In Search of a Sniffer

by Sumit Dhar

A sniffer is a program/device that eavesdrops on network traffic and grabs information traveling over the network. Sniffers are basically data interception programs. They work because the Ethernet was built around a principle of sharing.

Most networks use what is known as broadcast technology, meaning that every message transmitted by one computer on a network can be read by any other computer on that network. In practice, all the other computers, except the one for which the message is meant, will ignore that message. However, computers can be made to accept messages, even if they are not meant for them, by means of a sniffer.

How a Sniffer Works

Suppose the machine Venus wants to talk to the machine Cupid. Venus has the IP address, whereas Cupid has the IP address When Venus sends a packet on the network, it includes the destination media access control (MAC) address and the source MAC address. All the computers on the network compare the frame's destination MAC address with their own MAC addresses. When the two don't match, the frame is discarded. The machine running the sniffer breaks this rule and does not discard the frame. Such a machine is said to be in promiscuous mode and can effectively listen to all the network traffic.

Detecting Sniffers

A sniffer is usually passive, it only collects data. Hence, it becomes extremely difficult to detect sniffers. When installed on a computer, a sniffer will generate some small amount of traffic, though, and is therefore detectable. Here is an overview of the detection methods:

Ping Method: The trick used here is to send a ping request with the IP address of the suspect machine but not its MAC address. Ideally, no machine should see this packet, as each Ethernet adaptor will reject it since it does not match its own MAC address. If the suspect machine is running a sniffer, it will respond since it does not reject packets with a different destination MAC address. This is an old method and no longer reliable.

Address Resolution Protocol (ARP) Method: A machine caches ARPs, so what we do is send a non-broadcast ARP. A machine in promiscuous mode will cache your ARP address. Next, we send a broadcast ping packet with our IP address but a different MAC address. Only a machine that has our correct MAC address from the sniffed ARP frame will be able to respond to our broadcast ping request. Voilà!

On Local Host: Often, after your machine has been compromised, hackers will leave sniffers on it in order to compromise other hosts. On a local machine, run ifconfig. On a clean machine, the output will be:

        eth0      Link encap:Ethernet  HWaddr 52:54:05:F3:95:01  
                  inet addr:  Bcast:203.199.  ...
                  UP BROADCAST RUNNING MULTICAST  MTU:1500  ...
        But on a machine running a sniffer the output will be like this: 
        eth0      Link encap:Ethernet  HWaddr 52:54:05:F3:95:01  
                  inet addr:  Bcast:203.199.  ...

(Output slightly modified to fit screen.)

Latency Method: This method is based on the assumption that most sniffers do some parsing. Simply put, in this method, a huge amount of data is sent on the network, and the suspect machine is pinged before and during the data flooding. If the machine is in promiscuous mode, it will parse the data, increasing the load on it. It will therefore take extra time to respond to the ping packet. This difference in response times can be used as an indicator of whether or not a machine is in promiscuous mode. A point worth noting is the packets may be delayed because of the load on the wire, resulting in false positives.

Preventing Sniffing

The best way to secure yourself against sniffing is to use encryption. While this won't prevent a sniffer from functioning, it will ensure that what a sniffer reads is pure junk.

Switch to SSH. SSH is fast becoming the de facto standard method of connecting to a UNIX/Linux Machine. For more information on SSH, check out You might also want to check out the open-source implementation, OpenSSH, at

Sniffing Tools

Since I have been a Linux man for years, I will list some of the commonly available sniffers for Linux.

tcpdump: The granddaddy of packet sniffers. Ships by default on many Linux distributions.

sniffit: Robust packet sniffer with good filtering. Available at: the sniffit web site.

ethereal: A free network protocol analyzer for UNIX and Windows. It allows you to examine data from a live network or from a capture file on disk. Available at: the ethereal web site.

hunt: According to Pavel Krauz, the main goal of the HUNT project is to develop tools for exploiting well-known weaknesses in the TCP/IP protocol suite. With hunt, I think he comes pretty close to that goal. Available by FTP.

dsniff: I won't say much about dsniff except to point to an article by Kurt Seifried titled "The End of SSL and SSH". As Mark Joseph Edwards puts it in the article, "Dsniff is the Swiss army knife of privacy invasion." The package ships with a handful of nasties: urlsnarf, msgsnarf, mailsnarf, webspy, dsniff, etc.

Programs to Detect Sniffers

Anti Sniff: From L0pht Heavy Industries comes the new program, Anti Sniff. It has the ability to monitor a network and detect if a computer is in promiscuous mode. Available from

Neped: It detects network cards on the network that are in promiscuous mode by exploiting a flaw in the ARP protocol, as implemented on Linux machines. Outdated. Available by FTP.


Antisniff Technical Details

Robert Graham's Sniffing FAQ

Sumit Dhar is the Manager, Business Development and Products, for Design Expo Networks. He can be reached at

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