Resolve WSL DNS automatically

boot.sh

#!/bin/bash

# Remove existing "nameserver" lines from /etc/resolv.conf
sed -i '/nameserver/d' /etc/resolv.conf

# Run the PowerShell command to generate "nameserver" lines and append to /etc/resolv.conf
# we use full path here to support boot command with root user
/mnt/c/Windows/System32/WindowsPowerShell/v1.0/powershell.exe -Command '(Get-DnsClientServerAddress -AddressFamily IPv4).ServerAddresses | ForEach-Object { "nameserver $_" }' | tr -d '\r'| tee -a /etc/resolv.conf > /dev/null

resolve_wsl_dns.md

How to resolve DNS issue in WSL once and forever

There are tons of gists available for this problem:

• Fix DNS resolution in WSL2

However even solution is there - I was not satisfied that this problem always persists and I had to do same steps again from time to time. Also using a static DNS like 8.8.8.8 was not the option for me because we use pool of enterprise DNS servers and those are controlled by our IT on our Windows machines. So I wanted to find the way to get those DNS servers dynamically from Windows.

This guide describes the permanent solution using brand-new WSL boot command.

This gist removes nameservers every time your WSL starts and adds newly from powershell results. If it doesn't work for you - please feel free to adjust the script according to your needs.

Prepare boot config

• login to your wsl and run sudo bash. It's important that you do this with a root user because it's a user which executes this command during boot
• change to ~ folder ( or use any folder you like )
• create a new file for example boot.sh and update it with a content from boot.sh gist
• make it executable with chmod +x boot.sh
• create a symbolic link in one of PATH folders ( keep in mind, that for root user number of path folders is very limited by default )

/usr/local/sbin
/usr/local/bin
/usr/sbin
/usr/bin
/sbin
/bin

• i have used ln -s ~/boot.sh /usr/local/bin/boot.sh
• having such a limited list of folders makes root not aware of powershell.exe and rest of window utilities. I have preferred to use an absolute path like /mnt/c/Windows/System32/WindowsPowerShell/v1.0/powershell.exe in my script - but you may think of creating a symlink for powershell too.
• now let's add boot command to /etc/wsl.conf file. In my case it looks like this:

[network]
generateResolvConf=false
[boot]
systemd=true
command=boot.sh

• now just exit from bash, from wsl as well. Terminate your wsl with a command like wsl -t Ubuntu and start again
• if everything went well cat /etc/resolv.conf should have nameservers from your Windows

I also had the same issue. DNS on my WSL2 was not working when I was connected to my work VPN at home. I followed all the steps above but found that it was still working... problem was... I needed a PC restart! Shutting down and WSL does not seem to be sufficient. Very many thanks to put this in place without the need of admin accounts (as my work laptop is quite restricted)!!!!

Sometimes an interface can have a DNS value on it, but not be "Up", this takes care of that (User used as username for reference to next step update):
file (Windows): C:\Users\User\bootDNS.ps1

$upAdapters = Get-NetAdapter | Where-Object { $_.Status -eq 'Up' }
$adapterDNS = Get-DnsClientServerAddress
$upAdapters | ForEach-Object {
    $adapterName = $_.Name
    $ipAddresses = Get-NetIPAddress -InterfaceAlias $adapterName | Where-Object { $_.IPAddress -ne $null }
    if ($ipAddresses) {
        $ipAddresses | ForEach-Object {
            $adapterDNS | Where-Object { $_.InterfaceAlias -eq $adapterName } | Select-Object -ExpandProperty ServerAddresses | ForEach-Object { "nameserver $_" }
        }
    }
}

then update as such (fix your "User" reference for the -File path, and update example.com to your environment):
file (WSL): /root/boot.sh

#!/bin/bash

# Remove existing "nameserver" lines from /etc/resolv.conf
sed -i '/nameserver/d' /etc/resolv.conf

# Run the PowerShell command to generate "nameserver" lines and append to /etc/resolv.conf
# we use full path here to support boot command with root user
/mnt/c/Windows/System32/WindowsPowerShell/v1.0/powershell.exe -File C:\\Users\\User\\bootDNS.ps1 | sort | uniq | tr -d '\r'| tee -a /etc/resolv.conf > /dev/null

# Remove anything not having 'nameserver' in it
sed -i '/nameserver/!d' /etc/resolv.conf

# Add search domains
echo "search example.com" >> /etc/resolv.conf