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

[NotVivek12]

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

Summary

This PR fixes a regression introduced in Node.js v24.13.0 on Windows where DNS SRV lookups (commonly used by MongoDB connection strings) fail with ECONNREFUSED.

The issue stems from a change in c-ares behavior where the fallback resolver (loopback) is reported with port 53 rather than port 0. The existing Node.js glue layer strictly expected port 0 to identify a fallback scenario. Consequently, Node.js failed to detect the fallback, attempting to query a non-listening local stub resolver at 127.0.0.1:53 or [::1]:53.

Changes

1. **Updated ChannelWrap::EnsureServers**: Modified logic to treat any single loopback server (IPv4 127.0.0.1 or IPv6 ::1) as a fallback configuration, regardless of the port number reported by c-ares.
2. Proactive Validation: Added a call to EnsureServers() immediately before dispatching queries in the Query template to ensure the channel is correctly initialized before use.

Regression Details

• Affected Version: Node.js v24.13.0
• Platform: Windows 11 / Server
• Error: Error: querySrv ECONNREFUSED
• Last Known Good: v24.11.1

Reproduction

This issue is 100% reproducible on Windows with Node v24.13.0 using the following script (mimicking a standard Mongoose connection):

const express = require("express");
const mongoose = require("mongoose");
const app = express();

const connectDB = async () => {
  try {
    // Replace with a valid SRV URI
    const conn = await mongoose.connect("mongodb+srv://<user>:<pass>@cluster0.example.mongodb.net/db");
    console.log(`MongoDB Connected: ${conn.connection.host}`);
  } catch (error) {
    console.error('MongoDB connection failed:', error.message);
    process.exit(1);
  }
};
connectDB();

app.listen(3300, () => {
  console.log("mongodb connected at port 3300");
});

Output on v24.13.0:
Error: querySrv ECONNREFUSED _mongodb._tcp.cluster0.example.mongodb.net

Manual Verification

1. Built Node.js from source on Windows (VS 2022 Build Tools).
2. Ran the reproduction script above; connection succeeds.
3. Verified dns.getServers() no longer returns a single loopback address when the system resolver is active.

Risks

Low.

• The logic only activates when c-ares discovers a single loopback server and the user has not manually called setServers().
• If setServers() is called, is_servers_default_ becomes false, skipping this check entirely.
• The logic is platform-agnostic but primarily resolves the specific behavior observed on Windows.

Workaround for users

Users on v24.13.0 can temporarily work around this by forcing DNS servers before connection:

require('dns').setServers(['8.8.8.8', '1.1.1.1']);

---

Checklist

• Fixes regression in dns / c-ares
• Tested on Windows
• Tests added/verified (Manual verification performed)

[nodejs-github-bot]

Review requested:

• @nodejs/net

[Ethan-Arrowood]

Do you think you could add a tests so we don't see this regression again in the future? Thanks!