In TCP/IP networking, LAST_ACK is one of the possible states shown in netstat output. It indicates that a connection is in the final stage of termination, where one endpoint has sent a FIN packet and received an ACK for it, but is still waiting for the remote endpoint's FIN packet to complete the connection teardown.

When a Windows server shows thousands of connections in LAST_ACK state, it typically indicates one of these scenarios:

• The remote client crashed or disconnected improperly without completing the TCP handshake
• Network issues preventing the final ACK from reaching your server
• Application bugs where sockets aren't being properly closed
• Resource exhaustion preventing proper connection cleanup

Here's a PowerShell script to analyze LAST_ACK connections:

# Get all TCP connections in LAST_ACK state
$lastAckConnections = Get-NetTCPConnection -State LastAck

# Group by remote address to find problematic clients
$lastAckConnections | Group-Object RemoteAddress | Sort-Object Count -Descending

# Check if specific ports are affected
$lastAckConnections | Group-Object LocalPort | Sort-Object Count -Descending

Based on the diagnosis, consider these approaches:

# Adjust TCP timeout settings (requires admin)
Set-NetTCPSetting -SettingName InternetCustom -InitialRtoMs 3000
Set-NetTCPSetting -SettingName InternetCustom -MaxSynRetransmissions 2

# For application-level issues, implement proper socket cleanup:
try {
    $socket = New-Object System.Net.Sockets.TcpClient
    # ... socket operations ...
} finally {
    if ($socket -ne $null) {
        $socket.Close()
        $socket.Dispose()
    }
}

Implement regular monitoring with this Nagios check:

#!/bin/bash
LAST_ACK_COUNT=$(netstat -an | grep -c "LAST_ACK")
WARNING=100
CRITICAL=500

if [ "$LAST_ACK_COUNT" -ge "$CRITICAL" ]; then
    echo "CRITICAL: $LAST_ACK_COUNT connections in LAST_ACK"
    exit 2
elif [ "$LAST_ACK_COUNT" -ge "$WARNING" ]; then
    echo "WARNING: $LAST_ACK_COUNT connections in LAST_ACK"
    exit 1
else
    echo "OK: $LAST_ACK_COUNT connections in LAST_ACK"
    exit 0
fi

For persistent issues, consider these registry modifications:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters]
"TcpTimedWaitDelay"=dword:0000001e
"MaxUserPort"=dword:0000fffe
"TcpNumConnections"=dword:00fffffe

In TCP protocol terminology, LAST_ACK represents the final acknowledgment state in the connection termination sequence. When a connection enters this state, one endpoint has sent a FIN (finish) packet and received an ACK (acknowledgment) for it, and is now waiting for the remote endpoint's FIN packet before fully closing.

// Typical TCP state transition during connection termination
CLOSE_WAIT → LAST_ACK → CLOSED

When a Windows server shows numerous connections stuck in LAST_ACK, several scenarios could be at play:

• The remote client failed to properly close the connection (didn't send FIN)
• Network issues preventing FIN packets from reaching the server
• Application bugs where processes aren't properly closing sockets
• OS-level TCP stack issues or configuration problems

First, verify the state with netstat:

netstat -ano | findstr LAST_ACK
# Count occurrences:
netstat -ano | find /c "LAST_ACK"

Check associated processes:

tasklist /FI "PID eq [process_id_from_netstat]"

Immediate mitigation:

# Reset lingering connections (Windows Server 2012+)
netsh int ip reset
# Or adjust TCP timeout (temporary)
netsh int tcp set global rto=1000

Permanent fixes:

• Update network drivers and Windows patches
• Review application socket handling code
• Adjust TCP stack parameters in registry

This C# example shows correct socket termination:

try {
    // Normal shutdown sequence
    socket.Shutdown(SocketShutdown.Both);
    socket.Close();
}
catch (Exception ex) {
    // Force closure if normal shutdown fails
    socket.Dispose();
}
finally {
    if (socket != null)
        socket.Dispose();
}

Use Performance Monitor to track TCP connections:

Counter: TCPv4/Connections
Instance: _Total

Consider implementing connection health checks in your applications and setting appropriate keepalive timeouts.