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What Is a DNS Cache?

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Question: What Is a DNS Cache?
Answer: A DNS cache contains entries that translate Internet domain names *such as "compnetworking.about.com") to IP addresses. The Internet's Domain Name System (DNS) involves caching on both Internet DNS servers and on the client computers that contact DNS servers. These caches provide an efficient way for DNS to efficiently keep the Internet synchronized as the IP addresses of some servers change and as new servers come online.

DNS Cache Poisoning

A DNS cache becomes poisoned (sometimes also called polluted) when unauthorized domain names or IP addresses are inserted into it. Occasionally a cache may become corrupted due to technical glitches or administrative accidents, but DNS cache poisoning is typically associated with computer viruses or other attacks that insert invalid entries which redirect clients to malicious Web sites or other Internet servers.

Flushing a DNS Cache

When troubleshooting cache poisoning or other Internet connectivity issues, a computer administrator may wish to flush (meaning clear, reset, or erase) a DNS cache. In Microsoft Windows, flush a DNS cache using the ipconfig tool as follows:
ipconfig /flushdns
Other operating systems also provide command line options to flush a DNS cache:
dscacheutil -flushcache (on Mac OS X 10.5 and newer)
/etc/rc.d/init.d/nscd restart (on Linux)
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