From: https://raw.githubusercontent.com/zerotier/ZeroTierOne/master/COPYING

LICENSE

ZeroTier One is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or (at
your option) any later version.

See the file ‘LICENSE.GPL-3’ for the text of the GNU GPL version 3.
If that file is not present, see <http://www.gnu.org/licenses/>.