Implement DnsResolver for Windows by rzikm · Pull Request #129302 · dotnet/runtime

rzikm · 2026-06-11T16:57:33Z

Implements the API approved in #19443:

New Dns.Resolve*[Async] static methods for A/AAAA/SRV/MX/TXT/CNAME/PTR/NS records.
New DnsResolver / DnsResolverOptions for instance-based resolution with optional custom DNS servers.
Record types AddressRecord, SrvRecord, MxRecord, TxtRecord, CNameRecord, PtrRecord, NsRecord.
DnsResponseCode enum and DnsResult envelope carrying ResponseCode, Records, and NegativeCacheTtl.

Windows implementation uses DnsQueryEx. Non-Windows platforms get PlatformNotSupportedException stubs pending follow-up implementations (splitting to multiple stacked PRs for easier review).

Implements the API approved in dotnet#19443: * New Dns.Resolve*[Async] static methods for A/AAAA/SRV/MX/TXT/CNAME/PTR/NS records. * New DnsResolver / DnsResolverOptions for instance-based resolution with optional custom DNS servers. * Record types AddressRecord, SrvRecord, MxRecord, TxtRecord, CNameRecord, PtrRecord, NsRecord. * DnsResponseCode enum and DnsResult<T> envelope carrying ResponseCode, Records, and NegativeCacheTtl. Windows implementation uses DnsQueryEx (DNS_QUERY_REQUEST v1 by default; DNS_QUERY_REQUEST3 when the caller supplies non-default ports on Windows 11 build 22000+, throwing PlatformNotSupportedException otherwise). Non-Windows platforms get PlatformNotSupportedException stubs pending follow-up implementations. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Adds argument-validation tests and Windows-only OuterLoop network tests for the new DnsResolver / Dns.Resolve* APIs. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

… port 53 DnsQueryEx only contacts custom DNS servers on the standard port 53 and requires the sockaddr port field to be 0; any non-zero port (even 53) is rejected with ERROR_INVALID_PARAMETER. Simplify the Windows resolver to always use the v1 DNS_QUERY_REQUEST path, write sockaddr port 0, and throw PlatformNotSupportedException for server endpoints requesting a non-default port. Remove the now-unused v3 custom-server code path. Add an in-process loopback DNS server (bound to 127.0.0.1:53, skipped when the port is unavailable) and a comprehensive behavioral test suite covering address/SRV/MX/TXT/CNAME/PTR/NS resolution, NXDOMAIN vs NODATA, TTLs, SRV additional-address glue, port-0 acceptance, and in-flight cancellation. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

The synchronous Resolve* methods previously blocked on the async path via GetAwaiter().GetResult(). DnsQueryEx executes synchronously when no completion callback is supplied, so call it directly instead of going through the async state machine. Record-list parsing is factored into shared helpers reused by both the sync and async paths, and the loopback behavioral tests are parameterized over both APIs. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Move all platform-specific logic from DnsResolver.Windows.cs and DnsResolver.WindowsAsync.cs into a new DnsResolverPal.Windows.cs static PAL class, mirroring the existing NameResolutionPal pattern. The cross-platform Resolve*Core methods remain on DnsResolver and delegate to the PAL, providing a seam for future instrumentation and telemetry. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Instrument the Resolve*Core seam in DnsResolver with the existing NameResolutionTelemetry infrastructure (EventSource counters, the DnsLookup Activity span, and the dns.lookup.duration metric), matching the static Dns class. When no diagnostics consumer is enabled the PAL task is returned directly, keeping the common path allocation-free and preserving the synchronous-completion invariant the sync Resolve* methods depend on. Extend NameResolutionActivity.Stop to accept a string[] answer so the new record types can populate the dns.answers tag. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

The Resolve*Core methods invoked the PAL eagerly and only then wrapped the resulting task with telemetry. On the synchronous path the Windows PAL executes the query while creating the task, so BeforeResolution ran after most of the work was done and the recorded duration excluded the query time. Defer the PAL invocation behind a Func so telemetry brackets the entire query for both sync and async paths. Adds a regression test asserting the recorded dns.lookup.duration covers a delayed server response on both sync and async paths. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

dotnet-policy-service · 2026-06-11T16:59:14Z

Tagging subscribers to this area: @karelz, @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

Copilot

Pull request overview

This PR adds a new DNS query API surface area to System.Net.NameResolution (new Dns.Resolve* static entry points plus an instance-based DnsResolver), and implements the Windows PAL using DnsQueryEx. It also introduces functional tests, including deterministic loopback-server-driven tests for record parsing/handling.

Changes:

Adds new public APIs: Dns.Resolve* (A/AAAA/SRV/MX/TXT/CNAME/PTR/NS), DnsResolver, DnsResolverOptions, record structs, DnsResult<T>, and DnsResponseCode.
Implements Windows DNS querying/parsing via DnsQueryEx (sync + async paths), plus non-Windows PNSE stubs.
Adds functional tests (outer-loop network tests + loopback DNS server tests) and extends NameResolution telemetry to handle string[] answers.

Reviewed changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated 18 comments.

Show a summary per file

File	Description
src/libraries/System.Net.NameResolution/tests/FunctionalTests/System.Net.NameResolution.Functional.Tests.csproj	Adds new functional test sources to the project
src/libraries/System.Net.NameResolution/tests/FunctionalTests/LoopbackDnsServer.cs	Adds an in-process UDP/TCP loopback DNS server for deterministic tests
src/libraries/System.Net.NameResolution/tests/FunctionalTests/DnsResponseBuilder.cs	Adds a fluent builder for DNS response bytes used by loopback tests
src/libraries/System.Net.NameResolution/tests/FunctionalTests/DnsResolverTest.cs	Adds outer-loop functional tests for the new APIs (real DNS)
src/libraries/System.Net.NameResolution/tests/FunctionalTests/DnsResolverLoopbackTest.cs	Adds deterministic loopback-server-driven tests for parsing/behavior/metrics
src/libraries/System.Net.NameResolution/src/System/Net/NameResolutionTelemetry.cs	Extends telemetry answer handling to support `string[]`
src/libraries/System.Net.NameResolution/src/System/Net/DnsResult.cs	Introduces `DnsResult<T>` envelope type
src/libraries/System.Net.NameResolution/src/System/Net/DnsResponseCode.cs	Introduces `DnsResponseCode` enum
src/libraries/System.Net.NameResolution/src/System/Net/DnsResolverPal.Windows.cs	Implements Windows PAL via DnsQueryEx, record parsing, cancellation, glue, TTL extraction
src/libraries/System.Net.NameResolution/src/System/Net/DnsResolverPal.Unsupported.cs	Adds non-Windows PNSE PAL stubs
src/libraries/System.Net.NameResolution/src/System/Net/DnsResolverOptions.cs	Adds options type for custom server selection
src/libraries/System.Net.NameResolution/src/System/Net/DnsResolver.cs	Adds instance-based resolver API + telemetry wrapping
src/libraries/System.Net.NameResolution/src/System/Net/DnsRecords.cs	Adds record structs (Address/Srv/Mx/Txt/CName/Ptr/Ns)
src/libraries/System.Net.NameResolution/src/System/Net/Dns.Resolve.cs	Adds new static `Dns.Resolve*` APIs backed by a default resolver
src/libraries/System.Net.NameResolution/src/System/Net/Dns.cs	Makes `Dns` partial to host the new `Dns.Resolve*` partial
src/libraries/System.Net.NameResolution/src/System.Net.NameResolution.csproj	Wires new sources and Windows Dnsapi interop into the build
src/libraries/System.Net.NameResolution/src/Resources/Strings.resx	Adds SR string for unsupported custom DNS ports
src/libraries/System.Net.NameResolution/ref/System.Net.NameResolution.cs	Adds new public API contract entries
src/libraries/Common/src/Interop/Windows/Interop.Libraries.cs	Adds `Libraries.Dnsapi` constant
src/libraries/Common/src/Interop/Windows/Dnsapi/Interop.DnsTypes.cs	Adds DnsQueryEx/DNS_RECORD-related structs/constants
src/libraries/Common/src/Interop/Windows/Dnsapi/Interop.DnsApi.cs	Adds LibraryImport declarations for DnsQueryEx/DnsCancelQuery/DnsFree

MihaZupan

First pass through.

Just curious, will there be a way to opt-in to using the managed implementation even on Windows, or would there be no reason to want to do that anyway?

MihaZupan · 2026-06-11T17:07:10Z

+        public System.Net.DnsResult<System.Net.AddressRecord> ResolveAddresses(string name) { throw null; }
+        public System.Net.DnsResult<System.Net.AddressRecord> ResolveAddresses(string name, System.Net.Sockets.AddressFamily addressFamily) { throw null; }
+        public System.Net.DnsResult<System.Net.SrvRecord> ResolveSrv(string name) { throw null; }
+        public System.Net.DnsResult<System.Net.MxRecord> ResolveMx(string name) { throw null; }
+        public System.Net.DnsResult<System.Net.TxtRecord> ResolveTxt(string name) { throw null; }
+        public System.Net.DnsResult<System.Net.CNameRecord> ResolveCName(string name) { throw null; }
+        public System.Net.DnsResult<System.Net.PtrRecord> ResolvePtr(string name) { throw null; }
+        public System.Net.DnsResult<System.Net.NsRecord> ResolveNs(string name) { throw null; }


Suggested change

public System.Net.DnsResult<System.Net.AddressRecord> ResolveAddresses(string name) { throw null; }

public System.Net.DnsResult<System.Net.AddressRecord> ResolveAddresses(string name, System.Net.Sockets.AddressFamily addressFamily) { throw null; }

public System.Net.DnsResult<System.Net.SrvRecord> ResolveSrv(string name) { throw null; }

public System.Net.DnsResult<System.Net.MxRecord> ResolveMx(string name) { throw null; }

public System.Net.DnsResult<System.Net.TxtRecord> ResolveTxt(string name) { throw null; }

public System.Net.DnsResult<System.Net.CNameRecord> ResolveCName(string name) { throw null; }

public System.Net.DnsResult<System.Net.PtrRecord> ResolvePtr(string name) { throw null; }

public System.Net.DnsResult<System.Net.NsRecord> ResolveNs(string name) { throw null; }

I think we should talk about this with the team :)

My stance continues to be that exposing sync APIs here is a mistake, especially if we have real plans to eventually add support for resolution over HTTPS/QUIC.
The fact that e.g. Resolve silently has 2x the latency compared to the async variant makes the argument that we need sync APIs for perf wrong IMO.

I am also not entirely happy with having synchronous versions as well, we can bring it in a team discussion and push back if more of us feel strongly about this :)

- Conform instance DnsResolver to approved API: remove ResolvePtr(IPAddress) overloads (static Dns keeps them, delegating via BuildArpaName). - Validate DnsResolverOptions.Servers setter is non-null; resolver takes a defensive snapshot of the configured servers. - Fix negative-cache TTL: always extract from authority SOA and surface it on the success path (covers NODATA), plus merge max TTL when combining A/AAAA. - PAL cleanups: typed GCHandle<T>, bool completion flag, span-based address parsing, mixed-address-family validation, checked size arithmetic, simplified using-based query helpers. - Allocation-free telemetry path via TState + static delegates. - Simplify reverse-arpa name building (plain interpolation / string.Concat). - Add XML docs across the new public surface; reword/extend SR strings. - Tests: assert NegativeCacheTtl for NODATA/NXDOMAIN; skip custom-server test when no system DNS server is configured; move IPAddress PTR tests to static API. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

The approved API shape was corrected to include the IPAddress reverse-lookup overloads on the instance resolver, so restore ResolvePtr(IPAddress) and ResolvePtrAsync(IPAddress) on DnsResolver. The static Dns entry points now delegate to them. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 21 out of 21 changed files in this pull request and generated 4 comments.

Replace pointer-based record and sockaddr handling with safe Span<byte> and Marshal-based reads where possible: - TryParseAddress now reads DNS_A_DATA/DNS_AAAA_DATA via Marshal.PtrToStructure instead of byte* spans; DNS_AAAA_DATA uses an InlineArray field, removing the fixed buffer (and unsafe) from the interop struct. - BuildAddrArray populates the DNS_ADDR_ARRAY through a Span<byte> using BinaryPrimitives; WriteSockAddr takes Span<byte> instead of byte*. - Drop the unnecessary 'unsafe' modifier from PtrToString. Also fix a pre-existing compile break in MergeAddressResults (Math.Min has no TimeSpan overload). Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

MichalPetryka · 2026-06-17T16:29:50Z

I've worked with DNS apis here before so here's what I wanted to note:

This needs to account for this pre Windows 11 bug and fallback to DnsQueryW for affected cases (it's only when DnsQueryEx would return synchronously).
IIRC DnsQueryEx is also broken under impersonization, so this needs identical handling like the GetAddrInfoW handling today has.

- Dns.DefaultResolver: use LazyInitializer.EnsureInitialized for thread-safe one-time initialization instead of non-atomic ??=. - DnsResolver ctor: reject null entries in the server snapshot with an ArgumentException pointing at the public Servers property. - BuildAddrArray: surface mixed-address-family ArgumentException with the public-facing 'Servers' parameter name instead of the internal 'servers'. - LoopbackDnsServer: use ConcurrentDictionary for _responses to avoid races between test-thread mutations and listener-thread reads. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 21 out of 21 changed files in this pull request and generated 6 comments.

+        private static void WriteSockAddr(Span<byte> dest, IPEndPoint endpoint)
+        {
+            // IPEndPoint.Serialize() yields the platform SOCKADDR layout (family, port,
+            // address, and for IPv6 the flow info and scope id) via SocketAddressPal, so
+            // there's no need to lay out the bytes by hand. DnsQueryEx always queries DNS
+            // servers on port 53 and requires the sockaddr port field to be left as 0;
+            // serializing a port-0 endpoint satisfies that.
+            SocketAddress socketAddress = endpoint.Serialize();
+            socketAddress.Buffer.Span.Slice(0, socketAddress.Size).CopyTo(dest);
+        }


+        // DnsQueryEx only supports DNS servers reachable on the standard port 53.
+        // The sockaddr port field passed to the API must be 0 (the API always
+        // queries port 53); supplying any non-zero port - even 53 itself - results
+        // in ERROR_INVALID_PARAMETER. We therefore reject any server endpoint that
+        // requests a non-default port, since it cannot be honored on Windows.


+        // ---- DNS_RECORD (variable layout: header + Data union) ----
+        // We declare the fixed header layout and read the data area as a byte blob,
+        // re-interpreting per record type. The Data field begins at offset 24 on 32-bit
+        // and at offset 24 on 64-bit pointer layouts as documented; we use a
+        // Sequential struct with explicit Next/pName pointers.


+    public class WindowsLoopbackServer : IAsyncDisposable
+    {
+        private LoopbackDnsServer _server;
+
+        public WindowsLoopbackServer()
+        {
+            _server = LoopbackDnsServer.Start();
+        }
+
+        internal LoopbackDnsServer Server => _server;
+
+        public async ValueTask DisposeAsync()
+        {
+            if (_server != null)
+            {
+                await _server.DisposeAsync();
+                _server = null;
+            }
+        }
+    }


+    [OuterLoop("Binds the loopback DNS port 53 and issues real DnsQueryEx calls.")]
+    [Collection(nameof(DisableParallelization))]
+    [PlatformSpecific(TestPlatforms.Windows)]
+    public class DnsResolverLoopbackTest : IClassFixture<WindowsLoopbackServer>
+    {
+        private static DnsResolver CreateResolver(LoopbackDnsServer server)
+            => new DnsResolver(new DnsResolverOptions { Servers = { server.EndPoint } });


rzikm and others added 10 commits June 3, 2026 15:17

Add DnsResolver tests

77dd68c

Adds argument-validation tests and Windows-only OuterLoop network tests for the new DnsResolver / Dns.Resolve* APIs. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Polish tests

a589fad

Unify sync/async DnsResolver core methods into single bool-async methods

c375219

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Assert sync DnsResolver core tasks complete synchronously

8196da6

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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MichalPetryka commented Jun 17, 2026 •

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