The license agreements of most software companies keep you at the mercy of those companies. By contrast, our general public license is intended to give everyone the right to share NetHack. To make sure that you get the rights we want you to have, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. Hence this license agreement.
Specifically, we want to make sure that you have the right to give away copies of NetHack, that you receive source code or else can get it if you want it, that you can change NetHack or use pieces of it in new free programs, and that you know you can do these things.
To make sure that everyone has such rights, we have to forbid you to deprive anyone else of these rights. For example, if you distribute copies of NetHack, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must tell them their rights.
Also, for our own protection, we must make certain that everyone finds out that there is no warranty for NetHack. If NetHack is modified by someone else and passed on, we want its recipients to know that what they have is not what we distributed.
Therefore we (Mike Stephenson and other holders of NetHack copyrights) make the following terms which say what you must do to be allowed to distribute or change NetHack.
a) cause the modified files to carry prominent notices stating that you changed the files and the date of any change; and
b) cause the whole of any work that you distribute or publish, that in whole or in part contains or is a derivative of NetHack or any part thereof, to be licensed at no charge to all third parties on terms identical to those contained in this License Agreement (except that you may choose to grant more extensive warranty protection to some or all third parties, at your option)
c) You may charge a distribution fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee.
a) accompany it with the complete machine-readable source code, which must be distributed under the terms of Paragraphs 1 and 2 above; or,
b) accompany it with full information as to how to obtain the complete machine-readable source code from an appropriate archive site. (This alternative is allowed only for noncommercial distribution.)
For these purposes, complete source code means either the full source distribution as originally released over Usenet or updated copies of the files in this distribution used to create the object code or executable.
Stated plainly: You are permitted to modify NetHack, or otherwise use parts of NetHack, provided that you comply with the conditions specified above; in particular, your modified NetHack or program containing parts of NetHack must remain freely available as provided in this License Agreement. In other words, go ahead and share NetHack, but don't try to stop anyone else from sharing it farther.