Alert vs. log in the Snort /var/log/snort directory

A Linux security expert explains that the difference between the Snort alert and log logs in the Snort /var/log/snort directory is based on how rules are written.

Your article, "Improving Snort with Barnyard," was nice but i noticed two things. I have used Snort and Barnyard together for a while and there is some stuff Sourcefire never had the answer to. Maybe you can help. What is the difference between the "log" log and the alert logs that show up in the /var/log/snort directory? I was told by some people you don't need the alert log; the "log" log has everything that alert has and more. Anyways, it is awfully confusing and it seems that Barnyard only processes the /var/log/snort/*.log files and not the *.alert.

This is an interesting question. The difference between alert and log comes down to how you write your rules. Rules can have actions associated with them when they trigger. The possible actions are, to quote the Snort manual:

  • alert -- generate an alert using the selected alert method, and then log the packet
  • log -- log the packet
  • pass -- ignore the packet
  • activate -- alert and then turn on another dynamic rule
  • dynamic -- remain idle until activated by an activate rule, then act as a log rule

If a rule is configured to alert, then an alert will be generated and outputted to whatever alert method you have configured, like a file in /var/log/snort. The packet is then logged to your log output method; for example, the snort*.log files. So by processing the log files, you will get all of the entries. The best and clearest answer to this question, however, comes from Marty Roesch himself in this 2002 mailing list post.

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