Introduction to Network Security - Intrusion Detection
{LANG_NAVORIGIN} Security Basics Intrusion Detection
Mitchell Rowton
02/12/2005
Network Placement
In order for Network IDSs to function properly, they must monitor all traffic on the network segment that it is responsible for. This can be easier in a network segment containing a hub than one containing a switch.
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 broadcast every packet to every port on the hub, excluding the port the packet came in on. So in a hub network segment, you can place the IDS almost anywhere, while with switches specific workarounds must be used to assure the sensor is able to see the traffic required. The below figure shows a Network IDS attached to a hub, this IDS receives all traffic on its local network segment.
If the above diagram included a switch instead of a hub, then the IDS would not receive all the traffic in the network segment. That’s because a switch 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. Therefore, the IDS will not see all traffic destined to individual computers.
In order to use IDSs in a switched environment you must span the switch port, so that the IDS receives a copy of the traffic passing through the spanned port. Basically, a spanning port allows the switch to behave like a hub for a specific port.
The figure above shows the switch port to the router set to spanning mode, copying all network traffic to the IDS. You can span send traffic, receive traffic, or sometimes both. The drawback of this setup is that the IDS does not receive traffic among computers in its same network segment, it only receives traffic between the switch and router.
Using a hubs or TAPS is a very similar solution, the hub or tap is placed between the connections to be monitored. This is usually between two switches, a router and switch, or a server and switch, etc. In the below diagram 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. Also, hubs do not allow for full-duplex connections. 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 and often times are full duplex.
Conclusion
An Intrusion Detection System is very helpful in detecting threats to the confidentiality, integrity, and availability of organizations information. However without proper documented policies and procedures to outline monitoring and response steps this information is of little use.
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