Technically Untangle isn't selling Wireguard, they're selling a GUI they built that's cloud accessible that happens to configure Wireguard AND support for it.
There's a massive difference.
But yes, there is a ton of online whining that Untangle isn't giving away all that work. The only way to get it is via a Complete sub too, which if you already have you just GET this module. (Unless you're using the residential HomePro sub in a business setting, in which case BAD BAD Panda!)
So you aren't paying as it is... and expecting new toys? How does that work?
Also, the GPL says nothing about selling software, and no this isn't any sort of license violation even if they were to do so. Free doesn't mean free forever, in the GPLs case it just means code you write for the project must also be released as such. And Untangle's is, on their github. Source is right here:
https://github.com/untangle/ngfw_src/tree/master/wireguard-vpn
So you're free to compile and sideload the module yourself if you wish.
To be clear, I'm annoyed they didn't release this module to the free portion of their platform too. I'm also annoyed they continue to sell the IPSec module which is ALSO built on "free" VPN projects. The justification is the support costs, because Untangle provides "free" support for the "free" installs, they incurr cost based on those tickets.
My solution would be to sell support tickets for people that don't have complete subs, stop all free support for them, and leave the modules free... but that's me. In the meantime, how dare Untangle monetize their product shame on them!
