Restore dns on unclean shutdown

[lixmal]

Describe your changes

1. When the Netbird agent terminates ungracefully (kill, power outage, panic, etc.), the system DNS settings are not restored.

   This can result in broken DNS resolution (e.g. a DNS server provided by a peer), in which case the agent won't be able to connect to the management server.

   To fix this issue we create an indicator file after adjusting the system DNS settings.
   This file is deleted once the settings are restored.

   If the agent terminates ungracefully it won't be able to delete this file. Therefore when it is started again it will detect the existence of this file and restore the previous DNS state.

   • On Linux

     /var/lib/netbird/resolv.conf, contains the old /etc/resolv.conf
/var/lib/netbird/manager, contains the name of the previous host manager and last netbird IP address

     • manager file

       DNS settings are restored by copying the /var/lib/netbird/resolv.conf back to /etc/resolv.conf

       Test:

       • Uninstall systemd-resolved, openresolv/resolvconf
       • echo nameserver 9.9.9.9 | sudo tee /etc/resolv.conf
       • Watch /var/log/netbird/client.log for restore message and /etc/resolv.conf
       • Start agent
       • killall -9 netbird
       • Start agent

     • manager resolvconf

       DNS settings are restored by running restoreHostDNS() (runs resolvconf -d <interface>)

       Test:

       • Uninstall systemd-resolved
       • Install openresolv package
       • echo "nameserver 9.9.9.9" | sudo resolvconf -x -a <physical int>
       • Watch /var/log/netbird/client.log for restore message and /etc/resolv.conf
       • Start agent
       • killall -9 netbird
       • Start agent

     • manager systemd

       DNS settings are restored by running restoreHostDNS() (runs dbus methods`)

       Test:

       • Watch /var/log/netbird/client.log for restore message and resolvectl status
       • Start agent
       • killall -9 netbird
       • Start agent

     • manager networkManager

       DNS settings are restored by running restoreHostDNS() (runs dbus methods`)

       Test:

       • Uninstall systemd-resolved
       • Edit /etc/NetworkManager/NetworkManager.conf, add dns=dnsmasq in the main section
       • sudo nmcli general reload or sudo nmcli networking off; sudo nmcli networking on
       • Watch /var/log/netbird/client.log for restore message and nmcli device show | grep -i dns
       • Start agent
       • killall -9 netbird
       • Start agent

   • On Windows

     %PROGRAMDATA%\Netbird\unclean_shutdown_dns.txt, contains the interface guid that is required to set up the host manager.

     DNS settings are restored by running restoreHostDNS() (removes registry keys)

     Test:

     • Watch %PROGRAMDATA%\Netbird\client.log for restore message and SYSTEM\CurrentControlSet\Services\Dnscache\Parameters\DnsPolicyConfig\NetBird-Match registry
     • Start agent
     • Start PowerShell as admin
     • Get-Process -Name netbird | Stop-Process -Force
     • (Re)start agent

   • On macOS

     /var/lib/netbird/unclean_shutdown_dns, empty

     DNS settings are restored by running restoreHostDNS() (runs scutil remove).

     Test:

     • Watch /var/log/netbird/client.log for restore message and scutil --dns for DNS settings
     • Start agent
     • sudo killall -9 netbird
     • Start agent

   • mobile should not be affected

2. Moves the restore DNS logic before the remove interface logic so we don't get errors for missing interfaces

3. Improves error handling and error messages for debugging

Checklist

• Is it a bug fix
• Is a typo/documentation fix
• Is a feature enhancement
• It is a refactor
• Created tests that fail without the change (if possible)
• Extended the README / documentation, if necessary