Security Basics mailing list archives
RE: Caching a sniffer
From: "David Gillett" <gillettdavid () fhda edu>
Date: Thu, 25 Mar 2004 10:55:49 -0800
-----Original Message----- From: Shawn Jackson [mailto:sjackson () horizonusa com]1. Dump the entire MAC table. Switch acts as if power onreset justoccurred.Seams logical, but I've never seen it implemented. It would halt traffic while learning resumes, in addition if other checks needed to run (Spanning Tree/CDP) it would take much longer.No. Traffic will flood while learning restarts from scratch. Nothing was said about dumping STP/CDP statuses.Traffic will halt because the switch can't forward at the point it's trying to learn the network.
You've confused routine learning of source MAC addresses with the "learning" mode that ports go through while spanning tree is reconverging. It's true that packets won't be forwarded in the latter case, but resetting the MAC table (forgetting learned source MACs) need not force a spanning-tree reconvergence, and if it doesn't then the traffic will flood temporarily (as source MACs are re-learned) rather than be dropped.
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.Destinations not in the table normally get flooded, not dropped. Dropping this traffic is possible, but not a normal part of the action being suggested.If the system fires up on the afflicted switch. If it's in another segment of the network all systems and paths on the afflicted switch would be dark because the switch would use it's current MAC->Port table, and not add additional MAC's to the port.
Flooding unknown destinations to all ports (except the origin) *includes* forwarding to uplink/inter-switch ports. So hosts on the afflicted switch will still be able to reach hosts on other switches, and vice versa.
3. Partial Purge of table. Some portion of the tablegets purgedand the switch continues, treating those purged MACaddresses as ifthis 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 orsomethingelse - will cause the switch to act differently fordifferent users.Seams a better solution out of the bunch, could be a pain to implement.Some switches routinely age unused entries out of the table. Accelerating this process if the table fills shouldn't be too hard.You can usually modify aging in the configuration. But the aging time would have to outpace the flood to keep the table clear, this would result in more traffic and load for the switch.
I think this suggestion was that the MAC flood would trigger the aging, so it keeps pace automatically. That, in turn, requires the malicious host that's flooding bogus MAC addresses to continue doing so, in order to try to keep valid MACs from being retained in the table (long enough to keep their traffic from flooding and being seen by the sniffer). David Gillett
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Current thread:
- RE: Caching a sniffer, (continued)
- RE: Caching a sniffer Andrew Shore (Mar 25)
- RE: Caching a sniffer Paul Blackstone (Mar 25)
- RE: Caching a sniffer Byron Copeland (Mar 26)
- Re: Caching a sniffer Aaron (Mar 29)
- RE: Caching a sniffer Paul Blackstone (Mar 25)
- RE: Caching a sniffer David Gillett (Mar 25)
- RE: Caching a sniffer Andrew Shore (Mar 25)
- RE: Caching a sniffer Andrew Shore (Mar 25)
- RE: Caching a sniffer Shawn Jackson (Mar 25)
- RE: Caching a sniffer Shawn Jackson (Mar 25)
- RE: Caching a sniffer David Gillett (Mar 26)
- RE: Caching a sniffer Shawn Jackson (Mar 25)
- RE: Caching a sniffer David Gillett (Mar 25)
- RE: Caching a sniffer Shawn Jackson (Mar 25)
- RE: Caching a sniffer Andrew Shore (Mar 25)
- RE: Caching a sniffer Shawn Jackson (Mar 25)
- RE: Caching a sniffer David Gillett (Mar 26)
- RE: Caching a sniffer Shawn Jackson (Mar 26)
- RE: Caching a sniffer Shawn Jackson (Mar 26)
- RE: Caching a sniffer Nero, Nick (Mar 26)
- Re: Caching a sniffer aruna (Mar 29)
- Re: Caching a sniffer Mitchell Rowton (Mar 30)