V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

DUA
       Directory User Agent (X.500, DS)


The Free On-line Dictionary of Computing (30 December 2018):

Directory User Agent
DUA

   (DUA) The software that accesses the X.500 Directory Service
   on behalf of the directory user.  The directory user may be a
   person or another software element.

   (1994-12-07)