A packet analyzer (also known as a network analyzer, protocol analyzer, or sniffer, or for particular types of networks, an Ethernet sniffer or wireless sniffer) is a computer program or a piece of computer hardware that can intercept and log traffic passing over a digital network or part of a network.[1] As data streams flow across the network, the sniffer captures each packet and, if needed, decodes the packet's raw data, showing the values of various fields in the packet, and analyzes its content according to the appropriate RFC or other specifications.
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On wired broadcast LANs, depending on the network structure (hub or switch), one can capture traffic on all or just parts of the network 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 when systems (computers) are connected to a switch port. To use a network tap is an even more reliable solution than to use a monitoring port, since taps are less likely to drop packets during high traffic loads.[2]
On wireless LANs, one can capture traffic on a particular channel.
On wired broadcast and wireless LANs, to capture traffic other than unicast traffic sent to the machine running the sniffer software, multicast traffic sent to a multicast group to which that machine is listening, and broadcast traffic, the network adapter being used to capture the traffic must be put into promiscuous mode; some sniffers support this, others do not. On wireless LANs, even if the adapter is in promiscuous mode, packets not for the service set for which the adapter is configured will usually be ignored. To see those packets, the adapter must be in monitor mode.
The captured information is decoded from raw digital form into a human-readable format that permits users of the protocol analyzer to easily review the exchanged information. Protocol analyzers vary in their abilities to display data in multiple views, automatically detect errors, determine the root causes of errors, generate timing diagrams, etc.
Some protocol analyzers can also generate traffic and thus act as the reference device; these can act as protocol testers. Such testers generate protocol-correct traffic for functional testing, and may also have the ability to deliberately introduce errors to test for the DUT's ability to deal with error conditions.
Protocol Analyzers can also be hardware-based, either in probe format or, as is increasingly more common, combined with a disk array. These devices record packets (or a slice of the packet) to a disk array. This allows historical forensic analysis of packets without the user's having to recreate any fault.
The versatility of packet sniffers means they can be used to: