When working remotely through VPN connections, Windows authentication can behave unexpectedly. Here's what we've observed across different applications:

// Sample error message from SSMS
Msg 18452, Level 14, State 1, Server SERVERNAME, Line 1
Login failed. The login is from an untrusted domain and cannot be used with Windows authentication.

The core issue stems from how Windows handles credential delegation when multiple networks are involved. When VPN'd into a client network:

• Windows attempts to use the VPN connection's credentials first
• The local domain credentials get "pushed down" in the authentication queue
• Applications may receive the wrong security token

Instead of using runas manually each time, consider these more sustainable solutions:

# PowerShell script to launch SSMS with proper credentials
$cred = Get-Credential "DOMAIN\username"
Start-Process "ssms.exe" -Credential $cred

Alternatively, create a shortcut with this target:

runas /user:DOMAIN\username /netonly "C:\Program Files (x86)\Microsoft SQL Server\140\Tools\Binn\ManagementStudio\Ssms.exe"

For AD-authenticated websites, Internet Explorer and Edge have particular behaviors:

<!-- Example of adding trusted sites to IE -->
Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMap\Domains\yourdomain.com]
"https"=dword:00000001
"*"=dword:00000001

This registry tweak can help Windows prioritize your domain credentials:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa]
"DisableLoopbackCheck"=dword:00000001
"DisableDomainCreds"=dword:00000000

For Outlook authentication prompts, try these steps:

1. Close Outlook
2. Delete the profile from Mail in Control Panel
3. Recreate the profile while on VPN
4. Use the full domain\username format during setup

To avoid these issues system-wide:

:: Batch script to reset credential manager
cmdkey /list | findstr /I "Target" 
cmdkey /delete:TargetName

Consider implementing Always On VPN with device tunnels rather than user-level VPN connections for more consistent authentication behavior.

---

When working remotely via VPN, many Windows-authenticated services suddenly stop recognizing domain credentials. This manifests in several ways:

// Common symptoms:
// - SQL Server: "Login failed. The login is from an untrusted domain"
// - IIS Websites: Unexpected credential prompts
// - Office Apps: Repeated authentication requests

The root cause lies in how Windows handles credential delegation during VPN sessions. The system attempts to use the VPN connection's credentials (often NULL or cached) instead of your domain credentials.

// Typical authentication flow:
1. Local machine caches domain credentials
2. VPN connection establishes separate security context
3. Applications inherit VPN context by default
4. Domain resources reject VPN credentials

Method 1: RunAs with Explicit Credentials

The most reliable workaround is launching applications with explicit domain credentials:

runas /netonly /user:DOMAIN\\username "C:\\Program Files (x86)\\Microsoft SQL Server\\140\\Tools\\Binn\\ManagementStudio\\Ssms.exe"

Method 2: Registry Modification for Credential Delegation

Add this registry key to control credential behavior:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa]
"DisableLoopbackCheck"=dword:00000001

Method 3: PowerShell Script for Context Switching

Create a credential wrapper script:

# Save as VPN-AuthHelper.ps1
param(
    [Parameter(Mandatory=$true)]
    [string]$AppPath
)

$cred = Get-Credential -Message "Enter DOMAIN credentials"
Start-Process -FilePath $AppPath -Credential $cred

Group Policy Configuration

For domain-joined machines, configure these GPO settings:

Computer Configuration -> Administrative Templates -> System -> Credentials Delegation
- Allow Delegating Default Credentials = Enabled
- Add servers to list: TERMSRV/*, *.yourdomain.com

SQL Server Connection String Modification

When programmatically connecting to SQL:

// C# example with explicit credentials
string connectionString = @"Server=sqlserver.yourdomain.com;
                          Database=YourDB;
                          Integrated Security=SSPI;
                          Persist Security Info=True;
                          MultipleActiveResultSets=True";

Use these tools to diagnose authentication issues:

• klist purge - Clear Kerberos tickets
• whoami /all - View current security context
• Process Explorer - Check process tokens

The key is ensuring your applications run under the correct security context. While VPNs create temporary network bridges, they often break the native Windows authentication flow. The solutions above provide both immediate workarounds and long-term fixes depending on your environment.