WSL 2 Cisco AnyConnect Networking Workaround

Overview

WSL 2 uses a Hyper-V Virtual Network adapter. Network connectivity works without any issue when a VPN is not in use. However when a Cisco AnyConnect VPN session is established Firewall Rules and Routes are added which breaks connectivity within the WSL 2 VM. This issue is tracked WSL/issues/4277

Below outline steps to automatically configure the Interface metric on VPN connect and update DNS settings (/etc/resolv.conf) on connect/disconnect.

Manual Configuration

Set Interface Metrics

After connecting to the VPN, you'll want to modify the Interface Metric of the Cisco VPN Adapter

PS C:\Users\gyurgyik> Get-NetAdapter | Where-Object {$_.InterfaceDescription -Match "Cisco AnyConnect"} | Set-NetIPInterface -InterfaceMetric 6000

Run the following command in Powershell with Administrative permission.

At this point you should have connectivity in your container (but without name resolution). You can test this by running ping 8.8.8.8.

Set DNS servers in Linux VM

Once connected the VPN determine the DNS servers that are configured:

PS C:\Users\gyurgyik> (Get-NetAdapter | Where-Object InterfaceDescription -like "Cisco AnyConnect*" | Get-DnsClientServerAddress).ServerAddresses
10.10.0.124
10.10.0.132

Update /etc/resolv.conf

N-20S5PF20MB4R:~$ cat /etc/resolv.conf
nameserver 10.10.0.124
nameserver 10.10.0.132

Verify Connectivity

ping google.com -c 4

Automatic Configuration

Create Scripts

Save the following scripts to %homepath%\wsl\scripts

setCiscoVpnMetric.ps1

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

setDns.ps1

$dnsServers = (Get-NetAdapter | Where-Object InterfaceDescription -like "Cisco AnyConnect*" | Get-DnsClientServerAddress).ServerAddresses -join ','
$searchSuffix = (Get-DnsClientGlobalSetting).SuffixSearchList -join ','

function set-DnsWsl($distro) {
    if ( $dnsServers ) {    
        wsl.exe -d $distro -u root /opt/wsl_dns.py --servers $dnsServers --search $searchSuffix
    }
    else {
        wsl.exe -d $distro -u root /opt/wsl_dns.py
    }
}

set-DnsWsl fedora

wsl_dns.py

Download from: https://gist.githubusercontent.com/pyther/20bba4aee1a7e1485dd065adbf736891/raw/bb00f46d30da1b1eb7ba0fec4e6946a654c7b186/wsl_dns.py

Disable WSL Resolv Update

For each Linux instance:

Disable automatic updating of resolv.conf by WSL

$ cat <<EOF > /etc/wsl.conf
[network]
generateResolvConf = false
EOF

Restart/Shutdown WSL: wsl --shutdown (WARNING: this will kill all current sessions!)

Copy wsl_dns.py to Linux VMs

For each VM, run:

$ cp /mnt/c/Users/$username/wsl/scripts/wsl_dns.py /opt/wsl_dns.py
$ chmod +x /opt/wsl_dns.py

Create Scheduled Tasks

Windows Scheduled Tasks allows you to trigger an action when a certain log event comes in. The Cisco AnyConnect VPN client generates a number of log events.

We will create two tasks. The first task, will configure the interface metric when the VPN connects. The second task, will execute the dns update script inside of your Linux VM when the VPN Connects and Disconnects.

Cisco AnyConnect Events

2039: VPN Established and Passing Data
2061: Network Interface for the VPN has gone down
2010: VPN Termination
2041: The entire VPN connection has been re-established.

Procedure

Open Task Scheduler
Create a Folder called WSL (Optional, but easier to find rules later)
Create Rules
1. Update AnyConnect Adapter Interface Metric for WSL2
  - General: Check: Run with highest privileges
  - Triggers:
    - On an Event, Log: Cisco AnyConnect Secure Mobility Client, Source: acvpnagent, Event ID: 2039
    - On an Event, Log: Cisco AnyConnect Secure Mobility Client, Source: acvpnagent, Event ID: 2041
  - Action: Start a program, Program: Powershell.exe, Add arguments: -WindowStyle Hidden -NonInteractive -ExecutionPolicy Bypass -File %HOMEPATH%\wsl\scripts\setCiscoVpnMetric.ps1
  - Condition: Uncheck: Start the task only if the computer is on AC power
2. Update DNS in WSL2 Linux VMs
  - Triggers:
    - On an Event, Log: Cisco AnyConnect Secure Mobility Client, Source: acvpnagent, Event ID: 2039
    - On an Event, Log: Cisco AnyConnect Secure Mobility Client, Source: acvpnagent, Event ID: 2010
    - On an Event, Log: Cisco AnyConnect Secure Mobility Client, Source: acvpnagent, Event ID: 2061
    - On an Event, Log: Cisco AnyConnect Secure Mobility Client, Source: acvpnagent, Event ID: 2041
    - At log on: At log on of $USER
  - Action: Start a program, Program: Powershell.exe, Add arguments: -WindowStyle Hidden -NonInteractive -ExecutionPolicy Bypass -File %HOMEPATH%\wsl\scripts\setDns.ps1
  - Condition: Uncheck: Start the task only if the computer is on AC power
Test: Connect to the VPN, a powershell window should pop-up briefly

FAQ

Q: Does traffic orginating from the Linux VM still route through the VPN?
A: Yes, I believe so. I did not see any leaked traffic when running a tcpdump on my router.

Q: Are VPN resources accessible from the Linux VM?
A: Yes

Q: Can the Linux VM communicate with Windows?
A: No, it appears a firewall rule is preventing traffic between Windows and the Linux VM.

Q: Can I still run WSL1 instances?
A: Yes, you can run WSL1 and WSL2 insatnces simutaneously

Q: How do I revert/disable these changes?
A: Disable scheduled Tasks, remove/modify /etc/wsl.conf from each WSL Instance, Reboot

pyther/wsl2_ciscoanyconnect_workaround.md