Network Based IDS and IPS Issues
A network-based IPS offers an additional level of protection to complement that provided by a stateful inspection firewall. An IPS is closely related to an IDS. At first glance, the most obvious difference between the two is how they are deployed.
An IDS examines copies of network traffic, looking for malicious traffic patterns. It then identifies them and can sometimes be configured to take an automated response action, such as resetting TCP connections or configuring another network device to block traffic from an attacker.
NOTE It is important to remember that an IDS detects malicious traffic after it has already happened. Although automated response actions can take place, it is usually too late to stop the attack.
As shown in Figure 4-2, an IDS is typically deployed beside a traffic flow. It receives copies of network traffic from the network switches, hubs, taps, or routers. Because it does not sit in the flow of traffic, it does not break anything that MARS requires.
Figure 4-2 Intrusion Detection System
IDS receives copies of traffic
An IDS often issues a large number of alerts based on traffic generated from MARS, especially if you're using the internal vulnerability assessment feature. You need to tune your IDS so that it does not alert on the vulnerability scans that originate from MARS. You might want to adjust the IDS tuning so that scans from MARS to your CORP_NET are ignored, but scans directed to the Internet trigger an alert. It is generally considered a bad practice to automatically scan hosts outside your own network; the practice might even be illegal. Make sure that MARS is not configured to scan anything that is not on your own network. Your firewall egress rules should not allow this either. However, in the case of a misconfiguration, your IDS can alert the appropriate personnel so that the configuration errors can be corrected.
An IPS sits in the path of network traffic (see Figure 4-3), usually as a transparent device (like a bridge), and watches for many of the same behaviors as an IDS. A major difference between the two, though, is the capability of the IPS to act instantly when malicious traffic is seen.
Figure 4-3 Intrusion Prevention System
IPS inspects traffic
IPS inspects traffic
Network Traffic
Network Traffic
NOTE In addition to the automated actions an IDS can take, an IPS can also prevent the malicious traffic from passing through it.
Because traffic must pass through an IPS, the IPS can prevent MARS from functioning properly if it is misconfigured. Take time to closely watch alerts generated by your IPS and tune it appropriately. Like the IDS, you should tune the IPS to allow vulnerability scanning to occur from MARS to CORP_NET, while preventing it from scanning the Internet.
Some of the newest types of IPSs, such as the Cisco IPS, have a feature called traffic normalization. This feature, in particular, causes the MARS vulnerability assessment to fail. Traffic normalization enables several functions, including the following:
• Prevents illegal combinations of TCP flags from passing, or removes the illegal flags
• Prevents fragmented traffic from passing, or rebuilds it so that it is not fragmented
• Changes all packets in a traffic flow to have the same time to live (TTL)
This is just a small sampling of what a traffic normalizer does. In general, you can think of it as an engine that takes traffic that does not conform to standards, and either prevents the traffic from passing through the IPS or makes it conform to standards first.
By itself, traffic normalization breaks a large amount of attacks and reconnaissance activities. It also stops vulnerability assessment tools from being able to accurately determine information such as the operating system that a target host is running.
NOTE Cisco IPS 5.x and 6.x software, by default, does not generate alerts on most traffic normalization signatures. To properly tune the software, you need to enable alerts on that family of signatures.
If you're protecting your security management network with an IPS that supports traffic normalization, you need to tune it to either ignore the scans from MARS and Qualys (or other vulnerability scanners) or disable the traffic normalization capabilities.
Continue reading here: Restoring to a Reporting Appliance
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