Many Windows administrators encounter frustrating connection delays when their systems attempt IPv6 connectivity through AAAA records despite lacking proper IPv6 network infrastructure. The issue manifests most prominently with Microsoft Remote Desktop Protocol (RDP), where clients experience significant timeout delays before falling back to IPv4.

The core issue stems from Windows' Happy Eyeballs algorithm implementation. While RFC 6555 recommends simultaneous IPv4/IPv6 attempts with a 300ms fallback, Windows' implementation differs:

// Typical Windows IPv6 connection attempt sequence
1. Attempt IPv6 connection (waits for timeout)
2. Only after IPv6 fails does it attempt IPv4
3. Default timeout values create noticeable delays

Completely disabling IPv6 is neither recommended nor practical. Here are better approaches:

1. DNS-Level Solution: Prioritize IPv4 Records

Configure your DNS server to return IPv4 (A) records before IPv6 (AAAA) records:

# Windows DNS Server configuration
dnscmd /config /localnetpriority 1
dnscmd /config /localnetprioritynetmask ffffffff

2. Client-Side Registry Tweaks

Adjust the IPv6 preference on client machines via registry:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip6\Parameters]
"DisabledComponents"=dword:00000020

This sets IPv4 preference without completely disabling IPv6.

3. Disabling Problematic Transition Technologies

Disable specific IPv6 transition technologies causing issues:

netsh interface teredo set state disabled
netsh interface 6to4 set state disabled
netsh interface isatap set state disabled

For domain environments, implement these settings via GPO:

<GroupPolicy xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <Computer>
    <Network>
      <IPv6>
        <PreferIPv4OverIPv6>1</PreferIPv4OverIPv6>
      </IPv6>
    </Network>
  </Computer>
</GroupPolicy>

Verify your settings with these PowerShell commands:

Test-NetConnection -ComputerName server.example.com -TraceRoute
Get-NetTCPConnection -RemoteAddress server.example.com | Select-Object Local*,Remote*,State

For organizations planning IPv6 deployment:

1. Implement native IPv6 routing
2. Configure proper DNS resolution for both stacks
3. Test IPv6 connectivity before enabling in production

Essential tools for diagnosing IPv6-related issues:

• Wireshark with IPv6 filter (ipv6)
• Microsoft Message Analyzer
• Netsh trace (netsh trace start capture=yes IPv6.Address=*)

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Many Windows administrators encounter unexpected latency when IPv6-enabled clients attempt connections to servers with AAAA records, despite lacking proper IPv6 routing infrastructure. The core issue manifests when applications like Microsoft Remote Desktop (mstsc.exe) prioritize IPv6 connections, creating unnecessary delays before falling back to IPv4.

The behavior stems from Windows' dual-stack socket implementation which prefers IPv6 by default. When both A and AAAA records exist, the Happy Eyeballs algorithm should handle fallback, but implementation quirks cause delays:

// Typical Windows socket connection priority:
1. IPv6 native
2. 6to4
3. Teredo
4. IPv4

1. DNS-Level Control

For Windows DNS servers, configure DNS aging/scavenging to automatically remove stale AAAA records:

dnscmd /config /DefaultAgingState 1
dnscmd /config /DefaultNoRefreshInterval 3
dnscmd /config /DefaultRefreshInterval 3

2. Network Interface Configuration

Disable problematic IPv6 transition technologies while keeping core IPv6 stack active:

netsh interface teredo set state disabled
netsh interface 6to4 set state disabled
netsh interface isatap set state disabled

3. Application-Specific Fix for RDP

Create a registry tweak to modify RDP's connection behavior:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows NT\Terminal Services\Client]
"fClientDisableUDP"=dword:00000001
"AddressResolutionOrder"=dword:00000001

For large environments, implement automated AAAA record management:

# PowerShell script to audit and clean AAAA records
Import-Module DnsServer

$zones = Get-DnsServerZone | Where-Object {$_.ZoneType -eq "Primary"}
foreach ($zone in $zones) {
    $records = Get-DnsServerResourceRecord -ZoneName $zone.ZoneName -RRType "AAAA"
    foreach ($record in $records) {
        if (-not (Test-Connection $record.HostName -Count 1 -IPv6 -Quiet)) {
            Remove-DnsServerResourceRecord -ZoneName $zone.ZoneName -InputObject $record -Force
            Write-Host "Removed stale AAAA record for $($record.HostName)"
        }
    }
}

Verify IPv6 behavior using these diagnostic commands:

netsh interface ipv6 show prefixpolicies
nltest /dsgetsite
Test-NetConnection -ComputerName targetserver -TraceRoute

For packet-level analysis, use Wireshark with these display filters:

ipv6 || icmpv6 || tcp.port == 3389
!(ipv6.dst == ff02::1 || ipv6.dst == ff02::2)

For domain-joined systems, implement these settings via Group Policy:

• Computer Configuration > Policies > Administrative Templates > Network > TCPIP Settings > IPv6 Transition Technologies
• Disable "6to4 Relay Name" and "ISATAP Router Name"
• Set "Teredo State" to Disabled