Snort mailing list archives
detecting outgoing portscans
From: Andrew Daviel <andrew () andrew triumf ca>
Date: Fri, 12 Oct 2001 10:49:56 -0700 (PDT)
In the wake of NIMDA, when we had a supposedly patched machine infected, I thought I'd try to enable the portscan preprocessor (Snort 1.7) on everything. I initially tried to have two portscan entries, with different logfiles and different thresholds, but that doesn't work. If I have a threshold of 30 or so, as for detecting real inbound scans, I get a lot of bogus outbound scans from people websurfing. I *think* that SNort registers a SYN scan if a user aborts a page load in a browser, as for instance clicking a link before all the images have loaded. I've seen 190 SYN alerts in a couple of hours from someone using a search engine. Is there any way to treat this as "normal" , perhaps if an unacked SYN follows one or more acked packets ? Otherwise I guess I set a threshold in a postprocessor higher, like 500. I'd like to catch outbound scanning or worm activity for which I don't have a pattern, either because I'm lazy or where one hasn't been written yet. I'd found I was getting way too many false positives from the regular Snort pattern matching to bother investigating. Again, I suppose I can set a postprocessor to trigger on 500 outbound alerts or something. -- Andrew Daviel, TRIUMF, Canada Tel. +1 (604) 222-7376 security () triumf ca _______________________________________________ Snort-users mailing list Snort-users () lists sourceforge net Go to this URL to change user options or unsubscribe: https://lists.sourceforge.net/lists/listinfo/snort-users Snort-users list archive: http://www.geocrawler.com/redir-sf.php3?list=snort-users
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- detecting outgoing portscans Andrew Daviel (Oct 12)