Richard Bejtlich is founder of TaoSecurity, author of several books on network security monitoring, including Extrusion Detection: Security Monitoring for Internal Intrusions, and operator of the TaoSecurity blog. On the one hand, the rule for detecting traffic to the. We'll take some generic rules from the official Snort rules so that you can look. The objective of the exercise is to improve the rules proposed in the examples of rule creation. There is little point blaming the tool for doing what it was told to do. This is a rough example of the Snort rule language and its capabilities. I would blame the rule writer, IDS operator, and/or the IDS developer for setting the rule that resulted in the undesirable alert. If an operator tells an IDS to detect an event that matches certain characteristics, and the IDS performs that role properly, there is no false positive. I don't consider scenarios one and two to be false positives. That is a significant problem and can be justified as a false positive. Although that pattern did not pass on the wire, the IDS reported an alert. The IDS was told to provide an alert when detecting a certain traffic pattern. The reputation preprocessor loads these lists when Snort starts, and compares all traffic against those lists. Malicious IP addresses are stored in blacklists, and trusted IP addresses are stored in whitelists. An IDS reports seeing a security event, but the traffic it believes it saw never actually occurred on the wire.īy my standards, only scenario three represents an actual false positive. The reputation preprocessor was created to allow Snort to use a file full of just IP addresses to identify bad hosts and trusted hosts.These nf example files are distributed inside of the Snort Subscriber Rule Set. An IDS reports seeing an event that shows syntax used in an attack, but the syntax is used in a benign manner by an authorized user. As the nf that is contained inside the etc/ directory of the Snort tarball is a snapshot in time (at the time of the tarball release), it is necessary to occasionally update the nf in order to take advantage of updates for the preprocessors and include new rule files.An IDS reports an attack that targets Microsoft IIS Web servers, but the attack is directed against an Apache Web server.This one is the 7th revision of this rule. Rev - as rules are revised there are assigned revision numbrs just like software. ![]() alert, log, pass, activate, dynamic, drop, reject, sdrop. In our example, we can see that it is classified as an "attempted-recon". Rule Header + (Rule Options) Action - Protocol - Source/Destination IPs - Source/Destination Ports - Direction of the flow. In our example, we can find more info on this attack in the arachnids database, attack 198.Ĭlasstype - All the rules are classified into numerous categories to help the admin understand what type of attack has been attempted. Reference - This section is for referencing a security database for more information on the attack. ![]() This rule is looking for traffic that has both the SYN and FIN flags set (SF) and in addition, has the two reserved bits in the flags byte set (12). The following rule adds SID equal to 1000001. The only argument to this keyword is a number. Refer to the list of rules that came with your Snort distribution for examples. All numbers above 1,000,000 can be used for local rules. ![]() As you know, TCP flags can be SYN, FIN, PSH, URG, RST, or ACK. Range 100-1,000,000 is reserved for rules that come with Snort distribution. In our example, the rule is triggered on traffic with or without an established TCP connection.įlags - This couplet checks for TCP flags. It can have a number of values including established (TCP established), not established (no TCP connection established), stateless (either established or not established), etc. On the other hand, in Snort-2.6 it will be matched against only the tcp rules whose source and. In this case, Snort reports to the sysadmin "SCAN SYN FIN".įlow - This option allows the rule to check the flow of the traffic. For example, a tcp packet will be matched against all tcp rules. Msg - This is the message that's sent to the sysadmin if the rule is triggered.
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