Windows XP / Networking

Domain Name Service

Of the many other protocols inherent in the Internet, most are subject to attack or subversion. The Domain Name Service (DNS) has its own vulnerabilities.

DNS is used to resolve a friendly name, such as, to an IP address, such as 192.168,15.10. DNS is needed because while the Internet runs with IP addresses, people tend to think in words. The DNS service keeps a distributed directory handy, which allows you, the user, to type a uniform resource locator (URL) or Internet address, something that in most cases is fairly straightforward and easy to remember, into the address block on your web browser, and the computer will sweat the numbers.

DNS is not usually the first step in address resolution. To save time and prevent wasted bandwidth, a table of address and their URLs is usually stored or cached on the local machine. Your computer starts at this table when you make a web request, looking to see if it already has the IP address of the site you desire. When your local machine cannot find where to send a web request, it contacts the nearest DNS server, which tells the computer every thing it knows about the desired IP address. If if the address is unknown at the DNS server, that DNS server consults the next DNS server up the chain, until your address is found or you hit the top, come up empty, and are sent an error message.

This suggests three very convenient DNS attacks. First, if you seed the local machine's cache with incorrect data, it sends the user's communications to the wrong place, including possibly a decoy site of the attacker's own design. Second, if you pollute the database of one of the nation's big DNS servers, you may shut down a major portion of the Internet, which is always good for achieving status in the cracker underworld. Fortunately, the distributed nature of the DNS system makes this a little far-fetched because backup systems will likely kick in. Third, if you deny access to the DNS server that provides address resolution to a population of users, say the LAN that serves your company, users of that LAN are not going to be able to contact web sites for which they do not already have IP addresses. Attackers do this in at least two ways: take out the server with some kind of attack or change the place that your desktop computer looks for DNS resolution. It may be easier to force a DNS error by changing the place computers look for DNS by modifying the local information in cache than it would be to take down the server.

Poisoning the DNS system does not only slow down or prevent the access of web pages and services. Mail may not work, remote filesystems may be rendered inaccessible, and network printing may go down. Essentially everything that involves an external communication is at risk when DNS fails.

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