how to get unique identifier for a client

hi all
is there a way to get a unique identifier for a client? something like
a machine id. session_user does not seem to work since a user can log
on to many workstations. been through the list so i'm not searching
for getting the ip address since the list said something like it's not
supported in unix-sockets, though i'll take it since i'm using
postgres in windows environment. thanks in advance :)