Draft: RFC 6761 handling for invalid and *.localhost

[gbr-mendes]

This is a draft PR for issue #118569.

Motivation

RFC 6761 defines special-use domain names:

• invalid and *.invalid must always return NXDOMAIN
• localhost and *.localhost must always resolve to the loopback address

Currently, .NET relies fully on the OS resolver, which may allow invalid to resolve if defined in /etc/hosts or DNS, and may not handle *.localhost consistently across platforms.

Proposed approach

• Intercept invalid and *.invalid before OS resolution → throw SocketException with HostNotFound (simulate NXDOMAIN).
• Intercept localhost and *.localhost before OS resolution → return IPAddress.Loopback and IPAddress.IPv6Loopback.
• Otherwise, fallback to the platform resolver as today.

Status

• Draft PR to validate the idea/approach with maintainers.
• No tests included yet.
• Once approach is confirmed, I’ll proceed with the real implementation, unit tests and CI validation.

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