[Snort-devel] Average delay per packet observation

Navdeep Uniyal Navdeep.Uniyal at neclab.eu
Fri Jul 7 09:57:30 EDT 2017

Hi Steven,

Below are the rules I am using in my snort.conf:

alert udp any any -> any any (msg:"GENERAL UDP ALERT!!! UDP Packet Received";sid:278;rev:3;)
alert udp any -> any any (msg:"SENDER IP ALERT!!! IP Address found and now printing the same";sid:279;rev:3)
alert udp any any -> any any (msg:"TIME TO LIVE ALERT!!! TTL- TTL<=60.. FOUND!!!";ttl:<=60;sid:280;rev:3;)
alert udp any any -> any (msg:"RECEIVER IP ALERT!!!! Receiver IP Address Found...";sid:281;rev:3;)
alert udp any any -> any 8999 (msg:"PORT ALERT!!!! UDP packets sent to PORT 8999";sid:282;rev:3;)

These are the set of 5 rules which I am repeating (with different sid) to create 10,20,40 and 80 rules.
As I understood from your explanation, the matching algorithms will work only for 5 times in all the cases ( please correct me if I am wrong).

Now since I am logging the alerts on the screen, I am getting alerts for all 10,20,40 or 80 rules in each case. What could be the potential reason of getting the delay in a factor of 10^-4 in case of 80 rules but for 40,20 and 10, it is for a factor of 10^-8.

Best Regards,

From: Steven Sturges [mailto:ststurge at cisco.com]
Sent: Freitag, 7. Juli 2017 15:47
To: Navdeep Uniyal; snort-devel at lists.snort.org
Subject: Re: [Snort-devel] Average delay per packet observation

The matching algorithms in Snort do not repeat the work when the rule options are the same.
And further, as soon as one of the options does not match, evaluation of that entire group of
rules is halted.

As I noted, it depends on the makeup of the individual rules themselves.  If the rules have
'content' options, as is recommended, and that pattern is not present in the traffic being tested,
there is no additional evaluation on the rules at all.  For example, even with 10000 rules where
the pattern from the content option is not present in the traffic, you would get roughly the same
performance as if you had only 10 of those rules.
On 7/7/17 4:52 AM, Navdeep Uniyal wrote:
Thank you for your reply.

In my case I am using a set of 5 rules repeated over(with different sid). So approximately each set should take the same amount of time relatively.
Example: 80 rules have (16*5) rules
                    40 rules have (8*5) rules
                    20 rules have (4*5) rules
                    10 rules have (2*5) rules

By this way, I assume the delay should get halved in each case from 80 to 40. But this is not happening as we can see from the results. Could you please help me in getting the explanation.

Best Regards,

From: Steven Sturges [mailto:ststurge at cisco.com]
Sent: Mittwoch, 5. Juli 2017 13:43
To: Navdeep Uniyal; snort-devel at lists.snort.org<mailto:snort-devel at lists.snort.org>
Subject: Re: [Snort-devel] Average delay per packet observation

Rules are not processed sequentially.  Your expectations should depend on the nature of the

individual rules themselves.
On 7/4/17 10:16 AM, Navdeep Uniyal wrote:
Hello everyone,

I got some interesting results running snort (inline) for experiment with 80, 40, 20, 10 number of rules:
All rules are matching all the incoming UDP packets. Below are the average delay per packet I found in the 4 experiments:

80 rules:              Average delay:  0.000680666813409 seconds
40 rules:              Average delay:  2.06440535385e-08 seconds
20 rules:              Average delay:  1.6644513569e-08   seconds
10 rules:              Average delay:  1.43723338507e-08 seconds

These results are quite confusing as I expect, on decreasing from 80 to 40 rules the average delay should be approximately halved. But I can't see such behavior here.

What could be the possible reason, if someone could explain.

Best Regards,


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