sniffer

from WordNet (r) 3.0 (2006)

sniffer
    n 1: a person who sniffs

from The Free On-line Dictionary of Computing (8 July 2008)

packet sniffer
sniffer

   <networking, tool> 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)