Workaround for WSL2 network broken on VPN

WSL2_VPN_Workaround_Instructions.md

Overview

Internet connection and DNS routing are broken from WSL2 instances, when some VPNs are active.

The root cause seems to be that WSL2 and the VPN use the same IP address block, and the VPN routing clobbers WSL2's network routing.

This problem is tracked in multiple microsoft/WSL issues including, but not limited to:

• microsoft/WSL#5068
• microsoft/WSL#4277
• microsoft/WSL#4246

Change WSL2 subnet (Preferred Solution)

Permanently changing the IP address block used by WSL2 appears to prevent the routing conflict that breaks WSL2 networking.

More details and instructions in this gist: (Thanks @mikegerber for the explanation)

• https://gist.github.com/mikegerber/91fcea262028e09b2fd0969193c6c260

Interface Metric solution

Below is my original solution of modifying the VPN interface metric each time the VPN connects

Preferably, use the WSL subnet change above for a permanent fix.

The workaround breaks down into two problems:

1. Network connection to internet
2. DNS in WSL2

Network connection

When the VPN connection is active, network traffic out of WSL2 is not passed to the internet.

Changing the Interface Metric 1 -> 6000 for AnyConnect VPN Adapter resolves the connection issue, but this has to be done after each time the VPN connects.

By default, the Interface Metrics for AnyConnect are:

• IPv6: 6000
• IPv4: 1

ping times out from WSL Shell.

Changing the Interface Metrics for AnyConnect to:

• IPv6: 6000
• IPv4: 6000

ping to IP Addresses succeed, but still no DNS Resolution.

DNS Resolution

When the VPN is active, the autogenerated /etc/resolv.conf does not work. The list of nameservers must be manually built to include some sane default DNS Name Servers and the DNS from the VPN.

First, disable automatically generating /etc/resolv.conf. Add the following configuration, or create the file if it doesn't exist. The path to this file is from the shell prompt of your WSL2 instance.

/etc/wsl.conf

[network]
generateResolvConf = false

Next, manually add the corportate DNS Server as the first nameserver in /etc/resolv.conf.

/etc/resolv.conf

nameserver <corporateDNS1>
nameserver <corporateDNS2>
nameserver 1.1.1.1

To get <corporateDNS> addresses, use ipconfig /all from CMD or Powershell prompt, and check the details of the VPN adapter:

Description . . . . . . . . . . . : Cisco AnyConnect Secure Mobility Client Virtual Miniport Adapter for Windows x64
Physical Address. . . . . . . . . : XX-XX-XX-XX-XX-XX
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes
IPv6 Address. . . . . . . . . . . : xxxx:xxxx:xxxx:xxxx(Preferred)
Link-local IPv6 Address . . . . . : xxxx:xxxx:xxxx:xxxx(Preferred)
IPv4 Address. . . . . . . . . . . : 10.20.30.40(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.255
Default Gateway . . . . . . . . . : ::
                                    0.0.0.0
DHCPv6 IAID . . . . . . . . . . . :
DHCPv6 Client DUID. . . . . . . . : 
DNS Servers . . . . . . . . . . . : 123.45.67.89    <- Corporate DNS 1
                                    123.45.67.90    <- Corporate DNS 2
Primary WINS Server . . . . . . . : xxx.xx.xxx.xx
NetBIOS over Tcpip. . . . . . . . : Enabled

Automatically update Interface Metric

To automate this, I put the PS command in a script and created a Scheduled Task to run every time there is a network change.

Save the script in a file

First, create the script. I have a 'scripts' directory in my Windows user home, so I put it at:

%HOMEPATH%\scripts\UpdateAnyConnectInterfaceMetric.ps1

Get-NetAdapter | Where-Object {$_.InterfaceDescription -Match "Cisco AnyConnect"} | Set-NetIPInterface -InterfaceMetric 6000

You can save it where you want, just make sure to use that path in step 13 below.

Create the scheduled task:

1. Open 'Task Scheduler'
2. Click "Create Task" on Right Sidebar
3. Name: Update Anyconnect Adapter Interface Metric for WSL2
4. Set Security Options
   • Check box: 'Run with highest priveleges'
5. Select 'Triggers' Tab
6. Click 'New' at bottom of Window
7. Open 'Begin the task' drop-down
8. Select 'On an Event'
9. Configure Event:
   • option 1: Trigger on any Network Change
     • Log: 'Microsoft-Windows-NetworkProfile/Operational'
     • Source: 'NetworkProfile'
     • Event ID: '10000'
   • option 2: Trigger only when AnyConnect Client successfully connects to VPN
     • Anyconnect 4.x
       • Log: 'Cisco AnyConnect Secure Mobility Client'
       • Source: 'acvpnagent'
       • Event ID: '2039'
     • Anyconnect 5.x
       • Log: 'Cisco Secure Client - AnyConnect VPN'
       • Source: 'csc_vpnagent'
       • Event ID: '2039'
10. Click 'OK'
11. Select 'Actions' Tab
12. Click 'New'
13. Configure Action:
    • Action: 'Start a Program'
    • Program/script: 'Powershell.exe'
    • Add arguments: '-ExecutionPolicy Bypass -File %HOMEPATH%\scripts\UpdateAnyConnectInterfaceMetric.ps1'
14. Click 'OK'
15. Select 'Conditions' Tab
16. Uncheck box:
    • Power -> Start the task only if the computer is on AC Power
17. Click 'OK'

When AnyConnect finishes connecting, a Powershell window pops up for a couple seconds and WSL can reach the network.

(169.254.214.0/24 is the WSL2 NAT network I configured (see my comment above), .1 is the NAT gateway = my laptop) I would recommend first checking if you have general IPv4 connectivity using ping -n 8.8.8.8 first, then fixing DNS if it needs fixing.

I've added a note about DNS in my gist. WSL2 seems to bring a DNS proxy and autoconfigures it here, but I didn't find any documentation on it + I'm not a really a Windows expert.

Anyone coming here mightwant to attach anoter HyperV switch to the WSL machine

Here is a procedure

microsoft/WSL#4799 (comment)

microsoft/WSL#4799 (comment)