dns: fix Windows SRV ECONNREFUSED by adjusting c-ares fallback detection

Newer c-ares versions set tcp_port/udp_port to 53 instead of 0,
which caused the loopback detection to fail. This fix:

- Removes the port check from loopback detection
- Adds IPv6 loopback (::1) support
- Calls EnsureServers() before each DNS query

PR-URL: https://github.com/nodejs/node/pull/61453
Reviewed-By: Anna Henningsen <anna@addaleax.net>

src/cares_wrap.cc

@@ -983,19 +983,31 @@ void ChannelWrap::EnsureServers() {
 
   ares_get_servers_ports(channel_, &servers);
 
-  /* if no server or multi-servers, ignore */
+  /* if no server, ignore */
   if (servers == nullptr) return;
+
+  /* if multi-servers, mark as non-default and ignore */
   if (servers->next != nullptr) {
     ares_free_data(servers);
     is_servers_default_ = false;
     return;
   }
 
-  /* if the only server is not 127.0.0.1, ignore */
-  if (servers[0].family != AF_INET ||
-      servers[0].addr.addr4.s_addr != htonl(INADDR_LOOPBACK) ||
-      servers[0].tcp_port != 0 ||
-      servers[0].udp_port != 0) {
+  /* Check if the only server is a loopback address (IPv4 127.0.0.1 or IPv6
+   * ::1). Newer c-ares versions may set tcp_port/udp_port to 53 instead of 0,
+   * so we no longer check port values. */
+  bool is_loopback = false;
+  if (servers[0].family == AF_INET) {
+    is_loopback = (servers[0].addr.addr4.s_addr == htonl(INADDR_LOOPBACK));
+  } else if (servers[0].family == AF_INET6) {
+    static const unsigned char kIPv6Loopback[16] = {
+        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1};
+    is_loopback =
+        (memcmp(&servers[0].addr.addr6, kIPv6Loopback, sizeof(kIPv6Loopback)) ==
+         0);
+  }
+
+  if (!is_loopback) {
     ares_free_data(servers);
     is_servers_default_ = false;
     return;
@@ -1769,6 +1781,10 @@ static void Query(const FunctionCallbackInfo<Value>& args) {
   node::Utf8Value utf8name(args.GetIsolate(), string);
   auto plain_name = utf8name.ToStringView();
   std::string name = ada::idna::to_ascii(plain_name);
+
+  // Ensure c-ares did not fall back to loopback resolver.
+  channel->EnsureServers();
+
   channel->ModifyActivityQueryCount(1);
   int err = wrap->Send(name.c_str());
   if (err) {

test/common/dns.js

@@ -13,6 +13,7 @@ const types = {
   PTR: 12,
   MX: 15,
   TXT: 16,
+  SRV: 33,
   ANY: 255,
   CAA: 257,
 };
@@ -279,6 +280,15 @@ function writeDNSPacket(parsed) {
         buffers.push(Buffer.from('issue' + rr.issue));
         break;
       }
+      case 'SRV':
+      {
+        // SRV record format: priority (2) + weight (2) + port (2) + target
+        const target = writeDomainName(rr.name);
+        rdLengthBuf[0] = 6 + target.length;
+        buffers.push(new Uint16Array([rr.priority, rr.weight, rr.port]));
+        buffers.push(target);
+        break;
+      }
       default:
         throw new Error(`Unknown RR type ${rr.type}`);
     }

test/parallel/test-dns-resolvesrv-econnrefused.js

@@ -0,0 +1,147 @@
+'use strict';
+// Regression test for SRV record resolution returning ECONNREFUSED.
+//
+// This test verifies that dns.resolveSrv() properly handles SRV queries
+// and doesn't incorrectly return ECONNREFUSED errors when DNS servers
+// are reachable but the query format or handling has issues.
+//
+// Background: In certain Node.js versions, SRV queries could fail with
+// ECONNREFUSED even when the DNS server was accessible, affecting
+// applications using MongoDB Atlas (mongodb+srv://) and other services
+// that rely on SRV record discovery.
+
+const common = require('../common');
+const dnstools = require('../common/dns');
+const dns = require('dns');
+const dnsPromises = dns.promises;
+const assert = require('assert');
+const dgram = require('dgram');
+
+// Test 1: Basic SRV resolution should succeed, not return ECONNREFUSED
+{
+  const server = dgram.createSocket('udp4');
+  const srvRecord = {
+    type: 'SRV',
+    name: 'mongodb-server.cluster0.example.net',
+    port: 27017,
+    priority: 0,
+    weight: 1,
+    ttl: 60,
+  };
+
+  server.on('message', common.mustCall((msg, { address, port }) => {
+    const parsed = dnstools.parseDNSPacket(msg);
+    const domain = parsed.questions[0].domain;
+
+    server.send(dnstools.writeDNSPacket({
+      id: parsed.id,
+      questions: parsed.questions,
+      answers: [Object.assign({ domain }, srvRecord)],
+    }), port, address);
+  }));
+
+  server.bind(0, common.mustCall(async () => {
+    const { port } = server.address();
+    const resolver = new dnsPromises.Resolver();
+    resolver.setServers([`127.0.0.1:${port}`]);
+
+    try {
+      const result = await resolver.resolveSrv(
+        '_mongodb._tcp.cluster0.example.net'
+      );
+
+      // Should NOT throw ECONNREFUSED
+      assert.ok(Array.isArray(result));
+      assert.strictEqual(result.length, 1);
+      assert.strictEqual(result[0].name, 'mongodb-server.cluster0.example.net');
+      assert.strictEqual(result[0].port, 27017);
+      assert.strictEqual(result[0].priority, 0);
+      assert.strictEqual(result[0].weight, 1);
+    } catch (err) {
+      // This is the regression: should NOT get ECONNREFUSED
+      assert.notStrictEqual(err.code, 'ECONNREFUSED');
+      throw err;
+    } finally {
+      server.close();
+    }
+  }));
+}
+
+// Test 2: Multiple SRV records (common for MongoDB Atlas clusters)
+{
+  const server = dgram.createSocket('udp4');
+  const srvRecords = [
+    { type: 'SRV', name: 'shard-00-00.cluster.mongodb.net', port: 27017, priority: 0, weight: 1, ttl: 60 },
+    { type: 'SRV', name: 'shard-00-01.cluster.mongodb.net', port: 27017, priority: 0, weight: 1, ttl: 60 },
+    { type: 'SRV', name: 'shard-00-02.cluster.mongodb.net', port: 27017, priority: 0, weight: 1, ttl: 60 },
+  ];
+
+  server.on('message', common.mustCall((msg, { address, port }) => {
+    const parsed = dnstools.parseDNSPacket(msg);
+    const domain = parsed.questions[0].domain;
+
+    server.send(dnstools.writeDNSPacket({
+      id: parsed.id,
+      questions: parsed.questions,
+      answers: srvRecords.map((r) => Object.assign({ domain }, r)),
+    }), port, address);
+  }));
+
+  server.bind(0, common.mustCall(async () => {
+    const { port } = server.address();
+    const resolver = new dnsPromises.Resolver();
+    resolver.setServers([`127.0.0.1:${port}`]);
+
+    const result = await resolver.resolveSrv('_mongodb._tcp.cluster.mongodb.net');
+
+    assert.strictEqual(result.length, 3);
+
+    const names = result.map((r) => r.name).sort();
+    assert.deepStrictEqual(names, [
+      'shard-00-00.cluster.mongodb.net',
+      'shard-00-01.cluster.mongodb.net',
+      'shard-00-02.cluster.mongodb.net',
+    ]);
+
+    server.close();
+  }));
+}
+
+// Test 3: Callback-based API should also work
+{
+  const server = dgram.createSocket('udp4');
+
+  server.on('message', common.mustCall((msg, { address, port }) => {
+    const parsed = dnstools.parseDNSPacket(msg);
+    const domain = parsed.questions[0].domain;
+
+    server.send(dnstools.writeDNSPacket({
+      id: parsed.id,
+      questions: parsed.questions,
+      answers: [{
+        domain,
+        type: 'SRV',
+        name: 'service.example.com',
+        port: 443,
+        priority: 10,
+        weight: 5,
+        ttl: 120,
+      }],
+    }), port, address);
+  }));
+
+  server.bind(0, common.mustCall(() => {
+    const { port } = server.address();
+    const resolver = new dns.Resolver();
+    resolver.setServers([`127.0.0.1:${port}`]);
+
+    resolver.resolveSrv('_https._tcp.example.com', common.mustSucceed((result) => {
+      assert.strictEqual(result.length, 1);
+      assert.strictEqual(result[0].name, 'service.example.com');
+      assert.strictEqual(result[0].port, 443);
+      assert.strictEqual(result[0].priority, 10);
+      assert.strictEqual(result[0].weight, 5);
+      server.close();
+    }));
+  }));
+}

test/parallel/test-dns-resolvesrv.js

@@ -0,0 +1,102 @@
+'use strict';
+// Regression test for dns.resolveSrv() functionality.
+// This test ensures SRV record resolution works correctly, which is
+// critical for services like MongoDB Atlas that use SRV records for
+// connection discovery (mongodb+srv:// URIs).
+//
+// Related issue: dns.resolveSrv() returning ECONNREFUSED instead of
+// properly resolving SRV records.
+
+const common = require('../common');
+const dnstools = require('../common/dns');
+const dns = require('dns');
+const dnsPromises = dns.promises;
+const assert = require('assert');
+const dgram = require('dgram');
+
+const srvRecords = [
+  {
+    type: 'SRV',
+    name: 'server1.example.org',
+    port: 27017,
+    priority: 0,
+    weight: 5,
+    ttl: 300,
+  },
+  {
+    type: 'SRV',
+    name: 'server2.example.org',
+    port: 27017,
+    priority: 0,
+    weight: 5,
+    ttl: 300,
+  },
+  {
+    type: 'SRV',
+    name: 'server3.example.org',
+    port: 27017,
+    priority: 1,
+    weight: 10,
+    ttl: 300,
+  },
+];
+
+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, '_mongodb._tcp.cluster0.example.org');
+
+  server.send(dnstools.writeDNSPacket({
+    id: parsed.id,
+    questions: parsed.questions,
+    answers: srvRecords.map((record) => Object.assign({ domain }, record)),
+  }), port, address);
+}, 2));  // Called twice: once for callback, once for promises
+
+server.bind(0, common.mustCall(async () => {
+  const address = server.address();
+  const resolver = new dns.Resolver();
+  const resolverPromises = new dnsPromises.Resolver();
+
+  resolver.setServers([`127.0.0.1:${address.port}`]);
+  resolverPromises.setServers([`127.0.0.1:${address.port}`]);
+
+  function validateResult(result) {
+    assert.ok(Array.isArray(result), 'Result should be an array');
+    assert.strictEqual(result.length, 3);
+
+    for (const record of result) {
+      assert.strictEqual(typeof record, 'object');
+      assert.strictEqual(typeof record.name, 'string');
+      assert.strictEqual(typeof record.port, 'number');
+      assert.strictEqual(typeof record.priority, 'number');
+      assert.strictEqual(typeof record.weight, 'number');
+      assert.strictEqual(record.port, 27017);
+    }
+
+    // Verify we got all expected server names
+    const names = result.map((r) => r.name).sort();
+    assert.deepStrictEqual(names, [
+      'server1.example.org',
+      'server2.example.org',
+      'server3.example.org',
+    ]);
+  }
+
+  // Test promises API
+  const promiseResult = await resolverPromises.resolveSrv(
+    '_mongodb._tcp.cluster0.example.org'
+  );
+  validateResult(promiseResult);
+
+  // Test callback API
+  resolver.resolveSrv(
+    '_mongodb._tcp.cluster0.example.org',
+    common.mustSucceed((result) => {
+      validateResult(result);
+      server.close();
+    })
+  );
+}));