A packet sniffer, the network analyzer, is a wire-tap device that plugs into computer networks and eavesdrops on the network traffic. To capture the information going over the network is called sniffing. It is a "sniffing" program that lets someone listen in on computer conversations. However, computer conversations consist of apparently random binary data. Therefore, network wiretap programs also come with a feature known as "protocol analysis", which allow them to "decode" the computer traffic and make sense of it. These tools known as network sniffers are named after a product called the Sniffer Network Analyzer. Introduced in 1988 by Network General Corp. (now Network Associates Inc.), the Sniffer was one of the first devices that let managers sit at their desks and take the pulse of the larger network. The original sniffers read the message headers of data packets on the network, giving administrators details about the addresses of senders and receivers, file sizes and other low-level information about those packets, in addition to verifying transmission. Using graphs and text-based descriptions, sniffers helped network managers evaluate and diagnose performance problems with servers, the network wire, hubs and applications.
Types of Sniffers
Today, sniffers exist in two broad varieties:
The first is a stand-alone product incorporated into a portable computer that consultants can carry to customer sites and plug into the network to gather diagnostic data.
The second is part of a larger package of network-monitoring hardware and software for helping organizations keep tabs on their LANs, WANs and Web services.
Thus Commercial packet sniffers are used to help maintain networks. Underground packet sniffers are used to break into computers.
How a Packet Sniffer works
A sniffer must be located within the same network block (or net of trust) as the network it is intended to sniff. With relatively few exceptions, that sniffer could be placed anywhere within that block
Under many networking protocols, data that you transmit gets split into small segments, or packets, and the Internet Protocol address of the destination computer is written into the header of each packet. These packets then get passed around by routers and eventually make their way to the network segment that contains the destination computer. As each packet travels around that destination segment, the network card on each computer on the segment examines the address in the header. If the destination address on the packet is the same as the IP address of the computer, the network card grabs the packet and passes it on to its host computer
But Packet Sniffers set up on a computer work slightly differently. Instead of just picking up the packets that are addressed to them, they set their network cards to what's known as promiscuous mode and grab a copy of every packet that goes past. This lets the packet sniffers see all data traffic on the network segment to which they're attached - if they're fast enough to be able to process all that mass of data, that is. This means that it is looking at everything that comes through. The amount of traffic largely depends on the location of the computer in the network.
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