How to determine network interface cards for IDS sensors

After deciding on an operating system (OS) to use for your Snort IDS sensors, you will need to configure networking. Ideally, you should have a minimum of two network interface cards (NICs). One of these is used for sniffing and should be un-numbered -- that is, not have an IP address assigned to it. The other should have an IP address as usual and be used only for management. Also, you may have as many additional network interfaces as you like -- numbered or un-numbered -- provided the hardware and operating system can support them.

The management interface should be on a trusted network, usually your LAN, or a dedicated management VLAN or segment. You can configure it as you normally would for your OS and environment.

For un-numbered interfaces, having no IP address on the un-trusted or monitored segments adds a layer of security. Since there is no IP address to target, those segments are much harder to attack, but not foolproof. By definition, Snort sees the traffic. Therefore a vulnerability in Snort or the network packet capture library may still be exploited, and this has happened in the past. Remember, your sensor is a security device and should be configured, hardened and maintained with that in mind.

Windows, Unix and Linux all support un-numbered interfaces. For example, to bring up eth1 as an un-numbered interface on a Red Hat or derivative Linux distribution, use your favorite text editor to create or edit /etc/sysconfig/network-scripts/ifcfg-eth1

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DEVICE=eth1
ONBOOT=yes

Running an un-numbered interface under Windows is also easy, but counter intuitive. For example, under Windows 2000 simply right click on "My Network Places" and choose Properties. Right click the appropriate connection, e.g. "Local Area Connection 2" and choose Properties again. Verify that you are working with the correct physical interface by checking the name and/or properties (i.e. MAC address) of the network interface card, then uncheck all components, especially "Client for Microsoft Networks" and "Internet Protocol {TCP/IP}." You would think this action disables the card, but it doesn't. It will not show up under ipconfig /all, but it will if you use the snort –W command. Run snort –W and note the number of the interface you will use for sniffing (e.g. 2), then test that Snort is working by a command like snort –vi 2. If Snort suddenly stops working in the future, check snort –W again as Windows sometimes changes the interface numbers when you make changes to networking.

In any case, make sure you cable appropriately after configuring your un-numbered network interface. You don't want to plug the management interface into the un-trusted segment or vice versa.

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