` can be the integer `4` or `6`. Defaults to `null` that indicates both Ip v4 and v6 address family. The callback has arguments `(err, address, family)`. The `address` argument is a string representation of a IP v4 or v6 address. The `family` argument is either the integer 4 or 6 and denotes the family of `address` (not necessarily the value initially passed to `lookup`). 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 domain does not exist but also when the lookup fails in other ways such as no available file descriptors. ## dns.resolve(domain, [rrtype], callback) Resolves a domain (e.g. `'google.com'`) into an array of the record types specified by rrtype. Valid rrtypes 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) and `'CNAME'` (canonical name records). 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. On error, `err` is an `Error` object, where `err.code` is one of the error codes listed below. ## dns.resolve4(domain, callback) The same as `dns.resolve()`, but only for IPv4 queries (`A` records). `addresses` is an array of IPv4 addresses (e.g. `['74.125.79.104', '74.125.79.105', '74.125.79.106']`). ## dns.resolve6(domain, callback) The same as `dns.resolve4()` except for IPv6 queries (an `AAAA` query). ## dns.resolveMx(domain, 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'},...]`). ## dns.resolveTxt(domain, callback) The same as `dns.resolve()`, but only for text queries (`TXT` records). `addresses` is an array of the text records available for `domain` (e.g., `['v=spf1 ip4:0.0.0.0 ~all']`). ## dns.resolveSrv(domain, callback) The same as `dns.resolve()`, but only for service records (`SRV` records). `addresses` is an array of the SRV records available for `domain`. Properties of SRV records are priority, weight, port, and name (e.g., `[{'priority': 10, {'weight': 5, 'port': 21223, 'name': 'service.example.com'}, ...]`). ## dns.resolveNs(domain, 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 `domain` (e.g., `['ns1.example.com', 'ns2.example.com']`). ## dns.resolveCname(domain, 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 `domain` (e.g., `['bar.example.com']`). ## dns.reverse(ip, callback) Reverse resolves an ip address to an array of domain names. The callback has arguments `(err, domains)`. On error, `err` is an `Error` object, where `err.code` is one of the error codes listed below. ## Error codes 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 domain name. - `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.