Add support for DNS in non-Windows environments

[SteveSyfuhs]

Is your feature request related to a problem? Please describe.
DNS lookups only work on Windows today because it calls into win32 APIs directly. .NET doesn't provide a way to query for SRV records.

Describe the solution you'd like
The library needs a way to make DNS queries outside of Windows. The solution should be to provide a way to bring your own DNS implementation that way you can use something like https://github.com/MichaCo/DnsClient.NET on other platforms.

Describe alternatives you've considered
You could rely completely on a krb5 configuration file to provide all the necessary records, but that's complicated and messy.

Additional context
Design should be similar to the CryptoPal so callers can register their DNS mechanisms. A side-car nuget package could be created that wires in the DnsClient.NET implementation.

[SteveSyfuhs]

For folks running into this issue, there's a super simple platform-independent implementation available in the Bruce tool. It uses the DnsClient.NET library as mentioned above. It's kept out of the main project reduce total external dependencies.

Kerberos.NET/Bruce/Dns/PlatformIndependentDnsClient.cs

Lines 1 to 50 in 946daea

	using System;
	using System.Collections.Generic;
	using System.Linq;
	using System.Threading.Tasks;
	using DnsClient;
	using Kerberos.NET.Dns;
	namespace Kerberos.NET.CommandLine.Dns
	{
	internal class PlatformIndependentDnsClient : IKerberosDnsQuery
	{
	private static readonly WindowsDnsQuery WindowsDns = new WindowsDnsQuery();
	public async Task<IReadOnlyCollection<DnsRecord>> Query(string query, DnsRecordType type)
	{
	if (WindowsDns.IsSupported)
	{
	return await WindowsDns.Query(query, type);
	}
	var client = new LookupClient();
	var response = await client.QueryAsync(query, (QueryType)type);
	var srvRecords = response.Answers.SrvRecords().Select(a => new DnsRecord
	{
	Name = a.DomainName,
	Port = a.Port,
	Priority = a.Priority,
	Target = a.Target,
	TimeToLive = a.TimeToLive,
	Type = DnsRecordType.SRV,
	Weight = a.Weight
	}).ToList();
	var merged = srvRecords.GroupBy(r => r.Name);
	foreach (var srv in srvRecords)
	{
	var c1 = merged.Where(m => m.Key.Equals(srv.Target, StringComparison.InvariantCultureIgnoreCase));
	var canon = c1.SelectMany(r => r);
	srv.Canonical = canon.ToList();
	}
	return srvRecords;
	}
	}
	}

[MageFroh]

This class is very useful, but a bit hard to find. Is there a NuGet package that includes it? Would be great to be able to include something like Kerberos.NET.Portable, instead of copying the class in all projects that need it.

[SteveSyfuhs]

PR's are accepted to get that working.

[0x5ECF4ULT]

As requested by the contribution guidelines: I'm working on it

[SteveSyfuhs]

Awesome! The build process is somewhat of a pain to verify, but it should just be a matter of duplicating the kerberos.net lines in the build file and replacing the names with the new project/package name (https://github.com/dotnet/Kerberos.NET/blob/develop/build.yaml)