On Windows, dns resolves using the secondary nameserver

[spazmodius]

Version

v16.14.0

Platform

Microsoft Windows NT 10.0.19044.0 x64

Subsystem

dns

What steps will reproduce the bug?

My Primary DNS server configures for 127.0.0.1, a local instance of SmartDns. My secondary DNS server is my ISP's.

However, Node resolves using the secondary DNS server, not the primary:

$ node -e 'dns.resolve4("www.google.com", { ttl: true }, console.log);'
null [
  { address: '142.250.138.105', ttl: 63 },
  { address: '142.250.138.106', ttl: 63 },
  { address: '142.250.138.99', ttl: 63 },
  { address: '142.250.138.147', ttl: 63 },
  { address: '142.250.138.104', ttl: 63 },
  { address: '142.250.138.103', ttl: 63 }
]

I can tell because of the short TTLs, whereas my SmartDns server is configured with a minimum TTL of 2400. Thus:

$ node -e 'dns.setServers(["127.0.0.1"]); dns.resolve4("www.google.com", { ttl: true }, console.log);'
null [ { address: '172.217.13.4', ttl: 2397 } ]

And as confirmation:

$ node -e 'console.log(dns.getServers());'
[ '12.127.16.67', '127.0.0.1' ]

How often does it reproduce? Is there a required condition?

Always sorts local addresses after remote addresses. If both Primary and Secondary are remote, then re-ordering does not happen.

What is the expected behavior?

I'd expect dns.getServers() to return [ 'primary', 'secondary' ], and for name resolution to prefer the Primary DNS server.

What do you see instead?

dns.getServers() reverse the primary and secondary DNS servers, and resolution prefers the secondary server.

Additional information

[bnoordhuis]

It's a feature of c-ares, the resolver library: it sorts the DNS servers according to routing weight.

node/deps/cares/src/lib/ares_init.c

Lines 1158 to 1163 in 4508c8c

	/* For each interface, find any associated DNS servers as IPv4 or IPv6
	* addresses. For each found address, find the best route to that DNS
	* server address _on_ _that_ _interface_ (at this moment in time) and
	* compute the resulting total metric, just as Windows routing will do.
	* Then, sort all the addresses found by the metric.
	*/

There's no way to disable it but if it really bothers you, you could comment out the qsort() call a few lines below and recompile.

[spazmodius]

I suspected the same. But I replicated c-ares system calls and logic, and did not reproduce the reversal.

On my system, those DNS servers are returned (from GetAdaptersAddresses) in correct order, on the same interface, have the same metric (0, from GetBestRoute2), and qsort does not switch their order.

[spazmodius]

🤦🏼‍♂️ never mind. I am able to reproduce by calling into c-ares instead of (imperfectly, obviously) reconstructing the logic.