DNS Poisoning

DNS poisoning is an attack that’s becoming more widespread and it can affect most people using broadband.

Here’s a summary on what it is, how it can affect you, and what you can do about it.

Every time you use anything on the internet, including reading web pages, reading or sending emails, online chatting, etc, you use domain names (even if you don’t realise you’re using them). Domain names could be www.google.com, or www.fraudo.com, etc. They’re just addresses on the internet.

Your internet service provider (ISP) would have a machine in their network that your computer uses to look up these addresses. You won’t realise you’re using it but your computer definitely needs it (and it’s called a DNS server).

A DNS server is a phone book of web addresses.

Here’s how things work on a healthy environment (click to enlarge):

click to enlarge

You try to load up www.fraudo.com

  1. Your computer finds the DNS Server and asks it "where’s www.fraudo.com?"
  2. The DNS Server responds ("there it is –>")
  3. Your computer finds FraudO on the internet

All’s good and everyone’s happy browsing the internet. Along comes someone trying to hack your system. They make a change to your modem/router, telling it to use someone else’s DNS server.

How do they do this? The most common method today are viruses that break into routers and change settings. We’ll cover these another time.

Here’s how an environment looks when it’s been DNS poisoned (click to enlarge):

click to enlarge

Instead of using your ISP’s DNS server, it’s using a bad DNS server. The bad DNS server tells your computer how to find the evil websites instead of the real ones.

If this happens chances are you wouldn’t know how or why, it can be difficult to see what’s happening.

To prevent things like this happening here are some tips:

  • Change the password on your router. Everyone knows the default password (here’s a list of all the default passwords, find yours in the list)
  • Use a good virus scanner that scans all web pages, emails, and files
  • Keep your virus scanner up to date
  1. One way to try to minimise DNS attacks is to use a secure service such as OpenDNS, http://www.opendns.org. They also allow you to do simple content filtering, for free.

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