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I would like to understand if snort rules are applied for each packet in a flow between a source and destination separately? For instance, when packet 1 of the flow arrives at the IDS, all the rules in snort get evaluated, followed by packet 2 and so on? If this is not true, when do the rules actually get evaluated?

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Stream. Here is a good document regarding how snort uses libpcap to apply heuristic and signature based anomoly detection to network traffic; http://www.pearsonhighered.com/samplechapter/157870281X.pdf and when in doubt consult the manual https://www.snort.org/faq/readme-stream5

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    To casual viewers who may not understand what you mean. What is the difference, if any, between packet, flow, and steam? – Mark Buffalo Feb 2 '16 at 3:25
  • TCP, BGP, UDP, etc all generally include multiple data grams and to effectively determine if a payload includes a threat libpcap, (the packet creation and disassembly library under the hood of snort & surricata) it must utilize stream based analysis techniques. However I am not certain that all detection techniques use just a stream – jas- Feb 2 '16 at 12:04
  • Well here is the official documentation regarding the streams processing and configurable parameters snort.org/faq/readme-stream5 – jas- Feb 2 '16 at 13:16
  • It may help to update your answer to reflect the requested information in my comment. :] – Mark Buffalo Feb 2 '16 at 13:17

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