A Mullvad VPN user has discovered that Android devices leak DNS queries when switching VPN servers even though the "Always-on VPN" feature was enabled with the "Block connections without VPN" option.
"Always-on VPN" is designed to start the VPN service when the device boots and keep it running while the device or profile is on.
Enabling the "Block Connections Without VPN" option (also known as a kill switch) ensures that
ALL network traffic and connections pass through the always-connected VPN tunnel, blocking prying eyes from monitoring the users' web activity.
However, as Mullvad found out while
investigating the issue spotted on April 22, an Android bug leaks some DNS information even when these features are enabled on the latest OS version (Android 14).
This bug occurs while using apps that make direct calls to the
getaddrinfo C function, which provides protocol-independent translation from a text hostname to an IP address.
They discovered that Android leaks DNS traffic when a VPN is active (but no DNS server has been configured) or when a VPN app re-configures the tunnel, crashes, or is forced to stop.
"We have not found any leaks from apps that only use Android API:s such as
DnsResolver. The Chrome browser is an example of an app that can use getaddrinfo
directly," Mullvad explained.
"The above applies regardless of whether 'Always-on VPN' and 'Block connections without VPN' is enabled or not, which is not expected OS behavior and should therefore be fixed upstream in the OS."
