This post pertains to the Windscribe GUI app on Linux as encountered here on Debian and Ubuntu based Linux Mint with the latest version 2.6.14 as well as the previous version. To be fair the GUI app is still marked as in beta.
If you go to the download page it funny enough first attempted to serve me the older and more reliable CLI program, supposedly tried and tested, and only at the second attempt offered the GUI program for download. I'ld have to check if this problem is also present in the CLI version (it wasn´t previously when used around three years ago as I ran frequent tests) but for now with the move to a more comfortable graphical interface I've only got the new app installed.
Most sites like whatismyip.com will tell you that everything is fine. It seems like they're only running one test. Also, when tested on AirVPN's ipleak.net everything appears fine at first glance. However, they're running more tests and eventually after around 20-30 seconds suddenly a second DNS is detected and pops up as visible in the screenshot below. This is the DNS of the ISP.
