Resolve the localhost name family to loopback on Android (RFC 6761)

[simonrozsival]

Note

This pull request was authored with the assistance of GitHub Copilot.

Summary

On Android, the localhost name family did not resolve to loopback as required by RFC 6761 §6.3:

• localhost could return non-loopback addresses (interface addresses leaked into the result).
• localhost. (fully-qualified form with a trailing dot) and *.localhost subdomains failed with SocketException because Android's getaddrinfo does not strip the trailing dot and has no /etc/hosts entry for subdomains.

This PR makes the whole localhost family (localhost, localhost., *.localhost, *.localhost.) resolve to the loopback address, consistent with desktop glibc/Windows behavior.

Changes

Native (src/native/libs/System.Native/pal_networking.c) — Android-only

SystemNative_GetHostEntryForName augments getaddrinfo results with local interface addresses (from getifaddrs) when the queried name equals gethostname(). On Android, gethostname() can itself return "localhost", which caused non-loopback interface addresses to pollute localhost resolution. A new helper ShouldAugmentWithInterfaceAddresses skips augmentation for localhost on Android only.

Managed (src/libraries/System.Net.NameResolution/src/System/Net/Dns.cs) — all platforms

• Normalize an exact localhost. → localhost (NormalizeLocalhostName) at both the sync and async resolution cores, so the fully-qualified form resolves identically to localhost. This is platform-agnostic per the RFC; it only changes behavior on resolvers that don't strip the trailing dot themselves (Android), and is a no-op on glibc/Windows which already resolve both.
• RFC 6761 fallback for *.localhost subdomains: try the OS resolver first; if it fails or returns no addresses, fall back to resolving plain localhost (IsLocalhostSubdomain). The fallback target localhost is not a subdomain, so the fallback cannot recurse.

Tests (tests/FunctionalTests/GetHostAddressesTest.cs, GetHostEntryTest.cs)

*.localhost subdomains now require loopback on Android (requireLoopback = !PlatformDetection.IsAppleMobile), and added/clarified localhost.-with-trailing-dot and subdomain-with-trailing-dot coverage.

Validation

• Android emulator (API 36, Mono arm64): 107 passed / 0 failed; all localhost-family tests pass, including *_LocalhostWithTrailingDot_ReturnsLoopback (previously failing).
• macOS host: 131 / 0 failed — no desktop regression.

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[simonrozsival]

/azp run runtime-android

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[Copilot]

Pull request overview

This PR adjusts name resolution behavior so the localhost name family behaves consistently with RFC 6761 expectations on Android, and adds managed normalization/fallback logic to make localhost. and *.localhost variants resolve more reliably across platforms.

Changes:

• Native (Android-only): prevents interface-address augmentation from polluting localhost results when Android’s gethostname() is "localhost".
• Managed (all platforms): normalizes exact "localhost." → "localhost" and falls back from *.localhost to plain localhost when the OS resolver fails or returns no addresses.
• Tests: updates functional tests to require loopback for *.localhost on Android (while still allowing Apple mobile exceptions) and adds/clarifies trailing-dot coverage.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/native/libs/System.Native/pal_networking.c	Adds ShouldAugmentWithInterfaceAddresses to skip Android augmentation for localhost, preventing non-loopback leakage.
src/libraries/System.Net.NameResolution/src/System/Net/Dns.cs	Normalizes localhost. and implements RFC 6761-style fallback for *.localhost when OS resolution fails/returns empty.
src/libraries/System.Net.NameResolution/tests/FunctionalTests/GetHostEntryTest.cs	Updates localhost-subdomain expectations (Android now requires loopback) and documents/validates localhost. behavior.
src/libraries/System.Net.NameResolution/tests/FunctionalTests/GetHostAddressesTest.cs	Same as above for GetHostAddresses, including trailing-dot and subdomain coverage.

[github-actions]

System.Net Code Review — DNS / RFC 6761 localhost resolution on Android

Protocols: DNS resolution (RFC 6761 §6.3)
Platforms: Android (native fix), all platforms (managed normalization)

Summary

Severity	Finding
💡 Trivial	Non-Android Linux with hostname "localhost" would still augment — pre-existing, not a realistic config

Analysis

The two-layer fix is well-designed:

1. Native (pal_networking.c) — ShouldAugmentWithInterfaceAddresses correctly prevents interface addresses from leaking into localhost resolution when Android's gethostname() returns "localhost". The #ifdef TARGET_ANDROID guard is appropriately scoped.

2. Managed (Dns.cs) — NormalizeLocalhostName strips the trailing dot from "localhost." before calling the OS resolver. This is correct per DNS conventions (trailing dot = FQDN root notation) and fixes Android's getaddrinfo("localhost.") failure without regressing glibc/Windows which already handle both forms.

RFC 6761 compliance: ✅ Section 6.3 states localhost names SHOULD resolve to loopback. Both the FQDN normalization and the subdomain fallback correctly implement this.

Security: ✅ The change tightens behavior — prevents non-loopback addresses from appearing in localhost resolution.

Edge cases: The native strcasecmp comparisons are safe (constant strings, no buffer concerns). NormalizeLocalhostName is pure/stateless — no concurrency issues.

Tests: ✅ Android exclusions properly removed; loopback is now asserted on Android.

Verdict

✅ LGTM — Clean, well-documented fix with correct RFC references and appropriate platform scoping.

Generated by Code Review for issue #128771 · ● 4.4M · ◷