[Snort-users] Output question during FIN scan

Matt Kettler mkettler at ...4108...
Sun May 12 14:44:02 EDT 2002

1) I'll repeat, please don't post in HTML to this list, it munges the 
digest for digest mode subscribers, not to mention looks like complete 
sensless garbage to the numerous plain-text mail readers on this list.

2) The stats mean that the kernel dropped 1847 packets without being able 
to give them to snort. Of the 1305 packets that the kernel was able to give 
snort, snort processed all of them.

The total number of packets seen by the snort computer is 1847+1305.

Basically you missed more packets than you managed to process. Lighten your 
snort config up. Use non-text mode logging for starters. ie: tcpdump 
logging, etc.

Note that if you're doing fin scans across a local 100mbit ethernet segment 
this is not likely to be a realistic load for snort (unless you have an OC3 
you are monitoring). Compared to, say a typical cable-modem, T1, or common 
DSL line, which are typically under 2mbit/sec the 100mbit ethernet is 50 
times the load. Try to present snort with a load which isn't substantially 
greater than your real world setup and then tune.

If you really need to monitor a truly high-speed network, please RTFM on 
high performance setups:

(I've de-htmled tommy's message as best I can, and turned the html angle 
braces in the stray urn schemas tag to parens)


The summary from implementing a FIN scan to my own private network outputs 
part the following:

Snort analyzed 1305 out of 1305 packets, The kernel dropped 1847(141.533%) 
packets (?xml:namespace prefix = o ns = 
"urn:schemas-microsoft-com:office:office" /)

Breakdown by protocol:                Action Stats:

TCP: 1299              (99.540%)               ALERTS: 612

UDP: 0                    (0.000%) LOGGED: 611

ICMP: 2                  (0.153%) PASSED: 0

ARP: 4                    (0.307%)

IPv6: 0                    (0.000%)

IPX: 0                     (0.000%)

OTHER: 0              (0.000%)

DISCARD: 0          (0.000%)

My question is how is it possible to drop 1847 when the program analysed 1305?

Forgive me if it is an easy answer but I am a newbbie.

Thanks again.

Thomas Tsilalis

More information about the Snort-users mailing list