[Snort-users] Gigabit NIC's and snort hardware required??

Bennett Todd bet at ...6163...
Fri Jun 6 05:58:09 EDT 2003


2003-06-05T23:42:03 Zach Forsyth:
> When I use the command :> snort -vi2, shouldn't that be a very fast
> running version of snort?

When I check the man page, it says:

       -v     Be verbose.  Prints packets out to the console.   There  is  one
              big  problem with verbose mode: it's slow.  If you are doing IDS
              work with Snort, don't use the '-v' switch, you WILL drop  pack-
              ets.

That's for snort-2.0.0, but I really don't think this has changed.
It's very basic.

> It is only logging to the dos window I run it in. Is it using any rules
> when run in this way?

No, to get snort to run rules as an IDS you use "-c" to aim it at a
snort.conf. And in the snort manual (available at
<URL:http://www.snort.org/docs/writing_rules/>), right in the top,
at "1.4  Network Intrusion Detection Mode" it says:

	[...] if Snort is going to be used in a long term way as an
	IDS, the -v switch should be left off the command line for
	the sake of speed.  The screen is a slow place to write data
	to, and packets can be dropped while writing to the display.

> I was under the impression that if it could not keep up with that
> command when I tell it to log or alert to a DB it would be even worse.

No, the other way around, if you're telling snort to log every
packet to the screen, it'll be one slow little piggy. The starting
point command you want is something along the lines of "snort -b -A
fast -c .../snort.conf", modulo the differences between Unix and
Windows commandlines.

If you want to cram your alerts into a DB, and you also care about
performance, you will positively want to use the unified binary
output format feeding Barnyard. Or else to craft up something
custom, using either -A fast or syslog to another machine, and
feeding the DB on another machine with a file-tailer.

> P4 Xeon - does Xeon make a big difference? does it matter if it is dual
> or not?

If you only run snort on that machine, and do the DB feeding
somewhere else, dual should make less difference. If you tune to the
point where you don't have too many alerts dual will certainly not
make a difference. Snort itself is one single-threaded process.

> 512mb or 1gb ddr ram - ram speed help, or just amount?

I'm not up to date on different flavours and speeds of RAM. Slower
can't be better, though. With even the least bit of tuning 512M can
be fine, but if it's not too dear to expend 1gb that completely
removes any worry that snort might want a little more memory than it
can get (==> _slow_).

> SATA or SCSI raid? Does disk speed make a huge difference?

I built hot little snorters with simple plain IDE drives. You get
speed by ensuring snort doesn't need to write much. If snort is
having to write so much that disk speed matters, you'll have a slow
snorter; high alert volumes force snort to spend too much time
whinging and too little time analyzing. Now if you're cramming
alerts into a DB, the _separate_ machine the DB runs on should have
fast disks.

Windows instead of Linux may change this picture in some way, I
don't know.

> In order if importance to snort speed:
> 
> Tuning
> Pci bus and gb card speed
> memory
> Processor

I'd rank those

	Memory + Tuning
	Processor
	bus & card speed

Memory is mandatory. If snort doesn't fit in real memory, you're
doomed. Tuning is mandatory if you want to keep up with more than
roughly 50Mbps (in my experience, possibly newer hardware has moved
that point, I dunno). When I saw my little piggies starting to drop
packets for real, the processor utilization was right on up there,
so once the mandatory memory+tuning have been addressed I'd guess
CPU might come next. Bus and NIC performance would make more
difference on where the ceiling lies, I'd expect --- assuming you've
got a NIC with a good OS driver that doesn't chew the system up just
watching the net, before snort even starts looking at the packets.

Another factor would be the quality of the pcap lib. I don't know
about Windows there. On Linux I've heard that the ring-buffered
varient libpcap can be a win, although I've not used it myself (I
designed our snort deployment to avoid needing to handle more than
c. 50Mbps per box).

-Bennett
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