Security Basics mailing list archives
Re: VLAN Question
From: Bennett Todd <bet () rahul net>
Date: Wed, 20 Aug 2003 12:51:20 -0400
2003-08-20T03:09:24 Steven Williams:
I'm after some opinions of yours and your companies policy regarding the use of VLAN's as a method of isolating the internet to internal VLAN's on the same physical layer 2 / 3 switch and access controlled by ACL's or firewalls.
There are several sides to this question. Originally, VLANs were created solely to help mitigate the very high cost of early switches. Switches were being sold in multiples of 16 or 32 ports, and they were vastly more expensive than hubs. To help people get the most out of their switch investments, VLANs allowed partitioning broadcast domains, to buy the performance advantages of switch isolation while allowing multiple smaller networks to be implemented on the same expensive switch. In this context, leakage between vlans wasn't an issue as long as the amount of leakage didn't cause a performance impact. vlans leaked. Minor leakage was not considered a problem by the vendors. They weren't designed as security partitions. Customers started pressing vendors, and they've responded. I've spoken with a Cisco engineer who said that properly, carefully configured, current switches with current CatOS were not believed to leak between vlans, and a finding that they could so leak would be treated as a priority security bug. Cool says I, this enables something I've wanted to have for some time. Combine switches with vlans that are secure and 802.1q trunking, and you can have a firewall with a ludicrous number of firewall ports --- it becomes practical to consider building a fully-firewalled fully-routed network, where every host has its own dedicated firewall port. Not for everybody, perhaps, but I can think of places where it'd be worth doing. Say, hotels offering network jacks in the rooms. But there's another issue to consider. Even if the vlan implementation is truly secure in the switch, sharing multiple vlans representing different security domains on the same switch means that a config error on that switch could compromise your isolation. Config errors happen. Config errors that don't overtly break anything are often not detected for a long time. Switches are cheap. Use multiple switches unless there's a really compelling engineering requirement to use multiple vlans on the same switch. -Bennett
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Current thread:
- VLAN Question Steven Williams (Aug 20)
- RE: VLAN Question David Gillett (Aug 20)
- Re: VLAN Question Bennett Todd (Aug 20)
- RE: VLAN Question David Gillett (Aug 21)
- Re: VLAN Question Bennett Todd (Aug 21)
- RE: VLAN Question David Gillett (Aug 21)
- RE: VLAN Question David Gillett (Aug 21)
- <Possible follow-ups>
- RE: VLAN Question Meidinger Chris (Aug 22)
- RE: VLAN Question David Gillett (Aug 25)