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RE: Caching a sniffer
From: "Shawn Jackson" <sjackson () horizonusa com>
Date: Wed, 24 Mar 2004 11:23:20 -0800

In essence if you flood the MAC table of a switch the switch will
into a hub, thus "disabling the switch component of the ports".

Of course, that's not necessarily true.  The behavior of a switch when
the MAC 
address table is exceeded is not defined by any standard, nor is it
often specified 
by the manufacturer.

Of course your correct. But I've tested numerous switches, Cisco being
the most
tested. Bay Networks, SMC, HP/Compaq network gear have all responded in
a somewhat
similar fashion.

1. Dump the entire MAC table.  Switch acts as if power on reset just

Seams logical, but I've never seen it implemented. It would halt traffic
learning resumes, in addition if other checks needed to run (Spanning
it would take much longer.

2. Stop learning.  All previously learned MAC addresses remain, and so
only traffic 
for unrecognized MAC addresses gets sent to all ports.

That would damage the network. If a new client fires up, they would not
get added to
the switches tables and not receive any traffic.

3. Partial Purge of table.  Some portion of the table gets purged and
the switch 
continues, treating those purged MAC addresses as if this was the first
time they 
were seen.  Depending upon how the purged addresses are selected -
oldest first, 
youngest first, random, lowest MAC addresses, highest MAC addresses or
else - will cause the switch to act differently for different users.

Seams a better solution out of the bunch, could be a pain to implement.

4. Shutdown port - assume hostile intent and stop forwarding traffic.

Called Port-Security on Cisco switches, don't know about the others.

Do I know of which switches do what?  Nope.  But we should ALL have
learned the 
lessons of depending upon undocumented behaviors and unspecified
conditions with Y2K.

Agreed, but all we need to do is ask the manufacturer of the equipment
to see
how it deals with the problems/attacks we are describing. It's not, "the
way it works"
it's how they dealt with the problem. Cisco's do it by design, and so do
most manufactures.

SPAN/Port Mirroring/Roving Analysis Port(3Com) is intentional and
controlled by the 
administrator.  Also, how the port handles traffic in excess of it's
(say you are monitoring 3 100BaseT ports out a single 100BaseT port),
is completely 
Mfg dependent and undocumented.

Traffic gets queued, just like any other switch-port. FiFo, Weighted,
etc. That's not
undocumented, it's just not advertised on most switches. On the better
ones you can
select your queuing strategy.

Shawn Jackson
Systems Administrator
Horizon USA
1190 Trademark Dr #107
Reno NV 89521

Email: sjackson () horizonusa com
Phone: (775) 858-2338
       (800) 325-1199 x338

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