Fix SqlClient Kerberos on Linux

src/System.Data.SqlClient/src/System.Data.SqlClient.csproj

@@ -205,6 +205,8 @@
     <Compile Include="System\Data\SqlClient\SNI\SNINpHandle.cs" />
     <Compile Include="System\Data\SqlClient\SNI\SNIPacket.cs" />
     <Compile Include="System\Data\SqlClient\SNI\SNIProxy.cs" />
+    <Compile Include="System\Data\SqlClient\SNI\SNIProxy.Windows.cs" Condition="'$(TargetsNetCoreApp)' != 'true' OR '$(TargetsUnix)' != 'true'" />
+    <Compile Include="System\Data\SqlClient\SNI\SNIProxy.Unix.cs" Condition="'$(TargetsNetCoreApp)' == 'true' AND '$(TargetsUnix)' == 'true'" />
     <Compile Include="System\Data\SqlClient\SNI\SNITcpHandle.cs" />
     <Compile Include="System\Data\SqlClient\SNI\SslOverTdsStream.cs" />
     <Compile Include="System\Data\SqlClient\SNI\SNICommon.cs" />

src/System.Data.SqlClient/src/System/Data/SqlClient/SNI/SNIProxy.Unix.cs

@@ -0,0 +1,16 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace System.Data.SqlClient.SNI
+{
+    /// <summary>
+    /// Managed SNI proxy implementation. Contains many SNI entry points used by SqlClient.
+    /// </summary>
+    internal partial class SNIProxy
+    {
+        // On Unix/Linux the format for the SPN is SERVICE@HOST. This is because the System.Net.Security.Native
+        // GSS-API layer uses GSS_C_NT_HOSTBASED_SERVICE format for the SPN.
+        private const string SpnServiceHostSeparator = "@";
+    }
+}

src/System.Data.SqlClient/src/System/Data/SqlClient/SNI/SNIProxy.Windows.cs

@@ -0,0 +1,16 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace System.Data.SqlClient.SNI
+{
+    /// <summary>
+    /// Managed SNI proxy implementation. Contains many SNI entry points used by SqlClient.
+    /// </summary>
+    internal partial class SNIProxy
+    {
+        // On Windows the format for the SPN is SERVICE/HOST. So the separator character between the
+        // SERVICE and HOST components is "/".
+        private const string SpnServiceHostSeparator = "/";
+    }
+}

src/System.Data.SqlClient/src/System/Data/SqlClient/SNI/SNIProxy.cs

@@ -18,7 +18,7 @@ namespace System.Data.SqlClient.SNI
     /// <summary>
     /// Managed SNI proxy implementation. Contains many SNI entry points used by SqlClient.
     /// </summary>
-    internal class SNIProxy
+    internal partial class SNIProxy
     {
         private const int DefaultSqlServerPort = 1433;
         private const int DefaultSqlServerDacPort = 1434;
@@ -342,7 +342,7 @@ private static byte[] GetSqlServerSPN(string hostNameOrAddress, string portOrIns
                 // If the DNS lookup failed, then resort to using the user provided hostname to construct the SPN.
                 fullyQualifiedDomainName = hostEntry?.HostName ?? hostNameOrAddress;
             }
-            string serverSpn = SqlServerSpnHeader + "/" + fullyQualifiedDomainName;
+            string serverSpn = SqlServerSpnHeader + SpnServiceHostSeparator + fullyQualifiedDomainName;
             if (!string.IsNullOrWhiteSpace(portOrInstanceName))
             {
                 serverSpn += ":" + portOrInstanceName;