Issues with DAC connection

[Phaere]

Verified issue does not already exist?

I have searched and found no existing issue

What error did you receive?

Could not connect because the maximum number of '1' dedicated administrator connections already exists. Before a new connection can be made, the existing dedicated administrator connection must be dropped, either by logging off or ending the process

Steps to Reproduce

Issue 1:
Running Export-DbaLinkedServer -SqlInstance SRV will result in error mentioned above in server logs.
As described in #9921 we should open DAC connection before calling Get-DecryptedObject, so Export-DbaLinkedServer should open DAC connection too?
Issue 2:
After #9921 merged, Copy-DbaLinkedServer will always try to open DAC connection despite of "-ExcludePasswords" parameter. Should we check this parameter before opening connection?
Issue 3:
Root issue of error above is that System.Data.SqlClient.SQLConnection tries to connect to all IP's simultaneously.
So when we try to open DAC connection in Get-DecryptedObject it opens in "Invoke-Command2" function and executes on source server locally. Locally we have multiple IP's per server by default (local, ipv6, ipv4 etc) and DAC connection tries to open multiple times to each ip simultaneously

Please confirm that you are running the most recent version of dbatools

Major Minor Build Revision

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2 7 13 -1

Other details or mentions

I don't found how i can limit this behavior of System.Data.SqlClient.SQLConnection, so maybe DAC connection should perform directly to IP instead of DNS name if it connects to local machine?

What PowerShell host was used when producing this error

Windows PowerShell (powershell.exe)

PowerShell Host Version

Name Value

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PSVersion 5.1.20348.4294
PSEdition Desktop
PSCompatibleVersions {1.0, 2.0, 3.0, 4.0...}
BuildVersion 10.0.20348.4294
CLRVersion 4.0.30319.42000
WSManStackVersion 3.0
PSRemotingProtocolVersion 2.3
SerializationVersion 1.1.0.1

SQL Server Edition and Build number

Microsoft SQL Server 2016 (SP3-GDR) (KB5058718) - 13.0.6460.7 (X64) May 29 2025 21:37:21 Copyright (c) Microsoft Corporation Enterprise Edition: Core-based Licensing (64-bit) on Windows Server 2022 Datacenter 10.0 (Build 20348: )

.NET Framework Version

PSChildName Version

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Client 4.8.04161
Full 4.8.04161
Client 4.0.0.0

[potatoqualitee]

thank you for the report, @Phaere. @claude can you please do some deep research and ultrathink about the best solution? this is a challenging, recurring problem.

[claude]

Claude finished @potatoqualitee's task in 5m 8s —— View job

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Deep Analysis: DAC Connection Issues ✅

I've completed comprehensive research on the DAC connection problem. Here are my findings and recommended solutions.

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Problem Summary

Issue 1 - Export-DbaLinkedServer missing DAC connection:

• Location: Export-DbaLinkedServer.ps1:149
• Calls Get-DecryptedObject without first establishing a DAC connection
• Unlike Copy-DbaLinkedServer which opens DAC in the begin block (line 286)
• Forces Get-DecryptedObject to attempt DAC connection internally via Invoke-Command2

Issue 2 - Copy-DbaLinkedServer unnecessary DAC:

• Location: Copy-DbaLinkedServer.ps1:286
• Opens DAC connection unconditionally in begin block
• But Get-DecryptedObject is only called when -ExcludePassword is NOT set (line 136)
• Wastes the precious single DAC connection when passwords aren't being copied

Issue 3 - Root cause (SQL Client parallel connection attempts):

• When Get-DecryptedObject creates DAC connection string inside Invoke-Command2:
  • Non-clustered (line 110): "Server=ADMIN:$fullComputerName\$instance;Trusted_Connection=True;Pooling=false"
  • Clustered (line 127): "Server=ADMIN:$sourceName;Trusted_Connection=True;Pooling=false;"
• System.Data.SqlClient.SQLConnection resolves hostname to multiple IPs (IPv4, IPv6, localhost, etc.)
• SQL Client attempts connections to all IPs simultaneously
• DAC only allows 1 connection maximum, so parallel attempts trigger: "maximum number of '1' dedicated administrator connections already exists"

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Root Cause Analysis

The connection flow:

Export-DbaLinkedServer/Copy-DbaLinkedServer
  → Get-DecryptedObject (private function)
    → Invoke-Command2 (executes on target machine)
      → New System.Data.SqlClient.SQLConnection(connString)
        → Resolves hostname → [192.168.1.100, ::1, 127.0.0.1, fe80::...]
        → Attempts parallel connections to ALL IPs
        → ERROR: Only 1 DAC connection allowed!

On modern Windows servers with multiple network interfaces, a hostname typically resolves to:

• IPv4 loopback: 127.0.0.1
• IPv6 loopback: ::1
• Physical network IPv4: 192.168.x.x or 10.x.x.x
• Physical network IPv6: fe80::...

SQL Client's default behavior is to attempt connections to all resolved addresses simultaneously for faster connection establishment. This is fine for normal connections but fatal for DAC since only ONE DAC connection is allowed per SQL Server instance.

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Recommended Solutions

I recommend a multi-part fix addressing all three issues:

Solution for Issue 3 (Root Cause - MOST CRITICAL)

Modify Get-DecryptedObject.ps1 to use 127.0.0.1 for local DAC connections:

For non-clustered instances (line 110), change:

# CURRENT (problematic)
$connString = "Server=ADMIN:$fullComputerName\$instance;Trusted_Connection=True;Pooling=false"

# PROPOSED (use 127.0.0.1 to avoid multi-IP resolution)
$connString = "Server=ADMIN:127.0.0.1\$instance;Trusted_Connection=True;Pooling=false"

Rationale:

• 127.0.0.1 is a single IP address, preventing parallel connection attempts
• DAC connections are always local (executed via Invoke-Command2 on the target machine)
• Using loopback IP is standard practice for local SQL connections
• Avoids DNS resolution overhead

For clustered instances, the connection already uses the server name which is required for remote DAC.

Solution for Issue 1 (Export-DbaLinkedServer)

Add DAC connection establishment in Export-DbaLinkedServer.ps1:

Since Export-DbaLinkedServer processes multiple instances via pipeline, the DAC connection should be established in the process block for each instance when passwords are being exported (when -ExcludePassword is NOT specified).

The pattern should follow Copy-DbaLinkedServer but adapted for the pipeline pattern used in Export-DbaLinkedServer.

Solution for Issue 2 (Copy-DbaLinkedServer)

Conditionally open DAC in Copy-DbaLinkedServer.ps1:

Modify the begin block (currently line 286) to only establish DAC when passwords will be retrieved:

begin {
    if (-not $script:isWindows) {
        Stop-Function -Message "Copy-DbaCredential is only supported on Windows"
        return
    }
    $null = Test-ElevationRequirement -ComputerName $Source.ComputerName

    if ($Force) { $ConfirmPreference = 'none' }

    # Only establish DAC if we're actually retrieving passwords
    if (-not $ExcludePassword) {
        try {
            Write-Message -Level Verbose -Message "Opening dedicated admin connection for password retrieval."
            $sourceServer = Connect-DbaInstance -SqlInstance $Source -SqlCredential $SourceSqlCredential -DedicatedAdminConnection -WarningAction SilentlyContinue
        } catch {
            Stop-Function -Message "Failure" -Category ConnectionError -ErrorRecord $_ -Target $Source
            return
        }
        # ... rest of validation ...
    } else {
        # When ExcludePassword is used, don't waste the DAC connection
        try {
            $sourceServer = Connect-DbaInstance -SqlInstance $Source -SqlCredential $SourceSqlCredential
        } catch {
            Stop-Function -Message "Failure" -Category ConnectionError -ErrorRecord $_ -Target $Source
            return
        }
    }
    # ... rest of begin block ...
}

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Alternative Approaches Considered

Alternative 1: Add connection string parameters

• Add MultiSubnetFailover=False to prevent parallel attempts
• Rejected: Still attempts multiple IPs sequentially, slower and may still fail

Alternative 2: Resolve to IP before connecting

• Query DNS to get single IP, use that in connection string
• Rejected: Adds complexity, DNS resolution may still return multiple IPs

Alternative 3: Use (local) or . notation

• Change connection string to use ADMIN:(local)\$instance
• Rejected: Still resolves to multiple loopback addresses on some systems

Alternative 4: Refactor to pass DAC connection to Get-DecryptedObject

• Pass an already-opened DAC server object instead of opening inside
• Considered but complex: Would require significant refactoring of Get-DecryptedObject signature

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Implementation Priority

1. CRITICAL: Fix Issue 3 (Get-DecryptedObject using 127.0.0.1) - This solves the root cause
2. HIGH: Fix Issue 2 (Copy-DbaLinkedServer conditional DAC) - Prevents unnecessary DAC usage
3. MEDIUM: Fix Issue 1 (Export-DbaLinkedServer DAC pattern) - Improves reliability

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Additional Considerations

Testing Requirements:

• Test on servers with multiple network interfaces
• Test on both non-clustered and clustered instances
• Test Export-DbaLinkedServer with and without -ExcludePassword
• Test Copy-DbaLinkedServer with and without -ExcludePassword
• Verify DAC connection is properly released after use

Backward Compatibility:

• Using 127.0.0.1 is compatible with all SQL Server versions
• Changes to conditionally open DAC are internal logic improvements
• No breaking changes to public API

Documentation:

• Update function comments to clarify DAC usage
• Note that password decryption requires DAC connection
• Explain that only one DAC connection is allowed per instance

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Questions for Maintainers

1. Should we also update other commands that might use DAC? (e.g., Invoke-DbaDbDecryptObject, Copy-DbaCredential)
2. Should we add a common private function for DAC connection string generation to centralize this logic?
3. Do you want me to implement these fixes? I can create a PR with all three solutions if you approve this approach.

The user reported this as a "challenging, recurring problem" and you've correctly identified the root cause. The 127.0.0.1 fix is elegant, simple, and should resolve the core issue immediately.

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| Branch: claude/issue-10040-20251209-2329