[Snort-users] Pattern Matching

Rayne hjazz6 at ...14432...
Thu Oct 16 05:55:55 EDT 2008

Hi all,

I have a few questions regarding the pattern matching aspect of Snort.

1) If I have the following rule option (content:"ABC", content:"DEFGH"), am I right to say that the string "DEFGH" will be compared first to see if there is a match, and if there is, then "ABC" is compared, because "DEFGH" is the longer string?

2) Is it possible to have one rule activate another rule within the same packet, i.e. when a content match with "AB" is found, it will trigger another rule that consists of a content match with a longer string, e.g. "CDEFG". This would be something similar to activate/dynamic, except from what I understand, dynamic only logs a certain number of subsequent packets that match the first rule after being activated, which is not exactly what I want to do. If this is possible, does the second content match start from the beginning of the payload, or from where "AB" was matched?

3) Say I have 5 rules each with one content match. All the rule headers are the same, i.e. the 5 OTNs are under the same RTN, and they contain only the content match. Using the AC search method, does Snort build just one DFA that contains all 5 strings so each packet can be searched through only once for all 5 strings at a time, or is a DFA built for every OTN/string, resulting in searching through each packet 5 times? What if one of the rules has 3 content matches while the other 4 has only one content match each. How is the DFA built then?

4) Does the pattern matching algorithm return the position within the payload where the pattern is found? For example, if I'm matching for the string "GET" and the payload is "kas sdfGETjkdn", will I get something like "Pattern "GET" matched at position 8"? Also, in acsmx.c, it is mentioned that the AC algorithm "finds all occurrences of all patterns within a body of text". If there are, say, 5 occurences of a pattern string, do I get one alert/log per occurence, one alert/log per pattern matched (if there are multiple content strings in the rule option) or one alert/log per rule (regardless of the number of content strings in the rule option)?

5) How long does Snort hold fragments for reassembly in Frag3 and Stream5 before discarding the packets if they are incomplete?

Thank you.


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