Solution for Subnet Conflict Between VPN and LAN

We had an issue at work where VPN users are having intermittent difficulty reaching servers due to a subnet conflict, ie the VPN network is 192.168.1.X, and so is the LAN they’re connecting from. It displayed itself in a strange way: application connections would fail repeated attempts, but after pinging the servers in question the first ping would fail but the next 3 would work, and then the application would work, for a while. Finally I developed a solution for this. Here’s what I did: create a Powershell script that identifies the IP address of the VPN connection, then create a static route for the 192.168.1.0/24 and 192.168.0.0/24 subnets (since that’s another common home subnet, as well as one running at the office) going out the VPN connection’s IP with a lower metric number than everything else. Then, set a scheduled task to run automatically when the VPN connection occurs (task triggered by event). Save the script as “Fix_VPN.ps1” at the root of the C drive and import the scheduled task (you may need to make sure that the event I have triggering this is also being generated by whatever VPN solution you’re using; we’re using built in RAS (SSTP)).

Powershell Script:

 
$ErrorActionPreference= 'silentlycontinue'
$ip = $null
$nics = [System.Net.NetworkInformation.NetworkInterface]::GetAllNetworkInterfaces()
foreach ($nic in $nics) {
   if($nic.Name -like "*VPN*"){
      $props = $nic.GetIPProperties()
      $addresses = $props.UnicastAddresses
      foreach ($addr in $addresses) {
         $ip = $($addr.Address.IPAddressToString)
      }
      break
   }
}
if($ip -ne $null){
   route delete 192.168.1.0 METRIC 1 | Out-Null
   route delete 192.168.0.0 METRIC 1 | Out-Null
   route add 192.168.1.0 mask 255.255.255.0 $ip METRIC 1 | Out-Null
   route add 192.168.0.0 mask 255.255.255.0 $ip METRIC 1 | Out-Null
}

Scheduled Task:

   <?xml version="1.0" encoding="UTF-16"?>
    <Task version="1.2" xmlns="http://schemas.microsoft.com/windows/2004/02/mit/task">
      <RegistrationInfo>
        <Date>2016-01-28T12:11:28.5687701</Date>
        <Author>AGreenBHM</Author>
      </RegistrationInfo>
      <Triggers>
        <EventTrigger>
          <Enabled>true</Enabled>
          <Subscription>&lt;QueryList&gt;&lt;Query Id="0" Path="System"&gt;&lt;Select Path="System"&gt;*[System[Provider[@Name='Rasman'] and EventID=20267]]&lt;/Select&gt;&lt;/Query&gt;&lt;/QueryList&gt;</Subscription>
        </EventTrigger>
      </Triggers>
      <Principals>
        <Principal id="Author">
          <UserId>S-1-5-18</UserId>
          <RunLevel>HighestAvailable</RunLevel>
        </Principal>
      </Principals>
      <Settings>
        <MultipleInstancesPolicy>IgnoreNew</MultipleInstancesPolicy>
        <DisallowStartIfOnBatteries>false</DisallowStartIfOnBatteries>
        <StopIfGoingOnBatteries>true</StopIfGoingOnBatteries>
        <AllowHardTerminate>true</AllowHardTerminate>
        <StartWhenAvailable>false</StartWhenAvailable>
        <RunOnlyIfNetworkAvailable>false</RunOnlyIfNetworkAvailable>
        <IdleSettings>
          <StopOnIdleEnd>true</StopOnIdleEnd>
          <RestartOnIdle>false</RestartOnIdle>
        </IdleSettings>
        <AllowStartOnDemand>true</AllowStartOnDemand>
        <Enabled>true</Enabled>
        <Hidden>false</Hidden>
        <RunOnlyIfIdle>false</RunOnlyIfIdle>
        <WakeToRun>false</WakeToRun>
        <ExecutionTimeLimit>P3D</ExecutionTimeLimit>
        <Priority>7</Priority>
      </Settings>
      <Actions Context="Author">
        <Exec>
          <Command>powershell</Command>
          <Arguments>C:\Fix_VPN.ps1</Arguments>
        </Exec>
      </Actions>
    </Task>