A packet analyzer (also known as a packet sniffer) is a computer program or piece of computer hardware (such as a packet capture appliance) that can intercept and log traffic that passes over a digital network or part of a network.Packet capture is the process of intercepting and logging traffic. 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.
A packet analyzer used for intercepting traffic on wireless networks is known as a wireless analyzer or WiFi analyzer. A packet analyzer can also be referred to as a network analyzer or protocol analyzer though these terms also have other meanings.
On wired shared medias networks, such as Ethernet, Token Ring, and FDDI networks, depending on the network structure (hub or switch),[a] it may be possible to capture all traffic on the network from a single machine on the network. On modern networks, traffic can be captured using a network switch with a so-called monitoring port that mirrors all packets that pass through designated ports of the switch. 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 load.
On wireless LANs, traffic can be captured on one channel at a time, or by using multiple adapters, on several channels simultaneously.
On wired broadcast and wireless LANs, to capture unicast traffic between other machines, the network adapter capturing the traffic must be in promiscuous mode. On wireless LANs, even if the adapter is in promiscuous mode, packets not for the service set the adapter is configured for are usually ignored. To see those packets, the adapter must be in monitor mode. No special provisions are required to capture multicast traffic to a multicast group the packet analyzer is already monitoring, or broadcast traffic.
When traffic is captured, either the entire contents of packets are recorded, or just the headers are recorded. Recording just headers reduces storage requirements, and avoids some legal issues, yet often provides sufficient information to diagnose problems.
Captured information is decoded from raw digital form into a human-readable format that lets users easily review exchanged information. Protocol analyzers vary in their abilities to display and analyze data.
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 the DUT's ability to handle errors.
Protocol analyzers can also be hardware-based, either in probe format or, as is increasingly 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 users having to recreate any fault.
Packet capture can be used to fulfill a warrant from a law enforcement agency to wiretap all network traffic generated by an individual. Internet service providers and VoIP providers in the United States must comply with Communications Assistance for Law Enforcement Act regulations. Using packet capture and storage, telecommunications carriers can provide the legally required secure and separate access to targeted network traffic and are able to use the same device for internal security purposes. Collecting data from a carrier system without a warrant is illegal due to laws about interception. By using end-to-end encryption, communications can be kept confidential from telecommunication carriers and legal authorities.