North American Network Operators Group

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RE: Gb ethernet interface keeping dropping packet in ingress

  • From: Joe Johnson
  • Date: Tue Sep 14 00:07:00 2004

Now, we do try to monitor some things like that.  We have several crons
running checking the number of entries in the arp tables of our CPE
devices at customer locations, as well as several crons dedicated to
specific tell-tale signs of various worms and virii.

One that helped out a lot recently was Nachi/Welchia search.  Caught 40%
of our subscribers that were infected, and helped stop all but 3
specific broadcast storms on our network.  All the cron did was look for
the specific ICMP packet that the virus put out, and flagged the
connection in a list that is emailed to the NOC staff.

Joe Johnson

> -----Original Message-----
> From: owner-nanog@xxxxxxxxx [mailto:owner-nanog@xxxxxxxxx] On Behalf
Of
> Jeff Kell
> Sent: Monday, September 13, 2004 10:46 PM
> To: Joe Shen; nanog@xxxxxxxxx
> Subject: Re: Gb ethernet interface keeping dropping packet in ingress
> 
> 
> If you're sniffing one gigabit port from a switch with much higher
> bandwidth, you're going to lose something.  Our primary sensor sits on
> an aggregation switch just prior to hitting the net, and we have a 2Gb
> fast etherchannel span port defined and lose relatively little in
terms
> of packet loss.  If course, the more aggregate traffic you have, the
> higher the probability you will max out the span port and it's
buffers.
> Unless you're just drilling the heck out of the server farm(s) on that
> switch, you won't lose all that much with an etherchannel of 2 Gig
> ports.  We have 2Gb etherchannel uplinks back to the core, and the
most
> the switch could throw at us would be 2Gb etherchannel traffic.  So we
> are spanning the uplinks there.
> 
> Just as your switches/routers can be "over subscribed" the the 4506
> backplane is only 6Gb/slot, and we don't lose that much, and some of
> that loss is due to buffer constraints on the switch.  Not perfect,
but
> it works.  In less critical ennvironments, we can sniff with a 100Mb
> interface and still do well.
> 
> The only caution here is that you can seldom catch local traffic.  If
> there's a local scanner (like Blaster started out to be) it doesn't
show
> up except for excessive arps.  We have some cron'ed scripts that
> periodically (1) look at connection counts in the PIX, if they're out
of
> "range), we quarantine them to the Perfigo dungeon.  Similarly there
is
> a script that counts ARP requests (just the dorms specifically right
> now) and for every 1000 it forks itself to start anew, and analyzes
the
> numer of ARPS per station.  Local scanners get eaten up here really
> quickly and they are also quarantined.
> 
> Not how sure this fits into NANOG, this is more of a local
> ISP/Universiity setting.  I don't know that an ISP can do that much,
> they're too busy keeping the packets flowing and being only minimally
> intrusive on your traffic without special arrangements, at least as a
> usual case.  Special cases like Slammer, Blaster, and the initial
> Bagel/MyDoom mix some may have initiated ingress/egress filters for
> those, temporarily.
> 
> You should be able to handle an OC-12 with a gig interface or two on
the
> sensor.  I wouldn't make any claims for an OC-48 or above.  These
things
> don't scale well into the certral peering points (MAE, Abilene, etc);.
> 
> Jeff