Sometimes it is very practical to use grep, to find a specific regular expression in some files. It would also be very practical to have the grep functionality on the network. This feature is available.
Download ngrep. The package to get is ngrep-1.45.tar.bz2.
To get it to work on a mac do:
tar xvjf ngrep-1.45.tar.bz2 cd ngrep-1.45 ./configure --with-pcap-includes=/usr/include --prefix=/usr/local make sudo make install
To read the manual page you can run
Now run ngrep itself run
sudo ngrep <expression to find> <pcap filter expression>
<pcap filter expression> is used the same way as in tcdpdump.