Sniffing How It Is Done

Sniffing: How It Is Done

In a shared media network, such as Ethernet, all network interfaces on a network segment have access to all of the data that travels on the media.

Each network interface has a hardware-layer address (MAC Address) that should differ from all hardware-layer addresses of all other network interfaces on the network. Each network also has at least one broadcast address that corresponds not to an individual network interface, but to the set of all network interfaces. Normally, a network interface will only respond to a data frame carrying either its own hardware-layer address in the frame’s destination field or the “broadcast address” in the destination field. It responds to these frames by generating a hardware interrupt to the CPU. This interrupt gets the attention of the operating system, and passes the data in the frame to the operating system for further processing.

Note The term “broadcast address” is somewhat misleading. When the sender wants to get the attention of the operating systems of all hosts on the network, he or she uses the “broadcast address.” Most network interfaces are capable of being put into a “promiscuous mode.” In promiscuous mode, network interfaces generate a hardware interrupt to the CPU for every frame they encounter, not just the ones with their own address or the “broadcast address.” The term “shared media” indicates to the reader that such networks broadcast all frames—the frames travel on all the physical media that make up the network.

At times, you may hear network administrators talk about their networking trouble spots— when they observe failures in a localized area. They will say a particular area of the Ethernet is busier than other areas of the Ethernet where there are no problems. All of the packets travel through all parts of the Ethernet segment. Interconnection devices that do not pass all the frames from one side of the device to the other form the boundaries of a segment. Bridges, switches, and routers divide segments from each other, but low-level devices that operate on one bit at a time, such as repeaters and hubs, do not divide segments from each other. If only low-level devices separate two parts of the network, both are part of a single segment. All frames traveling in one part of the segment also travel in the other part.

The broadcast nature of shared media networks affects network performance and reliability so greatly that networking professionals use a network analyzer, or sniffer, to troubleshoot problems. A sniffer puts a network interface in promiscuous mode so that the sniffer can monitor each data packet on the network segment. In the hands of an experienced system administrator, a sniffer is an invaluable aid in determining why a network is behaving (or misbehaving) the way it is. With an analyzer, you can determine how much of the traffic is due to which network protocols, which hosts are the source of most of the traffic, and which hosts are the destination of most of the traffic. You can also examine data traveling between a particular pair of hosts and categorize it by protocol and store it for later analysis offline. With a sufficiently powerful CPU, you can also do the analysis in real time.

Most commercial network sniffers are rather expensive, costing thousands of dollars. When you examine these closely, you notice that they are nothing more than a portable computer with an Ethernet card and some special software. The only item that differentiates a sniffer from an ordinary computer is software. It is also easy to download shareware and freeware sniffing software from the Internet or various bulletin board systems.

Sniffing: How It Threatens Security

Sniffing data from the network leads to loss of privacy of several kinds of information that should be private for a computer network to be secure. These kinds of information include the following:

• Passwords
• Financial account numbers
• Private data
• Low-level protocol information