Cisco CCNP Certification / BCMSN Exam: Defending Against VLAN Hopping Attacks
- Date: 2007-05-23 - Word Count: 326
Share This!
During our Cisco CCNP BCMSN certification exam preparation, we've seen how intruders can use seemingly innocent ARP and DHCP processes can be used to harm our network, so it shouldn't come as any surprise that Dot1q tagging can be used against us as well!
One form of VLAN Hopping is double tagging, so named because the intruder will transmit frames that are "double tagged" with two separate VLAN IDs. As you'll see in our example, certain circumstances must exist for a double tagging attack to be successful:
The intruder's host device must be attached to an access port.
The VLAN used by that access port must be the native VLAN.
The term "native VLAN" tips us off to the third requirement - dot1q must be the trunking protocol in use, since ISL doesn't use the native VLAN.
When the rogue host transmits a frame, that frame will have two tags. One will indicate native VLAN membership, and the second will be the number of the VLAN under attack. In this example, we'll assume that to be VLAN 100, with the native VLAN set as VLAN 25.
The trunk receiving this double-tagged frame will see the tag for VLAN 25, and since that's the native VLAN, that tag will be removed and then transmitted across the trunk - but the tag for VLAN 100 is still there!
When the switch on the other side of the trunk gets that frame, it sees the tag for VLAN 100 and forwards the frame to ports in that VLAN. The rogue now has successfully fooled the switches and has hopped from one VLAN to another.
This is why you often see the native VLAN of a network set to a VLAN that no host on the network is a member of - that stops this version of VLAN Hopping right in its tracks.
Notice that I said "this version". We'll take a look at another VLAN Hopping tactic in the next installation of my CIsco CCNP BCMSN certification exam tutorial series!
One form of VLAN Hopping is double tagging, so named because the intruder will transmit frames that are "double tagged" with two separate VLAN IDs. As you'll see in our example, certain circumstances must exist for a double tagging attack to be successful:
The intruder's host device must be attached to an access port.
The VLAN used by that access port must be the native VLAN.
The term "native VLAN" tips us off to the third requirement - dot1q must be the trunking protocol in use, since ISL doesn't use the native VLAN.
When the rogue host transmits a frame, that frame will have two tags. One will indicate native VLAN membership, and the second will be the number of the VLAN under attack. In this example, we'll assume that to be VLAN 100, with the native VLAN set as VLAN 25.
The trunk receiving this double-tagged frame will see the tag for VLAN 25, and since that's the native VLAN, that tag will be removed and then transmitted across the trunk - but the tag for VLAN 100 is still there!
When the switch on the other side of the trunk gets that frame, it sees the tag for VLAN 100 and forwards the frame to ports in that VLAN. The rogue now has successfully fooled the switches and has hopped from one VLAN to another.
This is why you often see the native VLAN of a network set to a VLAN that no host on the network is a member of - that stops this version of VLAN Hopping right in its tracks.
Notice that I said "this version". We'll take a look at another VLAN Hopping tactic in the next installation of my CIsco CCNP BCMSN certification exam tutorial series!
Related Tags: certification, host, exam, cisco, ccnp, arp, native, double, rogue, tagging, dhcp, bcmsn, vlan, hopping, isl, dot1q
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of over 100 free certification exam tutorials, including Cisco CCNA certification test prep articles. His exclusive Cisco CCNA study guide and Cisco CCNA training is also available!Visit his blog and sign up for Cisco Certification Central, a daily newsletter packed with CCNA, Network+, Security+, A+, and CCNP certification exam practice questions! A free 7-part course, "How To Pass The CCNA", is also available, and you can attend an in-person or online CCNA boot camp with The Bryant Advantage! Your Article Search Directory : Find in Articles
Recent articles in this category:
- What Is An Sd Card? What Are Main Types Of Sd Cards
Manufactured as a competition offering to Sony's Memory Stick, the SD Card was developed by the SD C - Industrial Computers
In this day and age you can't afford to run an industry efficiently without a computer as it plays a - Computer Fix Services - Tips To Choose Good Quality Personal Computer Maintenance Online
Online computer repair services have become a fast and reliable problem solution for day to day tech - Reverse Engineering Romer For Aerospace
One of the most amazing things about reverse engineering is its significant contribution in any fiel - How To Buy The Best Projector Bulb
It is very important for you to know the right time to change the projector bulb before it will burn - Laptops Vs Desktops- Their Significance
Computers obviously are commonly used for more or less everything from entertainment to office work. - Cooling Your Laptop
It is common knowledge that laptops are commonly used by everyone but one must also know how to take - Dymo Labelwriter Fast Performance & Reliability
I don't think I'm the only one who has gotten tired of trips to the post office, standing in endless - Buying Dell Laptops A Long Time Investment
Today if you go to an office or a school, Dell laptops can be seen very commonly. Though we do not b - Hard Drives - Digital Space
The first hard drives (Hard Disk Drives, HDD) were developed by IBM in 1956. Since then the need for
Most viewed articles in this category:
- Video Killed the Beautiful View
"The sound brought our group to a stop; we turned around to see the ice mass collapse with a roar. A - Crossfire: The Dual GPU System Gamers Want
CrossFire is the new dual graphics card system from ATI. It offers remarkable image quality improvem - Canon PIXMA IP1600 Photo Printer Review & Its True Cost of Ownership
With Canon PIXMA iP1600 Inkjet Printer you can make high quality photo printings at home. No need to - Basic Information about Nero Burn DVD
Nero is a popular burning ROM designed to burn DVD's. The way this product got its name is very inte - The Development of the Vital IBM PC in Spite of the Corporate Culture of IBM
The public history of the PC began in August 1981, when IBM first announced "The IBM Personal Comput - PC Repairs
PC Repairs are often needed when we need our computer the most. One way to take away many problems o - Affordable Computer Printer Ink
The computer - once considered a luxury item - has easily become a staple in today's modern homes; a - Build your Own PC
Build your own PC! Simple and Fun. Just follow these instructions and you're on your way to a rel - Extending the Life of an Inkjet Printer with Proper Cleaning and Maintenance
Inkjet printers are becoming another "throw away" electronic device due to reducing costs and high i - Check Printers
A check printer is a printer that hooks up to a computer that has the capability of printing checks.