Expose a global switch to disable IPV6

src/libraries/Common/src/System/Net/SocketProtocolSupportPal.Unix.cs

@@ -7,12 +7,8 @@
 
 namespace System.Net
 {
-    internal static class SocketProtocolSupportPal
+    internal static partial class SocketProtocolSupportPal
     {
-        public static bool OSSupportsIPv6 { get; } = IsSupported(AddressFamily.InterNetworkV6);
-        public static bool OSSupportsIPv4 { get; } = IsSupported(AddressFamily.InterNetwork);
-        public static bool OSSupportsUnixDomainSockets { get; } = IsSupported(AddressFamily.Unix);
-
         private static unsafe bool IsSupported(AddressFamily af)
         {
             IntPtr invalid = (IntPtr)(-1);

src/libraries/Common/src/System/Net/SocketProtocolSupportPal.Windows.cs

@@ -9,12 +9,8 @@
 
 namespace System.Net
 {
-    internal static class SocketProtocolSupportPal
+    internal static partial class SocketProtocolSupportPal
     {
-        public static bool OSSupportsIPv6 { get; } = IsSupported(AddressFamily.InterNetworkV6);
-        public static bool OSSupportsIPv4 { get; } = IsSupported(AddressFamily.InterNetwork);
-        public static bool OSSupportsUnixDomainSockets { get; } = IsSupported(AddressFamily.Unix);
-
         private static bool IsSupported(AddressFamily af)
         {
             Interop.Winsock.EnsureInitialized();

src/libraries/Common/src/System/Net/SocketProtocolSupportPal.cs

@@ -0,0 +1,36 @@
+// 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.Sockets;
+
+namespace System.Net
+{
+    internal static partial class SocketProtocolSupportPal
+    {
+        private const string DisableIPv6AppCtxSwitch = "System.Net.DisableIPv6";
+        private const string DisableIPv6EnvironmentVariable = "DOTNET_SYSTEM_NET_DISABLEIPV6";
+
+        public static bool OSSupportsIPv6 { get; } = IsSupported(AddressFamily.InterNetworkV6) && !IsIPv6Disabled();
+        public static bool OSSupportsIPv4 { get; } = IsSupported(AddressFamily.InterNetwork);
+        public static bool OSSupportsUnixDomainSockets { get; } = IsSupported(AddressFamily.Unix);
+
+        private static bool IsIPv6Disabled()
+        {
+            // First check for the AppContext switch, giving it priority over the environment variable.
+            if (AppContext.TryGetSwitch(DisableIPv6AppCtxSwitch, out bool disabled))
+            {
+                return disabled;
+            }
+
+            // AppContext switch wasn't used. Check the environment variable.
+            string? envVar = Environment.GetEnvironmentVariable(DisableIPv6EnvironmentVariable);
+
+            if (envVar is not null)
+            {
+                return envVar == "1" || envVar.Equals("true", StringComparison.OrdinalIgnoreCase);
+            }
+
+            return false;
+        }
+    }
+}

src/libraries/System.Net.NameResolution/src/System.Net.NameResolution.csproj

@@ -31,6 +31,8 @@
              Link="Common\System\Net\IPAddressParserStatics.cs" />
     <Compile Include="$(CommonPath)System\Net\IPEndPointStatics.cs"
              Link="Common\System\Net\IPEndPointStatics.cs" />
+    <Compile Include="$(CommonPath)System\Net\SocketProtocolSupportPal.cs"
+             Link="Common\System\Net\SocketProtocolSupportPal.cs" />
   </ItemGroup>
   <ItemGroup Condition=" '$(TargetsWindows)' == 'true'">
     <Compile Include="System\Net\NameResolutionPal.Windows.cs" />

src/libraries/System.Net.NameResolution/src/System/Net/NameResolutionPal.Unix.cs

@@ -76,9 +76,11 @@ private static unsafe void ParseHostEntry(Interop.Sys.HostEntry hostEntry, bool
                     Interop.Sys.IPAddress* addressHandle = hostEntry.IPAddressList;
                     for (int i = 0; i < hostEntry.IPAddressCount; i++)
                     {
-                        if (Array.IndexOf(nativeAddresses, addressHandle[i], 0, nativeAddressCount) == -1)
+                        Interop.Sys.IPAddress nativeAddr = addressHandle[i];
+                        if (Array.IndexOf(nativeAddresses, nativeAddr, 0, nativeAddressCount) == -1 &&
+                            (!nativeAddr.IsIPv6 || SocketProtocolSupportPal.OSSupportsIPv6)) // Do not include IPv6 addresses if IPV6 support is force-disabled
                         {
-                            nativeAddresses[nativeAddressCount++] = addressHandle[i];
+                            nativeAddresses[nativeAddressCount++] = nativeAddr;
                         }
                     }
 

src/libraries/System.Net.NameResolution/tests/FunctionalTests/GetHostEntryTest.cs

@@ -6,7 +6,7 @@
 using System.Net.Sockets;
 using System.Threading;
 using System.Threading.Tasks;
-
+using Microsoft.DotNet.RemoteExecutor;
 using Microsoft.DotNet.XUnitExtensions;
 using Xunit;
 
@@ -21,9 +21,16 @@ public async Task Dns_GetHostEntryAsync_IPAddress_Ok()
             await TestGetHostEntryAsync(() => Dns.GetHostEntryAsync(localIPAddress));
         }
 
-        [ActiveIssue("https://github.com/dotnet/runtime/issues/1488", TestPlatforms.OSX)]
-        [ActiveIssue("https://github.com/dotnet/runtime/issues/51377", TestPlatforms.iOS | TestPlatforms.tvOS | TestPlatforms.MacCatalyst)]
-        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsNotArm64Process))] // [ActiveIssue("https://github.com/dotnet/runtime/issues/27622")]
+
+        public static bool GetHostEntryWorks =
+            // [ActiveIssue("https://github.com/dotnet/runtime/issues/27622")]
+            PlatformDetection.IsNotArmNorArm64Process &&
+            // [ActiveIssue("https://github.com/dotnet/runtime/issues/1488", TestPlatforms.OSX)]
+            PlatformDetection.IsNotOSX &&
+            // [ActiveIssue("https://github.com/dotnet/runtime/issues/51377", TestPlatforms.iOS | TestPlatforms.tvOS | TestPlatforms.MacCatalyst)]
+            !PlatformDetection.IsiOS && !PlatformDetection.IstvOS && !PlatformDetection.IsMacCatalyst;
+
+        [ConditionalTheory(nameof(GetHostEntryWorks))]
         [InlineData("")]
         [InlineData(TestSettings.LocalHost)]
         public async Task Dns_GetHostEntry_HostString_Ok(string hostName)
@@ -77,12 +84,10 @@ public async Task Dns_GetHostEntry_HostString_Ok(string hostName)
             }
         }
 
-        [ActiveIssue("https://github.com/dotnet/runtime/issues/1488", TestPlatforms.OSX)]
-        [ActiveIssue("https://github.com/dotnet/runtime/issues/51377", TestPlatforms.iOS | TestPlatforms.tvOS | TestPlatforms.MacCatalyst)]
-        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsNotArm64Process))] // [ActiveIssue("https://github.com/dotnet/runtime/issues/27622")]
+        [ConditionalTheory(nameof(GetHostEntryWorks))]
         [InlineData("")]
         [InlineData(TestSettings.LocalHost)]
-        public async Task Dns_GetHostEntryAsync_HostString_Ok(string hostName)
+        public async Task Dns_GetHostEntryAsync_HostString_Ok(string hostName)    
         {
             if (PlatformDetection.IsSLES)
             {
@@ -112,6 +117,44 @@ private static async Task TestGetHostEntryAsync(Func<Task<IPHostEntry>> getHostE
             Assert.Equal<IPAddress>(list1, list2);
         }
 
+        public static bool GetHostEntry_DisableIPv6_Condition = GetHostEntryWorks && RemoteExecutor.IsSupported;
+
+        [ConditionalTheory(nameof(GetHostEntry_DisableIPv6_Condition))]
+        [InlineData("")]
+        [InlineData(TestSettings.LocalHost)]
+        public void Dns_GetHostEntry_DisableIPv6_ExcludesIPv6Addresses(string hostnameOuter)
+        {
+            RemoteExecutor.Invoke(RunTest, hostnameOuter).Dispose();
+
+            static void RunTest(string hostnameInner)
+            {
+                AppContext.SetSwitch("System.Net.DisableIPv6", true);
+                IPHostEntry entry = Dns.GetHostEntry(hostnameInner);
+                foreach (IPAddress address in entry.AddressList)
+                {
+                    Assert.NotEqual(AddressFamily.InterNetworkV6, address.AddressFamily);
+                }
+            }   
+        }
+
+        [ConditionalTheory(nameof(GetHostEntry_DisableIPv6_Condition))]
+        [InlineData("")]
+        [InlineData(TestSettings.LocalHost)]
+        public void Dns_GetHostEntryAsync_DisableIPv6_ExcludesIPv6Addresses(string hostnameOuter)
+        {
+            RemoteExecutor.Invoke(RunTest, hostnameOuter).Dispose();
+
+            static async Task RunTest(string hostnameInner)
+            {
+                AppContext.SetSwitch("System.Net.DisableIPv6", true);
+                IPHostEntry entry = await Dns.GetHostEntryAsync(hostnameInner);
+                foreach (IPAddress address in entry.AddressList)
+                {
+                    Assert.NotEqual(AddressFamily.InterNetworkV6, address.AddressFamily);
+                }
+            }
+        }
+
         [Fact]
         public async Task Dns_GetHostEntry_NullStringHost_Fail()
         {

src/libraries/System.Net.NameResolution/tests/PalTests/System.Net.NameResolution.Pal.Tests.csproj

@@ -28,6 +28,8 @@
              Link="Common\System\Net\IPEndPointStatics.cs" />
     <Compile Include="$(CommonPath)System\Net\IPAddressParserStatics.cs"
              Link="Common\System\Net\IPAddressParserStatics.cs" />
+    <Compile Include="$(CommonPath)System\Net\SocketProtocolSupportPal.cs"
+             Link="Common\System\Net\SocketProtocolSupportPal.cs" />
     <Compile Include="$(CommonTestPath)System\Net\Configuration.cs"
              Link="Common\System\Net\Configuration.cs" />
     <Compile Include="$(CommonTestPath)System\Net\Configuration.Http.cs"

src/libraries/System.Net.Ping/src/System.Net.Ping.csproj

@@ -21,6 +21,8 @@
              Link="Common\System\Net\IPAddressParserStatics.cs" />
     <Compile Include="$(CommonPath)System\Net\SocketAddress.cs"
              Link="Common\System\Net\SocketAddress.cs" />
+    <Compile Include="$(CommonPath)System\Net\SocketProtocolSupportPal.cs"
+             Link="Common\System\Net\SocketProtocolSupportPal.cs" />
     <!-- Logging -->
     <Compile Include="$(CommonPath)System\Net\Logging\NetEventSource.Common.cs"
              Link="Common\System\Net\Logging\NetEventSource.Common.cs" />

src/libraries/System.Net.Sockets/src/System.Net.Sockets.csproj

@@ -65,6 +65,8 @@
              Link="Common\System\Net\SocketAddress.cs" />
     <Compile Include="$(CommonPath)System\Net\TcpValidationHelpers.cs"
              Link="Common\System\Net\TcpValidationHelpers.cs" />
+    <Compile Include="$(CommonPath)System\Net\SocketProtocolSupportPal.cs"
+             Link="Common\System\Net\SocketProtocolSupportPal.cs" />
     <Compile Include="$(CommonPath)System\Threading\Tasks\TaskToApm.cs"
              Link="Common\System\Threading\Tasks\TaskToApm.cs" />
     <!-- System.Net.Internals -->

src/libraries/System.Net.Sockets/tests/FunctionalTests/OSSupport.cs

@@ -3,6 +3,7 @@
 
 
 using System.Threading;
+using Microsoft.DotNet.RemoteExecutor;
 using Xunit;
 
 namespace System.Net.Sockets.Tests
@@ -25,6 +26,37 @@ public void SupportsIPv6_MatchesOSSupportsIPv6()
 #pragma warning restore
         }
 
+        [ConditionalFact(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
+        public void DisableIPv6_OSSupportsIPv6_False()
+        {
+            RemoteInvokeOptions options = new RemoteInvokeOptions();
+            options.StartInfo.EnvironmentVariables["DOTNET_SYSTEM_NET_DISABLEIPV6"] = "1";
+            RemoteExecutor.Invoke(RunTest, options).Dispose();
+
+            static void RunTest()
+            {
+                Assert.False(Socket.OSSupportsIPv6);
+            }
+        }
+
+        [ConditionalFact(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
+        public void DisableIPv6_SocketConstructor_CreatesIPv4Socket()
+        {
+            RemoteExecutor.Invoke(RunTest).Dispose();
+
+            static void RunTest()
+            {
+                AppContext.SetSwitch("System.Net.DisableIPv6", true);
+                using Socket socket1 = new Socket(SocketType.Stream, ProtocolType.Tcp);
+                using Socket socket2 = new Socket(SocketType.Dgram, ProtocolType.Udp);
+
+                Assert.Equal(AddressFamily.InterNetwork, socket1.AddressFamily);
+                Assert.Equal(AddressFamily.InterNetwork, socket2.AddressFamily);
+                Assert.False(socket1.DualMode);
+                Assert.False(socket2.DualMode);
+            }
+        }
+
         [Fact]
         public void IOControl_FIONREAD_Success()
         {