What Is Promiscuous Monitoring
In the realm of computer networking, promiscuous mode refers to the special mode of Ethernet hardware, in particular network interface cards (NICs), that allows a NIC to receive all traffic on the network, even if it is not addressed to this NIC. By default, a NIC ignores all traffic that is not addressed to it, which is done by comparing the destination address of the Ethernet packet with the hardware address (a.k.a. MAC) of the device. While this makes perfect sense for networking, non-promiscuous mode makes it difficult to use network monitoring and analysis software for diagnosing connectivity issues or traffic accounting.
In a wider sense, promiscuous mode also refers to network visibility from a single observation point, which doesn't necessarily have to be ensured by putting network adapters in promiscuous mode. Modern hardware and software provide other monitoring methods that lead to the same result. In this white paper, we'll discuss the techniques that are commonly used to ensure network visibility in different network environments, both wired and wireless, equipped with different kinds of hardware.
Practical Applications
So why would we need to monitor anything other than our own computer? Simply put, in a situation where your network consists of three computers, A, B, and C, and the network analyzer is running on computer A, you should be able to see not only the data being sent from/to computer A, but also the data being sent from B to C. This is practical if you use network analyzers, traffic accounting systems, content monitoring systems, and similar applications. In theory, you could run a monitoring application on each computer, but it's inconvenient, as you wouldn't have the complete picture in front of you.
This white paper was written to address frequently asked questions by the users of TamoSoft products CommView and CommView for WiFi, network analyzers for wired and wireless networks, CommTraffic, a traffic accounting system, and NetResident, a content monitoring system. However, the monitoring principles and network topologies discussed here are universally used, so this information, by large, is applicable to any other Ethernet and Wi-Fi monitoring and analysis software, regardless of the vendor and operating system.
Ethernet Networks, Hubs and Switches
In Ethernet networks, hubs and switches are central connection points for the networking of multiple computers or other network devices. Together, these computers form a network segment. On this segment, all computers can "talk" directly to each other. Hubs are less intelligent devices than switches: They simply receive incoming packets on one port and broadcast them to all other ports. This feature makes them ideal for promiscuous network monitoring.
Unlike hubs, switches inspect packets as they are received to check the source and destination MAC addresses and forward them to the correct port. In a switched network environment, a packet analyzer is limited to capturing broadcast and multicast packets and the traffic sent or received by the PC on which
You can see many broadcast packets being sent from the hosts in your LAN segment to the broadcast IP addresses, but you can't see normal, unicast traffic between these hosts or between these hosts and the Internet. Despite the fact that most switches prevent promiscuous monitoring, a lot of switches can be configured to forward packets to a special monitoring port. We'll discuss the usage of hubs and switches for monitoring purposes below.
Monitoring Using Hubs
Hubs are still popular devices for small networks because of their low cost, but attention should be paid to potential problems with using hubs for network monitoring. First, hubs are open to unauthorized monitoring from within the LAN segment, as any port can be used for promiscuous mode monitoring. Second, "auto-sensing", "dual-speed", "switching", or "intelligent" hubs may prevent you from monitoring the entire LAN segment. This problem will be discussed in the next chapter; meanwhile, we'll show a few alternative network layouts that use hubs for monitoring purposes.
This is probably the simplest and most obvious layout. Here, any computer connected to the hub can be a monitoring computer, as the hub replicates the data received/sent to/from the router to all ports. Additionally, data exchange between the local workstations can be monitored.
