dns: add setDefaultResolver and getDefaultResolver

doc/api/dns.md

@@ -103,6 +103,32 @@ added: REPLACEME
 Cancel all outstanding DNS queries made by this resolver. The corresponding
 callbacks will be called with an error with code `ECANCELLED`.
 
+## dns.getDefaultResolver()
+<!-- YAML
+added: REPLACEME
+-->
+
+Get the default resolver which may used by Node.js modules such as `http` and
+provides functions like `dns.resolve()`, `dns.setServers()`, etc.
+
+## dns.setDefaultResolver()
+<!-- YAML
+added: REPLACEME
+-->
+
+Set the default resolver which may used by Node.js modules such as `http` and
+provides functions like `dns.resolve()`, `dns.setServers()`, etc.
+
+The only parameter must be a `dns.Resolver` instance that to replace the default
+one.
+
+For example:
+
+```js
+const resolver = new dns.Resolver();
+dns.setDefaultResolver(resolver);
+```
+
 ## dns.getServers()
 <!-- YAML
 added: v0.11.3

lib/dns.js

@@ -75,6 +75,23 @@ const digits = [
   0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 96-111
   0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0  // 112-127
 ];
+const DEFAULT_RESOLVER_EXPORTED_FUNCTIONS = [
+  'getServers',
+  'setServers',
+  'resolve',
+  'resolveAny',
+  'resolve4',
+  'resolve6',
+  'resolveCname',
+  'resolveMx',
+  'resolveNs',
+  'resolveTxt',
+  'resolveSrv',
+  'resolvePtr',
+  'resolveNaptr',
+  'resolveSoa',
+  'reverse'
+];
 function isIPv4(str) {
   if (!digits[str.charCodeAt(0)]) return false;
   if (str.length === 1) return false;
@@ -377,28 +394,30 @@ function setServers(servers) {
   }
 }
 
-const defaultResolver = new Resolver();
+let defaultResolver;
+
+function getDefaultResolver() {
+  return defaultResolver;
+}
+
+function setDefaultResolver(resolver) {
+  if (!(resolver instanceof Resolver)) {
+    throw new errors.TypeError('ERR_INVALID_DNS_RESOLVER');
+  }
+
+  defaultResolver = resolver;
+  DEFAULT_RESOLVER_EXPORTED_FUNCTIONS.forEach((key) => {
+    module.exports[key] = defaultResolver[key].bind(defaultResolver);
+  });
+}
 
 module.exports = {
   lookup,
   lookupService,
 
   Resolver,
-  getServers: defaultResolver.getServers.bind(defaultResolver),
-  setServers: defaultResolver.setServers.bind(defaultResolver),
-  resolve: defaultResolver.resolve.bind(defaultResolver),
-  resolveAny: defaultResolver.resolveAny.bind(defaultResolver),
-  resolve4: defaultResolver.resolve4.bind(defaultResolver),
-  resolve6: defaultResolver.resolve6.bind(defaultResolver),
-  resolveCname: defaultResolver.resolveCname.bind(defaultResolver),
-  resolveMx: defaultResolver.resolveMx.bind(defaultResolver),
-  resolveNs: defaultResolver.resolveNs.bind(defaultResolver),
-  resolveTxt: defaultResolver.resolveTxt.bind(defaultResolver),
-  resolveSrv: defaultResolver.resolveSrv.bind(defaultResolver),
-  resolvePtr: defaultResolver.resolvePtr.bind(defaultResolver),
-  resolveNaptr: defaultResolver.resolveNaptr.bind(defaultResolver),
-  resolveSoa: defaultResolver.resolveSoa.bind(defaultResolver),
-  reverse: defaultResolver.reverse.bind(defaultResolver),
+  setDefaultResolver,
+  getDefaultResolver,
 
   // uv_getaddrinfo flags
   ADDRCONFIG: cares.AI_ADDRCONFIG,
@@ -430,3 +449,5 @@ module.exports = {
   ADDRGETNETWORKPARAMS: 'EADDRGETNETWORKPARAMS',
   CANCELLED: 'ECANCELLED'
 };
+
+setDefaultResolver(new Resolver());

lib/internal/errors.js

@@ -130,6 +130,7 @@ E('ERR_INVALID_CALLBACK', 'Callback must be a function');
 E('ERR_INVALID_CHAR', 'Invalid character in %s');
 E('ERR_INVALID_CURSOR_POS',
   'Cannot set cursor row without setting its column');
+E('ERR_INVALID_DNS_RESOLVER', 'Not a DNS resolver instance');
 E('ERR_INVALID_DOMAIN_NAME', 'Unable to determine the domain name');
 E('ERR_INVALID_FD', '"fd" must be a positive integer: %s');
 E('ERR_INVALID_FILE_URL_HOST',

test/parallel/test-dns-set-default-resolver.js

@@ -0,0 +1,68 @@
+'use strict';
+
+const common = require('../common');
+const dnstools = require('../common/dns');
+const dns = require('dns');
+const assert = require('assert');
+const dgram = require('dgram');
+
+const answers = [
+  { type: 'A', address: '1.2.3.4', ttl: 123 },
+  { type: 'AAAA', address: '::42', ttl: 123 },
+  { type: 'MX', priority: 42, exchange: 'foobar.com', ttl: 124 },
+  { type: 'NS', value: 'foobar.org', ttl: 457 },
+  { type: 'TXT', entries: [ 'v=spf1 ~all', 'xyz' ] },
+  { type: 'PTR', value: 'baz.org', ttl: 987 },
+  {
+    type: 'SOA',
+    nsname: 'ns1.example.com',
+    hostmaster: 'admin.example.com',
+    serial: 156696742,
+    refresh: 900,
+    retry: 900,
+    expire: 1800,
+    minttl: 60
+  },
+];
+
+const server = dgram.createSocket('udp4');
+
+server.on('message', common.mustCall((msg, { address, port }) => {
+  const parsed = dnstools.parseDNSPacket(msg);
+  const domain = parsed.questions[0].domain;
+  assert.strictEqual(domain, 'example.org');
+
+  server.send(dnstools.writeDNSPacket({
+    id: parsed.id,
+    questions: parsed.questions,
+    answers: answers.map((answer) => Object.assign({ domain }, answer)),
+  }), port, address);
+}));
+
+server.bind(0, common.mustCall(() => {
+  const address = server.address();
+  dns.setServers([`127.0.0.1:${address.port + 1}`]);
+  const resolver = new dns.Resolver();
+  resolver.setServers([`127.0.0.1:${address.port}`]);
+  dns.setDefaultResolver(resolver);
+
+  dns.resolveAny('example.org', common.mustCall((err, res) => {
+    assert.ifError(err);
+
+    // Compare copies with ttl removed, c-ares fiddles with that value.
+    assert.deepStrictEqual(
+      res.map((r) => Object.assign({}, r, { ttl: null })),
+      answers.map((r) => Object.assign({}, r, { ttl: null })));
+    server.close();
+
+    assert.strictEqual(dns.getDefaultResolver(), resolver);
+
+    common.expectsError(function() {
+      dns.setDefaultResolver(123);
+    }, {
+      code: 'ERR_INVALID_DNS_RESOLVER',
+      type: TypeError,
+      message: /^Not a DNS resolver instance$/
+    })
+  }));
+}));