What Are Packet Sniffers and How Do They Work?

Packet sniffing may sound like the latest street drug craze, but it's far from it. Packet sniffers or protocol analyzers are tools that are commonly used by network technicians to diagnose network-related problems. Packet sniffers can also be used by hackers for less than noble purposes such as spying on network user traffic and collecting passwords.

Let's take a look at what a packet sniffer is and what it does:

Packet sniffers come in a couple of different forms. Some packet sniffers used by network technicians are single-purpose dedicated hardware solutions while other packet sniffers are software applications that run on standard consumer-grade computers, utilizing the network hardware provided on the host computer to perform packet capture and injection tasks.

Packet sniffers work by intercepting and logging network traffic that they can 'see' via the wired or wireless network interface that the packet sniffing software has access to on its host computer.

On a wired network, what 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 of it, depending on how the network switches are configured, placed, etc. On wireless networks, packet sniffers can usually only capture one channel at a time unless the host computer has multiple wireless interfaces that allow for multichannel capture.

Once the raw packet data is captured, the packet sniffing software must analyze it and present it in human-readable form so that the person using the packet sniffing software can make sense of it. The person analyzing the data can view details of the 'conversation' happening between two or more nodes on the network. Network technicians can use this information to determine where a fault lies, such as determining which device failed to respond to a network request.

Hackers can 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 can also capture packets for later playback in replay, man-in-the-middle, and packet injection attacks that some systems may be vulnerable to.

Just like everybody else, both network engineers and hackers love free stuff, which is why open source and freeware sniffer software applications are often the tools of choice for packet sniffing tasks. One of the more popular open source offerings is ​Wireshark (previously known as Ethereal). Sniff your packets in the field, save them to a CAP file, and analyze them later.

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. Antisniff can detect if a network interface on your network has been put into 'promiscuous mode' (don't laugh that's the actual name for it), which is the required mode for packet capture tasks.

Another way to protect your network traffic from being sniffed is to use encryption such as Secure Sockets Layer (SSL) or Transport Layer Security (TLS). Encryption doesn't prevent packet sniffers from seeing source and destination information, but it does encrypt the data packet's payload so that all the sniffer sees is encrypted gibberish. Any attempt to modify or inject data into the packets would likely fail since messing with the encrypted data would cause errors that would be evident when the encrypted information was decrypted at the other end.

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