Network Question

[Gene Dascher]

I have redhat 6.2 on a Gateway 486 dx250 that I am using as my
Comcast at home internet gateway/firewall.  I have noticed over the last
few days that the data light on my modem is blinking very frequently
while none of the PCs on my network are surfing the web.  I have looked
for the obvious signs of a break in on the gateway, but cannot find any,
and do not see any odd processes running.  I want to know the nature of
the "data" that is hitting my machine.  What is the best tool for doing
this?  Would a packet sniffer (like Sniffit) be what I am looking for?

Thanks,
Gene