Environment variable & AppCtx switch to force-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 +1,31 @@
+// 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
+    {
+        public static bool OSSupportsIPv6 { get; } = !IsIPV6Disabled && IsSupported(AddressFamily.InterNetworkV6);
+        public static bool OSSupportsIPv4 { get; } = IsSupported(AddressFamily.InterNetwork);
+        public static bool OSSupportsUnixDomainSockets { get; } = IsSupported(AddressFamily.Unix);
+
+        private static bool IsIPV6Disabled
+        {
+            get
+            {
+                const string DisableIpv6AppContextSettingName = "System.Net.Sockets.DisableIPv6";
+                const string DisableIpv6EnvironmentVariableName = "DOTNET_SYSTEM_NET_SOCKETS_DISABLEIPV6";
+
+                if (AppContext.TryGetSwitch(DisableIpv6AppContextSettingName, out bool disableIpV6))
+                {
+                    return disableIpV6;
+                }
+
+                string? envVar = Environment.GetEnvironmentVariable(DisableIpv6EnvironmentVariableName);
+                return envVar != null && (envVar.Equals("true", StringComparison.OrdinalIgnoreCase) || envVar.Equals("1"));
+            }
+        }
+    }
+}

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

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <TargetFrameworks>$(NetCoreAppCurrent)-windows;$(NetCoreAppCurrent)-Unix;$(NetCoreAppCurrent)-Browser</TargetFrameworks>
@@ -72,6 +72,8 @@
              Link="Common\System\Net\RangeValidationHelpers.cs" />
     <Compile Include="$(CommonPath)System\Net\SocketAddress.cs"
              Link="Common\System\Net\SocketAddress.cs" />
+    <Compile Include="$(CommonPath)System\Net\SocketProtocolSupportPal.cs"
+             Link="Common\System\Net\SocketProtocolSupportPal.cs" />
     <Compile Include="$(CommonPath)System\Net\TcpValidationHelpers.cs"
              Link="Common\System\Net\TcpValidationHelpers.cs" />
     <Compile Include="$(CommonPath)System\Threading\Tasks\TaskToApm.cs"
@@ -109,7 +111,7 @@
     <Compile Include="$(CommonPath)System\Net\SocketAddressPal.Windows.cs"
              Link="Common\System\Net\SocketAddressPal.Windows.cs" />
     <Compile Include="$(CommonPath)System\Net\SocketProtocolSupportPal.Windows.cs"
-             Link="Common\System\Net\SocketProtocolSupportPal.Windows" />
+             Link="Common\System\Net\SocketProtocolSupportPal.Windows.cs" />
     <!-- Interop -->
     <Compile Include="$(CommonPath)Interop\Windows\Interop.Libraries.cs"
              Link="Common\Interop\Windows\Interop.Libraries.cs" />
@@ -208,7 +210,7 @@
     <Compile Include="$(CommonPath)System\Net\SocketAddressPal.Unix.cs"
              Link="Common\System\Net\SocketAddressPal.Unix.cs" />
     <Compile Include="$(CommonPath)System\Net\SocketProtocolSupportPal.Unix.cs"
-             Link="Common\System\Net\SocketProtocolSupportPal.Unix" />
+             Link="Common\System\Net\SocketProtocolSupportPal.Unix.cs" />
     <Compile Include="$(CommonPath)System\Net\Sockets\SocketErrorPal.Unix.cs"
              Link="Common\System\Net\Sockets\SocketErrorPal.Unix" />
     <Compile Include="$(CommonPath)Interop\Unix\Interop.Errors.cs"

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
@@ -206,5 +207,31 @@ public void IOControl_UnknownValues_Unix_Throws()
                 }
             }
         }
+
+        [Theory]
+        [InlineData(false)]
+        [InlineData(true)]
+        public void DisableIpv6(bool appContextOrEnvironmentVariable)
+        {
+            RemoteInvokeOptions options = null;
+            if (!appContextOrEnvironmentVariable)
+            {
+                options = new RemoteInvokeOptions();
+                options.StartInfo.EnvironmentVariables.Add("DOTNET_SYSTEM_NET_SOCKETS_DISABLEIPV6", "1");
+            }
+
+            RemoteExecutor.Invoke(static appContextOrEnvironmentVariableInner =>
+            {
+                if (bool.Parse(appContextOrEnvironmentVariableInner))
+                {
+                    AppContext.SetSwitch("System.Net.Sockets.DisableIPv6", true);
+                }
+
+                Assert.False(Socket.OSSupportsIPv6);
+
+                using Socket socket = new Socket(SocketType.Stream, ProtocolType.Tcp);
+                Assert.Equal(AddressFamily.InterNetwork, socket.AddressFamily);
+            }, appContextOrEnvironmentVariable.ToString(), options).Dispose();
+        }
     }
 }