UDT is copyrighted software and the copyright holder is the Board of Trustees of the University of Illinois. The current UDT version 4 is under BSD license, while all previous versions were under GNU Library or Lessor GPL ( LGPL).
BSD license is one of the most flexible open source licenses. It allows you to redistribute (including to sell) and use UDT in either source or binary form, with or without modification. However, you MUST keep or reproduce the complete license in your redistribution.