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A Packet sniffer (also known as network or protocol analyzer or Ethernet sniffer) is computer software (usually) or computer hardware that can intercept and log traffic passing over a digital network or part of a network. As data streams travel back and forth over the network, the sniffer captures each packet and eventually decodes and analyzes its content according to the appropriate RFC or other specifications. Depending on the network structure (hub or switch) one can sniff all or just parts of the traffic from a single machine within the network; however, there are some methods to avoid traffic narrowing by switches to gain access to traffic from other systems on the network (e.g. ARP spoofing). For network monitoring purposes it may also be desirable to monitor all data packets in a LAN by using a network switch with a so-called monitoring port, whose purpose is to mirror all packets passing through all ports of the switch.
The special network device driver used for some packet sniffing software is said to operate in "promiscuous mode" as it listens to everything on the wire.
The versatility of packet sniffers means they can be used to:
Analyse network problems.
Gain information for effecting a network intrusion.
Gather and report network statistics.
Filter suspect content from network traffic.
Spy on other network users and collect sensitive information such as passwords (depending on any content encryption methods which may be in use)
Debug client/server communications.
See also: Wireless sniffer.
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Example uses
A packet sniffer for a token ring network could detect that the token has been lost or the presence of too many tokens (verifying the protocol).
A packet sniffer could detect that messages are being sent to a network adapter; if the network adapter did not report receiving the messages then this would localize the failure to the adapter.
A packet sniffer could detect excessive messages being sent by a port, detecting an error in the implementation.
A packet sniffer could collect statistics on the amount of traffic (number of messages) from a process detecting the need for more bandwidth or a better method.
A packet sniffer could be used to extract messages and reassemble into a compete form the traffic from a process, allowing it to be reverse engineered.
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Well-known packet sniffers
Ethereal (There is no mention on the Ethereal website that it has become Wireshark.)
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See also
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