Two programs together Although psad provides

detection, alerting, and auto-response capabilities, the effectiveness of its detection engine is fundamentally limited by the characteristics of the iptables logging format. Better attack detection is offered by fwsnort, including detection for application layer attacks. And because iptables is always inline to network traffic,1 fwsnort can (optionally) prevent malicious packets from reaching their intended targets.

However, because an iptables policy derived from fwsnort runs entirely within the Linux kernel, it cannot perform various alerting functions that are typically possible with a userland application. We need a mechanism for tying the signature detection prowess of fwsnort together with psad's ability to issue whois queries, reverse DNS lookups, send email alerts, associate danger levels with malicious IP addresses, and communicate attack information to DShield.

1 This assumes that the system running iptables is not receiving packet data from a span port on a switch or via a similar mechanism. This is normally a good assumption because iptables is designed to enforce a security policy against live packet data that is destined for real systems; enforcing policy against passively collected packets is of little use.

In this chapter we'll discuss ways to maximize the effectiveness of both psad and fwsnort by using them to reinforce each other. The chapter culminates with a discussion of how to develop a signature to detect Metasploit updates and how to use both fwsnort and psad to interfere with such activity.

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