What Are Packet Sniffers and How Do They Work?

Intercept and log traffic over a network for good or evil

Packet sniffers or protocol analyzers are tools used by network technicians to diagnose network-related problems. Hackers use packet sniffers for less noble purposes, such as spying on network user traffic and collecting passwords.

Packet sniffers come in several forms. Some packet sniffers used by network technicians are single-purpose hardware solutions. In contrast, other packet sniffers are software applications that run on standard consumer-grade computers, using the network hardware provided on the host device to perform packet capture and injection tasks.

A dog "sniffing packets" from a user and a burglar
Lifewire / Tim Liedtke

How Packet Sniffers Work

Packet sniffers work by intercepting and logging network traffic via the wired or wireless network interface on its host computer.

On a wired network, the information that can be captured depends on the structure of the network. A packet sniffer might be able to see traffic on an entire network or only a certain segment; it depends on how the network switches are configured. On wireless networks, packet sniffers usually capture one channel at a time, unless the host computer has multiple wireless interfaces that allow for multichannel capture.

Although most packet sniffers in use these days are software, hardware packet sniffers still play a role in network troubleshooting. Hardware packet sniffers plug directly into a network and store or forward the information they collect.

Once the raw packet data is captured, the packet sniffing software analyzes it and presents it in a readable form so that the person using the software can make sense of it. The person analyzing the data can view details of the interaction between two or more nodes on the network.

Network technicians use this information to determine where a fault lies, such as determining which device failed to respond to a network request.

Hackers use sniffers to eavesdrop on unencrypted data in the packets to see what information is being exchanged between two parties. They can also capture information such as passwords and authentication tokens if they are sent in the clear. Hackers are also known to capture packets for later playback in replay, man-in-the-middle, and packet injection attacks that some systems are vulnerable to.

Software Tools Commonly Used in Packet Sniffing

Like most people, network engineers and hackers love free stuff, which is why open source and freeware sniffer applications are often their tools of choice. One popular open-source offering is ​Wireshark, previously known as Ethereal. Use it to sniff your packets in the field, save them to a CAP file, and analyze them later.

Protect a Network and Its Data From Hackers Using Sniffers

If you're a network technician or administrator and you want to see if anyone on your network is using a sniffer tool, check out a tool called Antisniff. It detects if a network interface on your network has been put into promiscuous mode. Don't laugh; that's the actual name for it, and it's the required mode for packet capture tasks.

Another way to protect your network traffic from being sniffed is with encryption, such as Secure Sockets Layer (SSL) or Transport Layer Security (TLS). Encryption won't prevent packet sniffers from seeing source and destination information, but it can encrypt the data packet's payload so that all the sniffer sees is gibberish.

Any attempt to modify or inject data into the packets fails because messing with the encrypted data causes errors that are evident when the encrypted information is decrypted at the other end.

Sniffers are great tools for diagnosing down-in-the-weeds network problems. Still, they are also useful for hacking purposes. It's essential for security professionals to familiarize themselves with these tools so they can see how a hacker might use them on their network.

Types of Information Packet Sniffers Gather

Although packet sniffers are tools of the trade for network engineers, they are also prevalent in some reputable antivirus software and as malware in nefarious email attachments.

Packet sniffers can gather almost any type of data. They can record passwords and login information, along with the websites visited by a computer user and what the user viewed while on the site. They can be used by companies to keep track of employee network use and scan incoming traffic for malicious code. In some cases, a packet sniffer can record all traffic on a network.

Packet sniffers are valuable because they limit malware and are valuable for troubleshooting network problems, but they should be used with robust security software to prevent their misuse.

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