Extend service discovery to support Consul-based DNS lookups:

src/Libraries/Microsoft.Extensions.ServiceDiscovery.Dns/DnsResolverOptions.cs

@@ -0,0 +1,33 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Net;
+
+namespace Microsoft.Extensions.ServiceDiscovery.Dns;
+
+/// <summary>
+/// Provides configuration options for DNS resolution, including server endpoints, retry attempts, and timeout settings.
+/// </summary>
+public class DnsResolverOptions
+{
+    /// <summary>
+    /// Gets or sets the collection of server endpoints used for network connections.
+    /// </summary>
+    public IList<IPEndPoint> Servers { get; set; } = new List<IPEndPoint>();
+
+    /// <summary>
+    /// Gets or sets the maximum number of attempts per server.
+    /// </summary>
+    public int MaxAttempts { get; set; } = 2;
+
+    /// <summary>
+    /// Gets or sets the maximum duration per attempt to wait before timing out.
+    /// </summary>
+    /// <remarks>
+    /// The maximum time for resolving a query is <see cref="MaxAttempts"/> * <see cref="Servers"/> count * <see cref="Timeout"/>.
+    /// </remarks>
+    public TimeSpan Timeout { get; set; } = TimeSpan.FromSeconds(3);
+
+    // override for testing purposes
+    internal Func<Memory<byte>, int, int>? _transportOverride;
+}

src/Libraries/Microsoft.Extensions.ServiceDiscovery.Dns/DnsResolverOptionsValidator.cs

@@ -0,0 +1,41 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using Microsoft.Extensions.Options;
+
+namespace Microsoft.Extensions.ServiceDiscovery.Dns;
+
+internal sealed class DnsResolverOptionsValidator : IValidateOptions<DnsResolverOptions>
+{
+    // CancellationTokenSource.CancelAfter has a maximum timeout of Int32.MaxValue milliseconds.
+    private static readonly TimeSpan s_maxTimeout = TimeSpan.FromMilliseconds(int.MaxValue);
+
+    public ValidateOptionsResult Validate(string? name, DnsResolverOptions options)
+    {
+        if (options.Servers is null)
+        {
+            return ValidateOptionsResult.Fail($"{nameof(options.Servers)} must not be null.");
+        }
+
+        if (options.MaxAttempts < 1)
+        {
+            return ValidateOptionsResult.Fail($"{nameof(options.MaxAttempts)} must be one or greater.");
+        }
+
+        if (options.Timeout != Timeout.InfiniteTimeSpan)
+        {
+            if (options.Timeout <= TimeSpan.Zero)
+            {
+                return ValidateOptionsResult.Fail($"{nameof(options.Timeout)} must not be negative or zero.");
+            }
+
+            if (options.Timeout > s_maxTimeout)
+            {
+                return ValidateOptionsResult.Fail($"{nameof(options.Timeout)} must not be greater than {s_maxTimeout.TotalMilliseconds} milliseconds.");
+            }
+        }
+
+        return ValidateOptionsResult.Success;
+    }
+}

src/Libraries/Microsoft.Extensions.ServiceDiscovery.Dns/DnsSrvServiceEndpointProviderFactory.cs

@@ -22,6 +22,8 @@ internal sealed partial class DnsSrvServiceEndpointProviderFactory(
     /// <inheritdoc/>
     public bool TryCreateProvider(ServiceEndpointQuery query, [NotNullWhen(true)] out IServiceEndpointProvider? provider)
     {
+        var optionsValue = options.CurrentValue;
+
         // If a default namespace is not specified, then this provider will attempt to infer the namespace from the service name, but only when running inside Kubernetes.
         // Kubernetes DNS spec: https://github.com/kubernetes/dns/blob/master/docs/specification.md
         // SRV records are available for headless services with named ports.
@@ -30,19 +32,26 @@ public bool TryCreateProvider(ServiceEndpointQuery query, [NotNullWhen(true)] ou
         // Otherwise, the namespace can be read from /var/run/secrets/kubernetes.io/serviceaccount/namespace and combined with an assumed suffix of "svc.cluster.local".
         // The protocol is assumed to be "tcp".
         // The portName is the name of the port in the service definition. If the serviceName parses as a URI, we use the scheme as the port name, otherwise "default".
-        if (string.IsNullOrWhiteSpace(_querySuffix))
+        if (optionsValue.ServiceDomainNameCallback == null && string.IsNullOrWhiteSpace(_querySuffix))
         {
             DnsServiceEndpointProviderBase.Log.NoDnsSuffixFound(logger, query.ToString()!);
             provider = default;
             return false;
         }
 
-        var portName = query.EndpointName ?? "default";
-        var srvQuery = $"_{portName}._tcp.{query.ServiceName}.{_querySuffix}";
+        var srvQuery = optionsValue.ServiceDomainNameCallback != null
+            ? optionsValue.ServiceDomainNameCallback(query)
+            : DefaultServiceDomainNameCallback(query, optionsValue);
         provider = new DnsSrvServiceEndpointProvider(query, srvQuery, hostName: query.ServiceName, options, logger, resolver, timeProvider);
         return true;
     }
 
+    private static string DefaultServiceDomainNameCallback(ServiceEndpointQuery query, DnsSrvServiceEndpointProviderOptions options)
+    {
+        var portName = query.EndpointName ?? "default";
+        return $"_{portName}._tcp.{query.ServiceName}.{options.QuerySuffix}";
+    }
+
     private static string? GetKubernetesHostDomain()
     {
         // Check that we are running in Kubernetes first.

src/Libraries/Microsoft.Extensions.ServiceDiscovery.Dns/DnsSrvServiceEndpointProviderOptions.cs

@@ -36,6 +36,11 @@ public class DnsSrvServiceEndpointProviderOptions
     /// </remarks>
     public string? QuerySuffix { get; set; }
 
+    /// <summary>
+    /// Gets or sets a delegate that generates a DNS SRV query from a specified <see cref="ServiceEndpointQuery"/> instance.
+    /// </summary>
+    public Func<ServiceEndpointQuery, string>? ServiceDomainNameCallback { get; set; }
+
     /// <summary>
     /// Gets or sets a delegate used to determine whether to apply host name metadata to each resolved endpoint. Defaults to <c>false</c>.
     /// </summary>

src/Libraries/Microsoft.Extensions.ServiceDiscovery.Dns/Resolver/DnsResolver.cs

@@ -11,6 +11,7 @@
 using System.Security.Cryptography;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
 
 namespace Microsoft.Extensions.ServiceDiscovery.Dns.Resolver;
 
@@ -19,43 +20,40 @@ internal sealed partial class DnsResolver : IDnsResolver, IDisposable
     private const int IPv4Length = 4;
     private const int IPv6Length = 16;
 
-    // CancellationTokenSource.CancelAfter has a maximum timeout of Int32.MaxValue milliseconds.
-    private static readonly TimeSpan s_maxTimeout = TimeSpan.FromMilliseconds(int.MaxValue);
-
     private bool _disposed;
-    private readonly ResolverOptions _options;
+    private readonly DnsResolverOptions _options;
     private readonly CancellationTokenSource _pendingRequestsCts = new();
     private readonly TimeProvider _timeProvider;
     private readonly ILogger<DnsResolver> _logger;
 
-    public DnsResolver(TimeProvider timeProvider, ILogger<DnsResolver> logger) : this(timeProvider, logger, OperatingSystem.IsLinux() || OperatingSystem.IsMacOS() ? ResolvConf.GetOptions() : NetworkInfo.GetOptions())
-    {
-    }
-
-    internal DnsResolver(TimeProvider timeProvider, ILogger<DnsResolver> logger, ResolverOptions options)
+    public DnsResolver(TimeProvider timeProvider, ILogger<DnsResolver> logger, IOptions<DnsResolverOptions> options)
     {
         _timeProvider = timeProvider;
         _logger = logger;
-        _options = options;
-        Debug.Assert(_options.Servers.Count > 0);
+        _options = options.Value;
 
-        if (options.Timeout != Timeout.InfiniteTimeSpan)
+        if (_options.Servers.Count == 0)
         {
-            ArgumentOutOfRangeException.ThrowIfLessThanOrEqual(options.Timeout, TimeSpan.Zero);
-            ArgumentOutOfRangeException.ThrowIfGreaterThan(options.Timeout, s_maxTimeout);
-        }
-    }
-
-    internal DnsResolver(ResolverOptions options) : this(TimeProvider.System, NullLogger<DnsResolver>.Instance, options)
-    {
-    }
+            foreach (var server in OperatingSystem.IsLinux() || OperatingSystem.IsMacOS()
+                ? ResolvConf.GetServers()
+                : NetworkInfo.GetServers())
+            {
+                _options.Servers.Add(server);
+            }
 
-    internal DnsResolver(IEnumerable<IPEndPoint> servers) : this(new ResolverOptions(servers.ToArray()))
-    {
+            if (_options.Servers.Count == 0)
+            {
+                throw new ArgumentException("At least one DNS server is required.", nameof(options));
+            }
+        }
     }
 
-    internal DnsResolver(IPEndPoint server) : this(new ResolverOptions(server))
+    // This constructor is for unit testing only. Does not auto-add system DNS servers.
+    internal DnsResolver(DnsResolverOptions options)
     {
+        _timeProvider = TimeProvider.System;
+        _logger = NullLogger<DnsResolver>.Instance;
+        _options = options;
     }
 
     public ValueTask<ServiceResult[]> ResolveServiceAsync(string name, CancellationToken cancellationToken = default)
@@ -365,7 +363,7 @@ internal struct SendQueryResult
         {
             IPEndPoint serverEndPoint = _options.Servers[index];
 
-            for (int attempt = 1; attempt <= _options.Attempts; attempt++)
+            for (int attempt = 1; attempt <= _options.MaxAttempts; attempt++)
             {
                 DnsResponse response = default;
                 try

src/Libraries/Microsoft.Extensions.ServiceDiscovery.Dns/Resolver/NetworkInfo.cs

@@ -8,8 +8,8 @@ namespace Microsoft.Extensions.ServiceDiscovery.Dns.Resolver;
 
 internal static class NetworkInfo
 {
-    // basic option to get DNS serves via NetworkInfo. We may get it directly later via proper APIs.
-    public static ResolverOptions GetOptions()
+    // basic option to get DNS servers via NetworkInfo. We may get it directly later via proper APIs.
+    public static IList<IPEndPoint> GetServers()
     {
         List<IPEndPoint> servers = new List<IPEndPoint>();
 
@@ -31,6 +31,6 @@ public static ResolverOptions GetOptions()
             }
         }
 
-        return new ResolverOptions(servers);
+        return servers;
     }
 }

src/Libraries/Microsoft.Extensions.ServiceDiscovery.Dns/Resolver/ResolvConf.cs

@@ -10,12 +10,12 @@ internal static class ResolvConf
 {
     [SupportedOSPlatform("linux")]
     [SupportedOSPlatform("osx")]
-    public static ResolverOptions GetOptions()
+    public static IList<IPEndPoint> GetServers()
     {
-        return GetOptions(new StreamReader("/etc/resolv.conf"));
+        return GetServers(new StreamReader("/etc/resolv.conf"));
     }
 
-    public static ResolverOptions GetOptions(TextReader reader)
+    public static IList<IPEndPoint> GetServers(TextReader reader)
     {
         List<IPEndPoint> serverList = new();
 
@@ -40,9 +40,9 @@ public static ResolverOptions GetOptions(TextReader reader)
         if (serverList.Count == 0)
         {
             // If no nameservers are configured, fall back to the default behavior of using the system resolver configuration.
-            return NetworkInfo.GetOptions();
+            return NetworkInfo.GetServers();
         }
 
-        return new ResolverOptions(serverList);
+        return serverList;
     }
 }

src/Libraries/Microsoft.Extensions.ServiceDiscovery.Dns/ServiceDiscoveryDnsServiceCollectionExtensions.cs

@@ -3,6 +3,7 @@
 
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.DependencyInjection.Extensions;
+using Microsoft.Extensions.Options;
 using Microsoft.Extensions.ServiceDiscovery;
 using Microsoft.Extensions.ServiceDiscovery.Dns;
 using Microsoft.Extensions.ServiceDiscovery.Dns.Resolver;
@@ -59,24 +60,10 @@ public static IServiceCollection AddDnsSrvServiceEndpointProvider(this IServiceC
 
         services.AddSingleton<IServiceEndpointProviderFactory, DnsSrvServiceEndpointProviderFactory>();
         var options = services.AddOptions<DnsSrvServiceEndpointProviderOptions>();
-        options.Configure(o => configureOptions?.Invoke(o));
+        options.Configure(configureOptions);
+
         return services;
 
-        static bool GetDnsClientFallbackFlag()
-        {
-            if (AppContext.TryGetSwitch("Microsoft.Extensions.ServiceDiscovery.Dns.UseDnsClientFallback", out var value))
-            {
-                return value;
-            }
-
-            var envVar = Environment.GetEnvironmentVariable("MICROSOFT_EXTENSIONS_SERVICE_DISCOVERY_DNS_USE_DNSCLIENT_FALLBACK");
-            if (envVar is not null && (envVar.Equals("true", StringComparison.OrdinalIgnoreCase) || envVar.Equals("1")))
-            {
-                return true;
-            }
-
-            return false;
-        }
     }
 
     /// <summary>
@@ -109,9 +96,55 @@ public static IServiceCollection AddDnsServiceEndpointProvider(this IServiceColl
         ArgumentNullException.ThrowIfNull(configureOptions);
 
         services.AddServiceDiscoveryCore();
+
+        if (!GetDnsClientFallbackFlag())
+        {
+            services.TryAddSingleton<IDnsResolver, DnsResolver>();
+        }
+        else
+        {
+            services.TryAddSingleton<IDnsResolver, FallbackDnsResolver>();
+            services.TryAddSingleton<DnsClient.LookupClient>();
+        }
+
         services.AddSingleton<IServiceEndpointProviderFactory, DnsServiceEndpointProviderFactory>();
         var options = services.AddOptions<DnsServiceEndpointProviderOptions>();
-        options.Configure(o => configureOptions?.Invoke(o));
+        options.Configure(configureOptions);
+
+        return services;
+    }
+
+    /// <summary>
+    /// Configures the DNS resolver used for service discovery.
+    /// </summary>
+    /// <param name="services">The service collection.</param>
+    /// <param name="configureOptions">The DNS resolver options.</param>
+    /// <returns>The provided <see cref="IServiceCollection"/>.</returns>
+    public static IServiceCollection ConfigureDnsResolver(this IServiceCollection services, Action<DnsResolverOptions> configureOptions)
+    {
+        ArgumentNullException.ThrowIfNull(services);
+        ArgumentNullException.ThrowIfNull(configureOptions);
+
+        var options = services.AddOptions<DnsResolverOptions>();
+        options.Configure(configureOptions);
+        services.AddTransient<IValidateOptions<DnsResolverOptions>, DnsResolverOptionsValidator>();
         return services;
     }
+
+    private static bool GetDnsClientFallbackFlag()
+    {
+        if (AppContext.TryGetSwitch("Microsoft.Extensions.ServiceDiscovery.Dns.UseDnsClientFallback", out var value))
+        {
+            return value;
+        }
+
+        var envVar = Environment.GetEnvironmentVariable("MICROSOFT_EXTENSIONS_SERVICE_DISCOVERY_DNS_USE_DNSCLIENT_FALLBACK");
+        if (envVar is not null && (envVar.Equals("true", StringComparison.OrdinalIgnoreCase) || envVar.Equals("1")))
+        {
+            return true;
+        }
+
+        return false;
+    }
+
 }

test/Libraries/Microsoft.Extensions.ServiceDiscovery.Dns.Tests.Fuzzing/Fuzzers/DnsResponseFuzzer.cs

@@ -19,10 +19,11 @@ public void FuzzTarget(ReadOnlySpan<byte> data)
         if (_resolver == null)
         {
             _buffer = new byte[4096];
-            _resolver = new DnsResolver(new ResolverOptions(new IPEndPoint(IPAddress.Loopback, 53))
+            _resolver = new DnsResolver(new DnsResolverOptions
             {
+                Servers = [new IPEndPoint(IPAddress.Loopback, 53)],
                 Timeout = TimeSpan.FromSeconds(5),
-                Attempts = 1,
+                MaxAttempts = 1,
                 _transportOverride = (buffer, length) =>
                 {
                     // the first two bytes are the random transaction ID, so we keep that
@@ -41,4 +42,4 @@ public void FuzzTarget(ReadOnlySpan<byte> data)
         Debug.Assert(task.IsCompleted, "Task should be completed synchronously");
         task.GetAwaiter().GetResult();
     }
-}
+}