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!

[NotVivek12]

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

Absolutely, happy to add a test. Did you have a specific scenario in mind, or should I just cover the case from the original bug report?

[NotVivek12]

Sure, I've added regression tests for this! Created two test files:

test-dns-resolvesrv.js- basic SRV resolution tests
test-dns-resolvesrv-econnrefused.js- specifically tests the ECONNREFUSED regression with MongoDB Atlas-style SRV lookups
Also added SRV record support to dns.js so we can mock SRV responses.

[addaleax]

Thank you @NotVivek12! This largely looks good, just have two questions here

[addaleax]

^ This file was included unintentionally, right?

[addaleax]

Do we need to check for host/network byte order conversion here like we do for IPv4 (via htonl)?

[codecov]

Codecov Report

❌ Patch coverage is 11.11111% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.79%. Comparing base (83893bb) to head (2a528e2).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
src/cares_wrap.cc	11.11%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61453      +/-   ##
==========================================
+ Coverage   89.78%   89.79%   +0.01%     
==========================================
  Files         672      672              
  Lines      203809   203814       +5     
  Branches    39183    39179       -4     
==========================================
+ Hits       182980   183020      +40     
+ Misses      13166    13113      -53     
- Partials     7663     7681      +18     

Files with missing lines	Coverage Δ
src/cares_wrap.cc	61.73% <11.11%> (+1.42%)	⬆️

... and 36 files with indirect coverage changes

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