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A packet analyzer (also known as a network analyzer, protocol analyzer or packet 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. 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.
Packet capture is the process of intercepting and logging traffic.
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 load.
On wireless LANs, one can capture traffic on a particular channel, or on several channels when using multiple adapters.
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.
When traffic is captured, either the entire contents of packets can be recorded, or the headers can be recorded without recording the total content of the packet. This can reduce storage requirements, and avoid legal problems, but yet have enough data to reveal the essential information required for problem diagnosis.
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, reconstruct TCP and UDP data streams, 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 users having to recreate any fault.
The versatility of packet sniffers means they can be used to:
- Analyze network problems
- Detect network intrusion attempts
- Detect network misuse by internal and external users
- Documenting regulatory compliance through logging all perimeter and endpoint traffic
- Gain information for effecting a network intrusion
- Isolate exploited systems
- Monitor WAN bandwidth utilization
- Monitor network usage (including internal and external users and systems)
- Monitor data-in-motion
- Monitor WAN and endpoint security status
- Gather and report network statistics
- Filter suspect content from network traffic
- Serve as primary data source for day-to-day network monitoring and management
- Spy on other network users and collect sensitive information such as login details or users cookies (depending on any content encryption methods that may be in use)
- Reverse engineer proprietary protocols used over the network
- Debug client/server communications
- Debug network protocol implementations
- Verify adds, moves and changes
- Verify internal control system effectiveness (firewalls, access control, Web filter, Spam filter, proxy)
Packet capture can be used to fulfill a warrant from a law enforcement agency (LEA) to produce all network traffic generated by an individual. Internet service providers and VoIP providers in the United States of America must comply with CALEA (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. Collection of data from a carrier system without a warrant is illegal due to laws about interception.
Notable packet analyzers 
- Capsa Network Analyzer
- Cain and Abel
- Carnivore (FBI)
- Microsoft Network Monitor
- Microsoft Message Analyzer
- ngrep, Network Grep
- Wireshark (formerly known as Ethereal)
See also 
- Bus analyzer
- Logic analyzer
- Network detector
- Network intrusion detection system
- Network tap
- Packet generation model
- Signals intelligence
|Wikimedia Commons has media related to: Computer data network analyzers|
- Protocol Analyzers at the Open Directory Project
- How-to Packet Sniff
- The Making of a Professional cTrace Packet Analyzer
- Packet Sniffing FAQ by Robert Graham
- A Quick Intro to Sniffers
- Multi-Tap Network Packet Capture
- How to Deploy a Packet Analyzer under Different Network Environment
- Microsoft Message Analyzer