Connect-DbaInstance: Use localhost for dedicated admin connections to avoid multiple IP resolution on multi-homed servers (#10199)

Author: andreasjordan

public/Connect-DbaInstance.ps1

@@ -640,7 +640,17 @@ function Connect-DbaInstance {
 
             if ($DedicatedAdminConnection -and $serverName) {
                 Write-Message -Level Debug -Message "Parameter DedicatedAdminConnection is used, so serverName will be changed and NonPooledConnection will be set."
-                $serverName = 'ADMIN:' + $serverName
+                if ($instance.IsLocalHost) {
+                    # Use localhost to avoid multiple IP resolution on multi-homed servers (issue #10151)
+                    if ($instance.InstanceName -ne 'MSSQLSERVER') {
+                        $serverName = "ADMIN:localhost\$($instance.InstanceName)"
+                    } else {
+                        $serverName = "ADMIN:localhost"
+                    }
+                    Write-Message -Level Debug -Message "IsLocalHost is true, using '$serverName' for DAC to avoid multi-IP resolution."
+                } else {
+                    $serverName = "ADMIN:$serverName"
+                }
                 $NonPooledConnection = $true
             }
 
@@ -704,7 +714,16 @@ function Connect-DbaInstance {
                         $connContext.StatementTimeout = $StatementTimeout
                     }
                     if ($DedicatedAdminConnection -and $inputObject.ConnectionContext.ServerInstance -notmatch '^ADMIN:') {
-                        $connContext.ServerInstance = 'ADMIN:' + $connContext.ServerInstance
+                        if ($instance.IsLocalHost) {
+                            # Use localhost to avoid multiple IP resolution on multi-homed servers (issue #10151)
+                            if ($instance.InstanceName -ne 'MSSQLSERVER') {
+                                $connContext.ServerInstance = "ADMIN:localhost\$($instance.InstanceName)"
+                            } else {
+                                $connContext.ServerInstance = "ADMIN:localhost"
+                            }
+                        } else {
+                            $connContext.ServerInstance = 'ADMIN:' + $connContext.ServerInstance
+                        }
                         $connContext.NonPooledConnection = $true
                     }
                     if ($Database) {

tests/Connect-DbaInstance.Tests.ps1

@@ -281,11 +281,30 @@ Describe $CommandName -Tag IntegrationTests {
     }
 
     Context "multiple dedicated admin connections are properly made using strings" {
-        # This might fail if a parallel test uses DAC - how can we ensure that this is the only test that is run?
         It "opens and closes the connections" {
+            $instance1 = [DbaInstanceParameter]$TestConfig.InstanceMulti1
+            if ($instance1.IsLocalHost) {
+                if ($instance1.InstanceName -ne 'MSSQLSERVER') {
+                    $name1 = "ADMIN:localhost\$($instance1.InstanceName)"
+                } else {
+                    $name1 = "ADMIN:localhost"
+                }
+            } else {
+                $name1 = 'ADMIN:' + $instance1.FullName
+            }
+            $instance2 = [DbaInstanceParameter]$TestConfig.InstanceMulti2
+            if ($instance2.IsLocalHost) {
+                if ($instance2.InstanceName -ne 'MSSQLSERVER') {
+                    $name2 = "ADMIN:localhost\$($instance2.InstanceName)"
+                } else {
+                    $name2 = "ADMIN:localhost"
+                }
+            } else {
+                $name2 = 'ADMIN:' + $instance2.FullName
+            }
             $server = Connect-DbaInstance -SqlInstance $TestConfig.InstanceMulti1, $TestConfig.InstanceMulti2 -DedicatedAdminConnection
-            $server[0].Name | Should -Be "ADMIN:$($TestConfig.InstanceMulti1)"
-            $server[1].Name | Should -Be "ADMIN:$($TestConfig.InstanceMulti2)"
+            $server[0].Name | Should -Be $name1
+            $server[1].Name | Should -Be $name2
             $null = $server | Disconnect-DbaInstance
             # DAC is not reopened in the background
             Start-Sleep -Seconds 10