Sniff a Small Network

Sniff a Small Network
How to sniff network traffic on a home network.
The proliferation of home networks sharing cable or DSL connections to the Internet has
brought about new challenges to the home guru. No longer is Mom or Dad just the
person with the credit card able to buy new games, they are now faced with being a
"Network Administrator".
Believe it or not, there are some sharp folks that have advanced beyond a few pings and
trace routes and want to sniff their network.
Please note that this document is intended for a reader that is not a full time, trained
network technician. Enterprise networks found in corporate environments have gear that
is capable of avoiding some of the makeshift environment described later.
Host- anything on the network that can send and receive data, including a computer,
router, firewall, etc.
Hub- a device that can have several computers connected. When network traffic from a
computer is sent to it, the traffic is sent to all of the ports. The computer that is supposed
to receive the traffic eventually filters out traffic bound for other computers and gets its
Sniffer- software that can gather data off of the network. Once gathered it can be
analyized for various reasons including troubleshooting, maintenance, or even
eavesdropping on data that is not encrypted (AIM traffic comes to mind <grin>). I
recommend Ethereal. It is available for Linux as well as Windows operating systems,
plus the price is right (free). More information about Ethereal can be found at
Switch- looks like a hub but has much better performance because each port has a
dedicated connection to all of the other ports. When a port receives traffic from a
computer it knows which port the receiving host is on and sends it to that port. This
eliminates collisons and all of the packets conglomerating in the hub’s shared traffic
The home networking gear is usually setup as follows:
Router/Firewall [1]
[1] This device is usually a switch with
one port to connect to the Internet.
To be able to gather data (sniff) the first thing that must be done is to put the computer
with the sniffer software and the computer to gather data from onto a hub. Doing so
removes the isolation that occurs in a switch.
Most switches or hubs have a port labeled "Uplink". Plug a cable from a port on a hub to
a port on the switch. NOTE: only one end must be plugged into an uplink port. There
should be active link lights on the two ports used.
Plug the Sniffer computer and the computer to be monitored into the hub. You are now
ready to start the sniffer program of your choice and begin capturing the packets.
Below is a diagram of the described setup.
Cable to
Uplink port
Hub [2]
PC with
Sniffer software
PC with traffic
to be analyzed
[2] As many computers as there are
available ports on the hub can be plugged
in and analyzed. Beware, the data can
become overwhelming!
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