Also see part 2Packet Sniffing is a fascinating subject. It wears both hats, the good and the evil. It's used by many (including myself) to detect network faults etc...but the same technology allows someone to "sniff" out passwords for your mail account or even your internet account. Now you understand why securing/encrypting your data is so important.
When a file is "deleted" what actually happens? Your operating system removes the reference to that file on the file system. This reference had details such as where on the disk the file was. Whilst marked and available as free space the old data didn't move, its just not seen on the file system but physically exisits on the disk. The entire file remains on the disk until another data is created over the physical area, and even then it may be possible to recover data by studying the magnetic fields on the platter surface.
What's a packet sniffer?
When you make contact with the Internet, data isn't sent in one continuous stream of data; this would be impractical and it would limit the performance of the Internet network. To keep the performance of the Internet as high as possible, the data is cut in slices. Such a slice of data (either inbound or outbound) is called "a packet". Now, you can't see atoms with your naked eye can you? No, I thought so. Sending information on a network means sending "packets" of data. Think of them like the atoms. A lot of packets will create the final information you will see on your screen, be it website or email. To "see" the atoms you would need a special device, some kind of electron microscope, to be able to see the "packets" you've sent or received... you also need a special device. This is a special type of monitoring program called... a packet sniffer. By using a packet sniffer you're able to see any bit of information entering or leaving your computer... even those you normally wouldn't see!
A packet sniffer can be considered as a sort of wire tap device. A device that can "plug" into computer networks and eavesdrops on the network traffic. Just as a telephone wiretap allows the CIA to listen to conversations, the same concept follows a packet sniffer in the sense that it allows someone to listen in on computer conversations.
How packet sniffers work
Packet sniffers capture "binary" data passing through the network, most if not all decent sniffers "decode" this data into a human readable form. To make it even easier (for humans) another step occurs known as "protocol analysis". There is a varying degree of the analysis that takes place, some are simple, just breaking down the "packet" information. Others are more complex giving "detailed" information about what it sees on the packet (i.e., highlights a password for a service).
One very important (and very simple) point to understand is that the sniffer has to be on the same "wire" on which the data is travelling to. In short the "probing" device that "captures" the data has to be on the same wire. The data can then be relayed to a decoding computer on a different network.
Situation: Bob and John are engaged in a internet chat session. You are in a city far apart from where the two men reside. Bob and John are talking top secret details on a cocaine deal. You (the law abiding citizen) decide to sniff their chat session (from your location) to help the feds bust Bob and John.
The simple answer is you CAN'T do that as you don't have access to the path that the data travels from! Of course if you are a good hacker (or well Cracker) then you could install a Trojan on Bob or John's computer and run a sniffer from their system, thus the sniffer it self is on the same wire.
Go to Part 2