As organizations begin piloting Windows 11 24H2 deployments across their enterprise environments, unexpected anomalies may surface during early adoption. One such issue has emerged in a recent pilot: a workstation, upgraded via Windows Update for Business (WUfB) from Windows 10 22H2 to Windows 11 24H2, experienced a loss of trust relationship with the Active Directory domain.

While this issue affected only 1 out of 12 devices in the pilot cohort, it raises significant concerns for wider rollouts. This post provides a deep dive into the root cause analysis, troubleshooting methodology, and a preventive configuration to mitigate the risk in future deployments.

Observed Symptoms

In the context of a Windows 11 24H2 pilot upgrade via Intune’s WUfB mechanism:

  • One workstation became unable to authenticate to the domain post-upgrade.

  • Users were presented with a “The trust relationship between this workstation and the primary domain failed” error upon login.

  • Rejoining the domain temporarily resolved the issue, but concerns remained about recurrence across the fleet.

Given that the upgrade path was formally supported, this behaviour was unexpected and warranted a thorough investigation.

Technical Background

This problem typically indicates that the secure channel between the workstation and the domain controller (DC) has been broken. The secure channel is used for machine authentication and is established during domain join. Failures can occur due to:

  • A mismatch in the machine account password (rotated every 30 days by default).

  • Time synchronization issues.

  • Kerberos authentication failures, especially in response size or protocol negotiation.

In this case, evidence pointed to an issue during the post-upgrade reinitialization of secure communications, particularly Kerberos ticket exchanges that failed under specific network transport conditions.

Troubleshooting and Root Cause Analysis

Upon examining the affected workstation’s event logs and comparing them with network captures and domain controller diagnostics, the root cause was identified:

The Kerberos Key Distribution Center (KDC) on the DC attempted to respond to the workstation’s request, but the response packet size exceeded the limit for UDP transport, triggering a protocol-level error.

Specifically:

  • The client initiated a Kerberos Authentication Service Request (AS_REQ) over UDP.

  • The KDC’s response (AS_REP) was too large to fit within the default MaxDatagramReplySize (1465 bytes).

  • The KDC responded with KRB_ERR_RESPONSE_TOO_BIG, signaling that the client should switch to TCP.

  • The workstation failed to retry over TCP in a timely or expected manner, leading to authentication failure and a broken secure channel.

This behaviour was not present in prior builds (e.g., 22H2), suggesting a change in network stack behaviour, Kerberos token size inflation (possibly due to group memberships or SID history), or handling differences introduced in Windows 11 24H2.

Network Layer Insight

Network analysis revealed that UDP fragmentation was being mishandled in some parts of the network. If the network drops fragmented UDP packets or improperly reassembles them, Kerberos replies exceeding the size threshold fail to reach the client.

Furthermore, many enterprise environments use VPN tunnels, MTU restrictions, or firewalls that interfere with large UDP responses, making Kerberos over TCP the more reliable transport.

In this pilot, the issue only surfaced on one device likely due to:

  • A unique token size for that machine/user combination.

  • A path through the network with suboptimal UDP handling.

  • A Windows 11 24H2 client configuration that did not gracefully fallback to TCP.

Recommended Workaround: Forcing Kerberos to Prefer TCP

To mitigate this, a registry modification can be implemented on domain controllers to allow larger Kerberos responses over UDP—or encourage fallback to TCP.

Registry Key: MaxDatagramReplySize

Path:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Kdc

Entry:
MaxDatagramReplySize

Type:
REG_DWORD

Default Value:
1465 (decimal, bytes)

Guidance:

  • Increasing this value may delay triggering the TCP fallback, but can increase UDP fragmentation risk.

  • Conversely, reducing this value will encourage clients to use TCP more often, at the cost of slight latency increase during authentication.

PowerShell to Configure (Dev Environment First):

$keyPath = "HKLM:\SYSTEM\CurrentControlSet\Services\Kdc"
$entryName = "MaxDatagramReplySize"
$entryType = "DWord"
$entryValue = 1KB # Suggested: Set this lower than 1465 to force earlier TCP fallback

# Apply Registry Change
New-ItemProperty -Path $keyPath -Name $entryName -PropertyType $entryType -Value $entryValue

Write-Output "Registry key $keyPath with entry $entryName of type $entryType and value $entryValue created successfully."
⚠️ Important: Validate any change in a non-production (Dev/Test) environment first. Network conditions and authentication token sizes vary, and premature optimization can result in unexpected consequences in legacy or hybrid identity setups.

Alternative Client-Side Mitigation

To address fallback behaviour on the client side, the following group policy or registry can be used to force the workstation to use TCP for Kerberos:

Registry for Clients:

[HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Lsa\Kerberos\Parameters]
"MaxPacketSize"=dword:00000001

Setting MaxPacketSize to 1 forces the system to always use TCP for Kerberos communications.

  • Not recommended enterprise-wide unless UDP is universally unreliable across the network.

Preventing Issues in Future 24H2 Rollouts

Based on this incident, the following preventive steps are advised before proceeding with broader deployment of Windows 11 24H2:

1. Validate Kerberos Packet Size Profiles

  • Review token sizes of service accounts and power users using:

    whoami /groups

  • For users with many group memberships (over 100), token sizes can grow significantly.

2. Inspect Network Path MTU and UDP Handling

  • Run tools like ping -f -l and tracert to validate the MTU across VPNs and internal paths.

  • Use Wireshark or similar to monitor Kerberos UDP fragments.

3. Adjust MaxDatagramReplySize on DCs (If Needed)

  • Lower the value to force TCP fallback for environments where UDP reliability is questionable.

4. Test and Stage

  • Continue testing Windows 11 24H2 on varied hardware, user profiles, and network segments.

  • Monitor secure channel health post-upgrade using Test-ComputerSecureChannel PowerShell command.

5. Automation with Intune

  • Deploy registry settings via custom PowerShell scripts or Intune Remediations in proactive deployments.

  • Monitor registry compliance using endpoint reporting tools.

While the issue in this case study affected a minority of devices, it underscores the subtle interplay between operating system updates, network transport behaviour, and identity protocols like Kerberos. Trust relationship failures are particularly disruptive, and their intermittent nature can complicate root cause analysis.

By understanding the mechanics of Kerberos transport, specifically the implications of MaxDatagramReplySize, organizations can proactively mitigate the risk during major OS rollouts like Windows 11 24H2, keeping in mind:

  • The issue is not fundamentally a flaw in 24H2, but rather a byproduct of how identity negotiation operates under real-world network constraints.

  • With appropriate registry tuning and testing, the deployment process can be made resilient, predictable, and scalable.

Further Reading

Hope this helps!
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