Indy is dual licensed. You can review which license better suits your needs, and use that license. You can even change your mind later if you have previously chosen one.
Indy Modified BSD License – The Indy Modified BSD license is a very no nonsense license that allows you to do almost anything you want with Indy, provided you provide proper attribution. License Details.
Indy MPL (Mozilla Public License) License – To make it easier and consistent for JEDI users, we also offer an MPL license v1.1. License Details.
Which license should I use?
You need to investigate them and see which one fits your needs. The BSD license is simpler and allows much broader use, but requires attribution (See below). The MPL license is more restrictive, but does not require attribution. If in doubt, choose the BSD license and just provide proper attribution.
Do I have to provide attribution in the about box?
No. While we appreciate attribution in the about box, it is not a requirement. A mention in the documentation (or in multiple places), or any reasonable visible place to the users of the product is acceptable. There are also Indy graphics and logos you can use.