Fix WSL 2 DNS resolution when connected to Cisco AnyConnect VPN

wsl-vpn-fix.sh

#!/bin/bash
#--------------------------------------------------------------------------------#
#                                                                                #
# Fix WSL DNS resolution with Cisco AnyConnect                                   #
#                                                                                #
# ! Don't forget to set this configuration in /etc/wsl.conf:                     #
#        [network]                                                               #
#        generateResolvConf = false                                              #
#                                                                                #
# Based on:                                                                      #
#        https://askubuntu.com/a/15856                                           #
#        https://gist.github.com/coltenkrauter/608cfe02319ce60facd76373249b8Aca6 #
#        https://gist.github.com/nfekete/7a277bf9e25e89e1c8bfb8b64dcc08ed        #
#        https://github.com/microsoft/WSL/issues/4277#issuecomment-921087670     #
#                                                                                #
# Enjoy, ~ Mateus Mercer <contact@matbm.net> 2021                                #
#                                                                                #
#--------------------------------------------------------------------------------#

if [[ $EUID -ne 0 ]]; then
   echo "This script must be run as root"
   exit 1
fi

TMP_DIR=`mktemp -d`
TMP_SCRIPT="$TMP_DIR/network-metrics.ps1"
TMP_RESOLV="$TMP_DIR/resolv.conf"
POWERSHELL="powershell.exe"

touch "$TMP_SCRIPT"
# this starts the script as admin in powershell
pcmd="Start-Process -FilePath $POWERSHELL -verb runas -ArgumentList $POWERSHELL,-noprofile,-executionpolicy,bypass,-file,$(wslpath -aw $TMP_SCRIPT)"

# generate the fix script
echo 'Get-NetIPInterface -InterfaceAlias "vEthernet (WSL)" | Set-NetIPInterface -InterfaceMetric 1
Get-NetAdapter | Where-Object {$_.InterfaceDescription -Match "Cisco AnyConnect"} | Set-NetIPInterface -InterfaceMetric 6000' > $TMP_SCRIPT

# required to allow windows do read the folder contents
chmod 777 $TMP_DIR

trap ctrlC INT

removeTempFiles() {
        # powershell continues in the background, trying to use a file that was deleted. TODO: fix the racing condition
        # rm -rf $TMP_DIR
        true
}

ctrlC() {
        echo
        echo "Trapped Ctrl-C, removing temporary files"
        removeTempFiles
        stty sane
}

echo "Current resolv.conf"
echo "-------------------"
cat /etc/resolv.conf

echo
echo "Creating new resolv.conf"
echo "------------------------"

{
        head -1 /etc/resolv.conf | grep '^#.*generated'
        for i in `$POWERSHELL -Command "Get-DnsClientServerAddress -AddressFamily ipv4 | Select-Object -ExpandProperty ServerAddresses"`; do
                echo nameserver $i
        done
        tail -n+2 /etc/resolv.conf | grep -v '^nameserver'

} | tr -d '\r' | tee $TMP_RESOLV

(set -x; cp -i $TMP_RESOLV /etc/resolv.conf; set +x)

echo
echo "Fixing network metrics for cisco anyconnect"
echo "-------------------------------------------"
cat $TMP_SCRIPT

# run the network metrics fix script
$POWERSHELL -command "$pcmd"

removeTempFiles