Fix DAC connections to localhost in managed SNI

src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/SNI/SNIProxy.cs

@@ -579,10 +579,12 @@ internal static DataSource ParseServerName(string dataSource)
         private void InferLocalServerName()
         {
             // If Server name is empty or localhost, then use "localhost"
-            if (string.IsNullOrEmpty(ServerName) || IsLocalHost(ServerName))
+            if (string.IsNullOrEmpty(ServerName) || IsLocalHost(ServerName) ||
+                (Environment.MachineName.Equals(ServerName, StringComparison.CurrentCultureIgnoreCase) &&
+                 _connectionProtocol == Protocol.Admin))
             {
-                ServerName = _connectionProtocol == Protocol.Admin ?
-                    Environment.MachineName : DefaultHostName;
+                // For DAC use "localhost" instead of the server name.
+                ServerName = DefaultHostName;
             }
         }
 

src/Microsoft.Data.SqlClient/tests/ManualTests/SQL/ConnectivityTests/ConnectivityTest.cs

@@ -457,5 +457,41 @@ public static void ConnectionAliasTest()
                 key.DeleteValue(b.DataSource);
             }
         }
+
+        private static bool CanUseDacConnection()
+        {
+            if (!DataTestUtility.IsTCPConnStringSetup())
+            {
+                return false;
+            }
+
+            SqlConnectionStringBuilder b = new(DataTestUtility.TCPConnectionString);
+            if (!DataTestUtility.ParseDataSource(b.DataSource, out string hostname, out int port, out string instanceName))
+            {
+                return false;
+            }
+
+            if ("localhost".Equals(hostname.ToLower()) && (port.Equals(-1) || port.Equals(1433)) &&
+                string.IsNullOrEmpty(instanceName) && b.UserID != null && b.UserID.ToLower().Equals("sa"))
+            {
+                return true;
+            }
+
+            return false;
+        }
+
+        [ConditionalFact(nameof(CanUseDacConnection))]
+        public static void DacConnectionTest()
+        {
+            if (!CanUseDacConnection())
+            {
+                throw new Exception("Unable to use a DAC connection in this environment. Localhost + sa credentials required.");
+            }
+
+            SqlConnectionStringBuilder b = new(DataTestUtility.TCPConnectionString);
+            b.DataSource = "admin:localhost";
+            using SqlConnection sqlConnection = new(b.ConnectionString);
+            sqlConnection.Open();
+        }
     }
 }

src/Microsoft.Data.SqlClient/tests/tools/Microsoft.Data.SqlClient.TestUtilities/Microsoft.Data.SqlClient.TestUtilities.csproj

@@ -8,10 +8,9 @@
     <Copy SourceFiles="config.default.json" DestinationFiles="config.json" Condition="!Exists('config.json')" />
   </Target>
   <ItemGroup>
-    <Content Include="config.json">
+    <None Update="config.json">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
-      <Link>config.json</Link>
-    </Content>
+    </None>
     <PackageReference Include="Newtonsoft.Json" Version="$(NewtonsoftJsonVersion)" />
   </ItemGroup>
 </Project>