Init.d script for keeping WSL resolv.conf in-sync with Windows

dns-sync.sh

#! /bin/bash
### BEGIN INIT INFO
# Provides:          dns-sync
# Required-Start:
# Required-Stop:
# Default-Start:     S
# Default-Stop:
# Short-Description: Synchronizes /etc/resolv.conf in WLS with Windows DNS - Matthias Brooks
### END INIT INFO

PATH=/sbin:/bin
PS=/mnt/c/Windows/System32/WindowsPowerShell/v1.0/powershell.exe

. /lib/init/vars.sh
. /lib/lsb/init-functions

do_start () {
        while true
        do
          #Retrieve nameservers from via Powershell
          TEMPFILE=$(mktemp)
          $PS -Command "Get-DnsClientServerAddress -AddressFamily IPv4 | Select-Object -ExpandProperty ServerAddresses" > $TEMPFILE
          /usr/bin/awk '!x[$0]++' $TEMPFILE > $TEMPFILE.2
          IFS=$'\r\n' GLOBIGNORE='*' command eval  'UNIQUE_NAMESERVERS=($(cat $TEMPFILE.2))'
          rm -f $TEMPFILE $TEMPFILE.2
          
          #Retrive search domains via powershell
          IFS=$'\r\n' GLOBIGNORE='*' command eval  'SEARCH_DOMAIN=($($PS -Command "Get-DnsClientGlobalSetting | Select-Object -ExpandProperty SuffixSearchList"))'
          UNIQUE_SEARCH_DOMAIN=($(/usr/bin/tr ' ' '\n' <<< "${SEARCH_DOMAIN[@]}" | /usr/bin/sort -u | /usr/bin/tr '\n' ' '))

          
          #Modify /etc/resolv.conf
          touch /etc/resolv.conf
          sed -i '/nameserver/d' /etc/resolv.conf > /dev/null  2>&1 || true
          sed -i '/search/d' /etc/resolv.conf > /dev/null  2>&1 || true

          for i in "${UNIQUE_NAMESERVERS[@]}"
          do
          	echo "nameserver ${i}" >> /etc/resolv.conf
          done
          if [ ${#UNIQUE_SEARCH_DOMAIN[@]} -ne 0 ]; then
            echo "search ${UNIQUE_SEARCH_DOMAIN[@]}" >> /etc/resolv.conf
          fi
          sleep 15
        done
}

do_status () {
        PID=$(cat /var/run/dns-sync.pid 2>/dev/null)
        if [ "$PID" == "" ]; then
          echo "dns-sync is not running"
          return
        fi
        
        if ps -p $PID > /dev/null
        then
           echo "dns-sync is running"
        else
           echo "dns-sync is not running"
        fi
}

case "$1" in
  start|"")
        kill $(cat /var/run/dns-sync.pid >/dev/null 2>&1) > /dev/null  2>&1 || true
        do_start &
        DO_SYNC_PID=$!
        echo "${DO_SYNC_PID}" > /var/run/dns-sync.pid
        ;;
  restart|reload|force-reload)
        echo "Error: argument '$1' not supported" >&2
        exit 3
        ;;
  stop)
        PID=$(cat /var/run/dns-sync.pid)
        if ps -p $PID > /dev/null
        then
           kill -9 $(cat /var/run/dns-sync.pid)
           echo "dns-sync stopped"
        else
           echo "dns-sync is not running"
        fi
        ;;
  status)
        do_status
        exit $?
        ;;
  *)
        echo "Usage: dns-sync.sh [start|stop]" >&2
        exit 3
        ;;
esac

Steps to run (as root)

This init.d script will keep Ubuntu WSL dns config in-sync with windows with a 15 sec lag-time.

Be warned: If this service is not running and the nameservers in windows userspace changed, then you may lose connectivity in WSL - make sure the daemon is running at all times

wget https://gist.github.com/matthiassb/9c8162d2564777a70e3ae3cbee7d2e95/raw/b204a9faa2b4c8d58df283ddc356086333e43408/dns-sync.sh -O /etc/init.d/dns-sync.sh
chmod +x /etc/init.d/dns-sync.sh
unlink /etc/resolv.conf
service dns-sync.sh start

Note: WSL does not support service auto-start, when you boot your system you'll need to start the service manually

wget https://gist.githubusercontent.com/matthiassb/9c8162d2564777a70e3ae3cbee7d2e95/raw/56640fbb50ec870d2a2f62b1f188081c29d45337/dns-sync.sh -O /etc/init.d/dns-sync.sh

Disable Init.d and restore original WSL functionality (as root)

service dns-sync.sh stop
unlink /etc/resolv.conf 
ln -s /run/resolvconf/resolv.conf /etc/resolv.conf

@regisbsb fixed

It's possible to autostart this script with bash. Just add this lines to ~/.bashrc:

if service dns-sync.sh status| grep -q 'dns-sync is not running'; then
   sudo service dns-sync.sh start
fi

Thanks for the great tip to get this working!

The script enables networking with the outside world (yay!). Unfortunatelly, that script prevents Linux to connect to a local X Server, so I can't use a local xfce :(

@estebarb. Can you elaborate a little more on your configuration/setup? I might be able to update this GIST to support this scenario.

Thank you for this... weirdly enough using this I saw in task manager init using a ton of CPU%... I will just stick to manually configuring.

Weird... everytime the script runs it changes the font used in the WSL console window. Has anyone else seen this?

Weird... everytime the script runs it changes the font used in the WSL console window. Has anyone else seen this?

This seems to happen when PowerShell, called from bash (maybe other shells, too), in a WSL instance is instructed to redirect its stdout to a file.

A workaround for the font switching problem is explained here -> https://patrickwu.space/2019/08/03/wsl-powershell-raster-font-problem/

congrats on the solution! I mean, REALLY!

you rock

Still works fantastically. Thanks!

@matthiassb is this required https://gist.github.com/matthiassb/9c8162d2564777a70e3ae3cbee7d2e95#gistcomment-2769071 ?

Seems to work without it and give me an error when enabled :)

To prevent yourself from having to type your password every time you open a new shell after putting

if service dns-sync.sh status| grep -q 'dns-sync is not running'; then
   sudo service dns-sync.sh start
fi

in your ~/.bashrc to start the service automatically as described by @alexey-krylov you can use the following lines to modify allow sudo users to modify the service without a password:

echo -e "%sudo   ALL=(ALL) NOPASSWD: /usr/sbin/service dns-sync.sh *\n%sudo   ALL=(ALL) NOPASSWD: /usr/sbin/service dns-sync.sh\n" | sudo tee /etc/sudoers.d/dns-sync
sudo chmod 0440 /etc/sudoers.d/dns-sync

This script seems to finally have fixed my DNS issues on Ubuntu 20.04 under WSL2! Thank you!
Not sure why, but when the script is started, my terminal font changes...

I needed to change line 22 to $PS -Command 'Get-DnsClientServerAddress -AddressFamily IPv4 -InterfaceIndex $(Get-NetIPInterface -AddressFamily IPv4 | Where-Object ConnectionState -EQ "Connected" | Sort-Object InterfaceMetric | Select-Object -ExpandProperty ifIndex) | Select-Object -ExpandProperty ServerAddresses' > $TEMPFILE to avoid DNS servers from disconnected interfaces showing up first

Does anyone have any updates to this.

I started at the top and tried every thing listed as I went down the list of comments and nothing works.

I still get this, I applied every mod/patch that everyone mentioned all the way down to jan-glx one at a time and nothing changed.

I did wsl --shutdown after every change to make sure it was getting a fresh start and data.

I still get:

❯ go mod tidy
go: downloading github.com/spf13/cobra v1.6.1
adotools/cmd imports
        github.com/spf13/cobra: github.com/spf13/cobra@v1.6.1: Get "https://proxy.golang.org/github.com/spf13/cobra/@v/v1.6.1.zip": proxyconnect tcp: dial tcp: lookup my.corp.proxy.com: i/o timeout

when I try to do sudo apt update all I get is:

Temporary failure resolving 'my.corp.proxy.com'

my.corp.proxy.com is anonymized obviously :-)

Anything else anyone can think out would help.

Now WSL2 supports systemd, so we can run script as a service.

Download script to home directory

wget https://gist.github.com/matthiassb/9c8162d2564777a70e3ae3cbee7d2e95/raw/b204a9faa2b4c8d58df283ddc356086333e43408/dns-sync.sh -O ~/dns-sync.sh
chmod +x ~/dns-sync.sh

Add these lines to the /etc/wsl.conf if not exists (sudo nano /etc/wsl.conf)

[boot]
systemd=true

[network]
generateResolvConf=false

Reboot.

Create a unit file

sudo touch /etc/systemd/system/dns-sync.service

Edit (sudo nano /etc/systemd/system/dns-sync.service) and fix path to script inside

[Unit]
Description=dns sync

[Service]
ExecStart=PUT_PATH_HERE
Type=forking

[Install]
WantedBy=multi-user.target

Reboot

Run service

sudo systemctl enable dns-sync.service
sudo systemctl start dns-sync.service
sudo systemctl status dns-sync.service

@cantti : enabling systemd prevent any .execommand to work, including the powershell command required by the script. That resulted in... an empty /etc/resolv.conf ! ❌

Related interesting thread : https://www.reddit.com/r/bashonubuntuonwindows/comments/11vx61n/wsl2_error_cannot_execute_binary_file_exec_format/

I'm looking for a workaround but for now I have to (re)disable systemd 😭