WordNet (r) 3.0 (2006):

sniffer
    n 1: a person who sniffs

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

packet sniffer
sniffer

    A network monitoring tool that captures
   data packets and decodes them using built-in knowledge of
   common protocols.  Sniffers are used to debug and monitor
   networking problems.

   (1995-03-28)