I'm new to setting up my own VPN, but I'd like some guidance to
software which meets these criteria:

Server runs on NetBSD (preferably from pkgsrc)

Client runs on NetBSD, Linux, OSX, and Windows.

If different client programs are required, that's fine I guess as long
as it works.

Any suggestions? I have used VPN software, but I don't really know
anything about it. If I need to study first, suggestions on that would
be good too.

Thanks.

Andy