passive finger printing in linux

i need two passive finger printing programs.

i've tried p0f, but the results i am getting are not consistent --maybe just wrong ...

i load up p0f, and use the line p0f -f p0f.fp log1.log

then i either use netcat to try and open a connection or a browser to open a connection, but, i get all sorts of results, but not the ip of the target i'm shooting for

i need to identify the target(yahoo, google, microsoft, whatever) operating system through a passive program

so, i dig the domain name or ping the site and retreive an ip address, and then wait for it to populate in p0f, but
i never actually see a packet come back from that site identifying an OS

this is for an ethical hacking class

thanks

-dave.j