The difficulty of implementing IDS into a switched environment stems from the basic differences between standard hubs and switches. Hubs have no concept of a connection and thus will echo every packet to every port on the hub, excluding only the port the packet came in on. A switch however is based on connections, when a packet comes in a temporary connection in the switch is made to the destination port, and the packets are forwarded on. So in a hub environment we can place our sensors almost anywhere, while with switches specific workarounds must be used to assure the sensor is able to see the traffic required.
The current options for this are TAPS, Hubs and Spanning ports. A spanning port configures the switch to behave like a hub for a specific port. For instance in Figure-1 we wish to monitor the connection between the switch and the Resource Machine. To do this we tell the switch to span the data from the resource machines port to the IDS port. This can be done with Transmit Data, Receive Data or both. Some current switches cannot be relied on to pass 100% of the traffic to the spanned port, so attacks could go un-noticed even when the IDS system is configured to look for the attack. Switches also only allow one port to be spanned at a time, so monitoring multiple machines can be difficult or impossible.
Using Hubs or TAPS is a very similar solution, the hub or tap is placed between the connection to be monitored. This is usually between either two switches, a router and switch, or a server and switch, etc. In Figure 2 a hub has been placed between the resource machine and the switch. This allows traffic to still flow between the switch and the Resource while the properties of the hub cause a copy of the traffic to be copied off to the IDS. This, like the span port is only suitable for single machines. Multiple machines on the hub would cause network problems and remove the benefits of a switched solution. In addition, to get a fault tolerant hub would increase the cost of the solution dramatically. Taps are by design fault tolerant having the main connection (i.e. the connection between the resource and the switch), hardwired into the device, preventing failure.
Figure 3 shows a tap monitoring a single Resource machine. The tap is unidirectional and passes traffic from the switch and resource machine to the IDS only. This prevents traffic passing from the IDS to the switch or Resource machine nor can traffic be directed at the IDS. Since the Tap is unidirectional we can route the traffic from several taps back to a hub to be monitored by the IDS system, without causing network problems, this is shown in Figure 4.
www.shomiti.com, www.ods.com, are makers of taps.
Brian W. Laing
Internet Security Systems