Security Basics mailing list archives
RE: Caching a sniffer
From: "David Gillett" <gillettdavid () fhda edu>
Date: Wed, 24 Mar 2004 08:58:47 -0800
Incorrect. A switch is basically a hub and router in one.
... if you redefine "router" to include a concept similar to "layer 2 router", at which many people will look at you rather strangely. The term normally refers to a layer 3 packet-forwarder which rewrites packets, whereas switches are multiport bridges that forward frames, without rewriting, based on the destination MAC address.
You can flood the MAC address table of the switch, where it decides what port has what MAC's on it so it knows what port to route the traffic to. Once the table is full switches then 'turn-off' the routing/switching systems and the switch then becomes a hub. There is a program called macoff that does this. So you don't need to have access to the switch to sniff the entire network.
Switches "learn" what MAC addresses are on what port by collecting source addresses from frames into a table. Traffic will be flooded to all ports if the destination MAC address is not in the table. I presume that some switches, faced with something like macoff, will overflow the table such that legitimate addresses that should have been learned start flooding to all ports as well. But this is not the only possible reaction of a switched network to macoff! If Cisco's port security is enabled, the switch may just shut down the port running macoff. If the network consists of multiple switches, something like macoff may prompt a spanning-tree reconvergence, disrupting the entire network for 30 seconds or so. I'm sure there are other possibilities depending on manufacturer/model/firmware of the switches in the network. Personally, if I had to sniff traffic on a switched network without admin access, I'd prefer to use arp poisoning, a la ettercap. The MAC address tables on the switches go right on functioning normally, just all of the traffic to/from the client you're interested in gets sent to the sniffer machine's MAC address and forwarded to the intended destination from there, with minimal impact on other network traffic or performance. About the only visibility is if the victim happens to run "arp -a" and understands what they're seeing. David Gillett
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Current thread:
- RE: Caching a sniffer, (continued)
- RE: Caching a sniffer David Gillett (Mar 23)
- Re: Caching a sniffer Fernando Gont (Mar 24)
- RE: Caching a sniffer Chris Merkel (Mar 11)
- RE: Caching a sniffer Shawn Jackson (Mar 23)
- RE: Caching a sniffer David Gillett (Mar 24)
- Re: Caching a sniffer Patrick Toomey (Mar 24)
- RE: Caching a sniffer Shawn Jackson (Mar 24)
- RE: Caching a sniffer Burton M. Strauss III (Mar 25)
- RE: Caching a sniffer Fernando Gont (Mar 25)
- RE: Caching a sniffer Shawn Jackson (Mar 24)
- RE: Caching a sniffer David Gillett (Mar 24)
- RE: Caching a sniffer Fernando Gont (Mar 25)
- RE: Caching a sniffer David Gillett (Mar 25)
- RE: Caching a sniffer David Gillett (Mar 24)
- RE: Caching a sniffer Fernando Gont (Mar 25)
- 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 David Gillett (Mar 25)
- RE: Caching a sniffer Shawn Jackson (Mar 25)
- RE: Caching a sniffer Andrew Shore (Mar 25)
- RE: Caching a sniffer Paul Blackstone (Mar 25)