Use Windows API to get dns nameservers

[blink1073]

Fixes #1191

As far as I can tell the only two ways of getting the search list are using COM or the registry, so I subclassed the registry getter.

Testing with a PR against my fork so I can run actions: blink1073#1

• Get it to pass in GitHub Actions
• Get it to pass in PyMongo CI
• Establish how to configure the usage of this getter

[blink1073]

@jaraco might you be able to verify that this fixes the issue for you as well? For PyMongo the relevant change was that we needed to ignore a loopback adapter.

[rthalley]

This patch is looking good so far. Ideally we want all three DnsInfo attributes. You've gotten the nameservers attribute. The DnsSuffix field is the domain attribute of DnsInfo (if not NULL or an empty string); if NULL or empty, the DnsInfo value should be None. The list at FirstDnsSuffix at the very bottom of the structure is what should go in the search field (or empty list if not there).

[blink1073]

I did some more digging, and the FirstDnsSuffix is only set if the adapter is configured directly. I'm getting a null pointer for the value. If it is null, we need to fall back to using the registry. Here's what ipconfig looks like on our EC2 host:

$ ipconfig /all

Windows IP Configuration

   Host Name . . . . . . . . . . . . : <snip>
   Primary Dns Suffix  . . . . . . . :
   Node Type . . . . . . . . . . . . : Hybrid
   IP Routing Enabled. . . . . . . . : No
   WINS Proxy Enabled. . . . . . . . : No
   DNS Suffix Search List. . . . . . : us-east-1.ec2-utilities.amazonaws.com
                                       ec2.internal

Ethernet adapter Ethernet 4:

   Connection-specific DNS Suffix  . : ec2.internal
   Description . . . . . . . . . . . : Amazon Elastic Network Adapter #2
  <snip>
  DNS Servers . . . . . . . . . . . : 10.122.0.2

[blink1073]

Actually, I can't find any indication that FirstDnsSuffix is what we want, but we know that the registry approach works, so I think the best compromise is to use the registry for the search list unconditionally.