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When a VPN app is doing DNS with its own tunnel servers, protecting that traffic can result in the tunnel and tunnel DNS server IP addresses leaking. Any VPN app that is setting tunnel DNS servers that aren't reachable through the tunnel is already broken, as apps would be unable to do DNS when leak blocking is enabled.
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This CL stops default (fallback) DNS servers being used for uids that have a lockdown VPN. This eliminates a serious DNS leak that can occur at tunnel creation time regardless of how the VPN is configured, and additional serious leaks that can occur when a VPN does not have DNS servers configured.
It also includes miscellaneous improvements to the VPN, such as stopping network identifiers from wrapping and crashing system_server if there are errors when setting up the lockdown VPN firewall.
This will not introduce any compatibility issues. Using fallback DNS is unreliable when there's a lockdown VPN, so VPN apps that depend on it already don't work in many cases.
GrapheneOS/platform_packages_modules_Connectivity#39
GrapheneOS/platform_packages_modules_DnsResolver#14
GrapheneOS/os-issue-tracker#5273