Snort mailing list archives
Re: New Snort 2.2 Rules
From: Alex Kirk <alex.kirk () sourcefire com>
Date: Wed, 14 Sep 2005 18:02:39 -0400
This is partly correct. The flow preprocessor still handles keeping track of a TCP connection's state & direction, as always. The distinction you're seeing is that many older rules used things like flags:AP to attempt to detect established connections -- not a particularly reliable method, given that it's possible to set the ACK & PUSH flags on a packet that is not part of an established TCP connection -- while modern rules use the flow:established keyword/value pair to use the capabilities of the flow preprocessor to do this type of checking. Since the preprocessor is much, much more accurate when determining connection state, it filters out an even larger number of malicious packets which are not part of an existing TCP connection than flags:AP or the like, and as a result your IDS will be correspondingly more quiet.
Alex Kirk Research Analyst Sourcefire, Inc.
I've noticed the same thing in my configuration where Snort is much more quiet than it used to be... False positives and "noise" seem to be at a minimum now. This is definitely not at the expense of solid detection however. I really put Snort 2.4 through some heavy tests with Nessus and other tools, and it does detect everything just fine.In looking at the rules, I noticed that many of the rules now use the /flow:established/ option. I might be mistaken, but I don't think this was always the case with the rules. I think a preprocessor used to handle the flow conditions. In a rule with /flow:established/, Snort will only detect the anomalies that occur during an established connection. It doesn't alert on the packets that are simply aimed at your network segment, but not actually traversing an existing connection.Do I have this right? ------------------------------------------------------------------------*From:* snort-users-admin () lists sourceforge net [mailto:snort-users-admin () lists sourceforge net] *On Behalf Of *Walt Rich*Sent:* Wednesday, September 14, 2005 4:27 PM *To:* snort-users () lists sourceforge net *Subject:* [Snort-users] New Snort 2.2 RulesI updated the Snort rules to the latest available on Souceforge's site. They wre auite out of date, and almost a year old. Snort is up and running, but has become very queit! It used to detect alot of false positives, which were a pain, but at least I knew it was working. Now it is very, very quiet, and hasn't detected anything in over 2 hours. Is it possible that the rule writers have become so good that the detection of false positives has been almost eliminated? Has anyone else experienced anything similar? Any input is greatly appreciated. Thanks!Parago Logo ------------------------------------------------------------------------| *Walt Rich* | Sr. Network Engineer | Parago, Inc. | 972.538.7253 | walt.rich () parago com <mailto:walt.rich () parago com> |
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Current thread:
- New Snort 2.2 Rules Walt Rich (Sep 14)
- Re: New Snort 2.2 Rules Eric Hines (Sep 14)
- Re: New Snort 2.2 Rules Eric Hines (Sep 14)
- RE: New Snort 2.2 Rules Andre' M. DiMino (Sep 14)
- Re: New Snort 2.2 Rules Alex Kirk (Sep 14)
- RE: New Snort 2.2 Rules Andre' M. DiMino (Sep 15)
- Re: New Snort 2.2 Rules Alex Kirk (Sep 14)
- Re: New Snort 2.2 Rules Eric Hines (Sep 14)