Enable DnsResolver tests on all Unix platforms

[rzikm]

Follow-up to #129846, which added DnsResolver with a managed stub resolver implementation.

That implementation is compiled through the shared -unix target framework, which is a catch-all for every Unix-family TargetOS that has no explicit entry in TargetFrameworks. System.Net.NameResolution lists only windows;unix;browser;wasi, so macOS, the BSDs, illumos, Haiku, iOS, tvOS, MacCatalyst and Android already compile the same managed resolver as Linux — no per-platform PAL work is needed to support them. The tests were the only thing still gated to Windows.

Test gating

• DnsResolverTest: the network tests move from IsWindows to "everywhere the resolver is implemented" (that is, everything except Android, Browser and WASI). Three tests that exercise genuinely DnsQueryEx-specific behavior stay Windows-only, and two Unix counterparts were added for the constructor-validation ones — the managed resolver talks to each server endpoint directly, so it accepts non-standard ports and mixed IPv4/IPv6 server lists that DnsQueryEx rejects.
• DnsResolverLoopbackTest: drops IsNotMobile. These tests pass an explicit Servers list and talk to an in-process loopback server, so they never touch the system resolver configuration and can run on Android and Apple mobile.
• DnsResolver_PreCanceledToken_ReturnsCanceled now asserts ThrowsAnyAsync<OperationCanceledException>. The managed PAL cancels via CancellationToken.ThrowIfCancellationRequested(), which produces a plain OperationCanceledException, whereas the Windows PAL produces a TaskCanceledException.

Android

Android is the one Unix platform that cannot use the system-configured servers. There is no accessible /etc/resolv.conf, and IPInterfaceProperties.DnsAddresses throws PlatformNotSupportedException there for the same reason, so ResolvConf.GetNameServers() comes back empty and GetServers() falls back to 127.0.0.1:53 — every query would silently time out against an unreachable address.

Rather than fail that way, DnsResolverPal.Managed.ValidateServers now rejects the configuration up front with PlatformNotSupportedException, and the affected surface is annotated:

• the parameterless DnsResolver() constructor
• all 18 Dns.Resolve* statics, which route through it

DnsResolver(DnsResolverOptions) with an explicit Servers list keeps working on Android, which is why the annotation cannot go on the type. Tracked by #132212.

Browser and WASI

DnsResolver is unsupported on both — they compile DnsResolverPal.Unsupported.cs (and on Browser the whole assembly is a generated PNSE stub). Browser needs no per-member attribute because Directory.Build.props already sets UnsupportedOSPlatforms=browser, which emits an assembly-level attribute; WASI was not covered by anything, so [UnsupportedOSPlatform("wasi")] is applied to the DnsResolver type and the Dns.Resolve* statics, following the Zstandard* precedent in System.IO.Compression.

Reusing the managed resolver on WASI is possible in principle — WASI does have TCP and UDP sockets — but it has no resolver configuration to read and no threads, so the synchronous query path (which blocks on IAsyncResult.AsyncWaitHandle.WaitOne and blocking Socket.Send/Receive) would deadlock the only thread. Tracked by #132215.

A new test asserts the PlatformNotSupportedException contract on each of Android, Browser and WASI. The argument-validation tests were switched to a resolver built with an explicit (never-contacted) server so that they keep running on Android instead of being skipped there.

API

No new API surface. The only ref/ changes are [UnsupportedOSPlatform] attributes on API already approved and merged in #129846.

Validation

Built and tested on Linux x64: System.Net.NameResolution.Functional.Tests — 201 total, 0 failed, 6 skipped. The library builds clean (0 warnings) across all five target frameworks, including -browser and -wasi, and cross-compiles clean for TargetOS=android, osx and maccatalyst.

The mobile, macOS, Browser and WASI legs could not be exercised locally, so CI is the first execution there.

Note

This pull request was authored with GitHub Copilot.

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[rzikm]

/azp run runtime-android

[rzikm]

/azp run runtime-extra-platforms

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[Copilot]

Pull request overview

Expands System.Net.NameResolution’s DnsResolver functional test coverage beyond Windows by removing overly strict platform gating, while clarifying and enforcing the unsupported/PNSE contract on platforms where the default (system-server) resolver path cannot work.

Changes:

• Enable DnsResolverTest network coverage on non-Android Unix platforms (and keep Browser/WASI excluded), while preserving Windows-only DnsQueryEx-specific behaviors.
• Enable DnsResolverLoopbackTest on mobile platforms by relying on explicit loopback servers (no system resolver configuration dependency).
• Enforce and document platform support contracts: throw early on Android when system DNS servers can’t be discovered, and annotate APIs as unsupported on Android (default constructor + Dns.Resolve*) and WASI (DnsResolver / Dns.Resolve*).

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/libraries/System.Net.NameResolution/tests/FunctionalTests/DnsResolverTest.cs	Broadens test gating to run on Unix platforms where DnsResolver is implemented; adds explicit unsupported-platform and Android contract tests.
src/libraries/System.Net.NameResolution/tests/FunctionalTests/DnsResolverLoopbackTest.cs	Removes IsNotMobile gating so loopback-based deterministic resolver tests run on mobile where applicable.
src/libraries/System.Net.NameResolution/src/System/Net/DnsResolverPal.Unsupported.cs	Adds clarifying comment about Browser/WASI lack of implementation and tracking issue.
src/libraries/System.Net.NameResolution/src/System/Net/DnsResolverPal.Managed.cs	Rejects Android default (system-server) configuration up front with a clear PlatformNotSupportedException.
src/libraries/System.Net.NameResolution/src/System/Net/DnsResolver.cs	Annotates DnsResolver as unsupported on WASI; marks parameterless ctor unsupported on Android and documents PNSE behavior.
src/libraries/System.Net.NameResolution/src/System/Net/Dns.Resolve.cs	Annotates Dns.Resolve* statics as unsupported on Android/WASI and documents PNSE contract.
src/libraries/System.Net.NameResolution/src/Resources/Strings.resx	Adds a dedicated SR message for “system-configured DNS servers cannot be determined”.
src/libraries/System.Net.NameResolution/ref/System.Net.NameResolution.cs	Updates public contract with [UnsupportedOSPlatform] attributes consistent with implementation.

[rzikm]

/azp run runtime-android

[rzikm]

/azp run runtime-extra-platforms

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Azure Pipelines:
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[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/libraries/System.Net.NameResolution/tests/FunctionalTests/DnsResolverTest.cs:56

• The Android-specific PNSE contract is only asserted for the synchronous Dns.ResolveAddresses path. Since the change also affects the async static APIs (they route through the same default resolver), add an assertion that ResolveAddressesAsync throws PlatformNotSupportedException as well to prevent regressions specific to the async entry point.

            // Android exposes no readable resolver configuration, so a resolver that would
            // have to use the system-configured servers cannot be created.
            Assert.Throws<PlatformNotSupportedException>(() => new DnsResolver());
            Assert.Throws<PlatformNotSupportedException>(() => Dns.ResolveAddresses(TestHost));
        }