How Does DNS Cache Work?
In order to save time and reduce traffic on the Internet, the concept of DNS cache was developed. This system permits your Internet Service Provider (ISP) to cache DNS resolution requests for a set timeframe on its DNS server. The cached or stored information is referred to as the DNS cache. The length of time that a DNS loopup may be stored in DNS cached is defined as the TTL (time to live) and is determined by the authoritative server which has control over the DNS resolution for the domain name. If the TTL value for the cached information of the web page has expired, the caching server will delete the current entry and request new information from the Authoritative DNS server. Although this will result in detectable delay by the end-user for unique website lookups or lookups on expired DNS entries, subsequent queries for pages on the same URL will then be faster.
DNS Cache Issues
Although DNS caching increases the speed for the domain name resolution process, if a major change has been made for a domain name it can take up to a day or more for the new changes to be reflected world-wide. For example, if you register a new website at a domain registrar, the domain name may not resolve to the IP address of the web host for up to a day. The same issue can arise when transferring a domain name to a new Internet host. In recent years, Operating System manufacturers have also implemented DNS client caching which stores the resolution for websites that the user has surfed on recently in order to further speed up the name resolution process. If a major change has occurred to a web site’s name or IP address since the last visit, flushing the DNS cache on the client computer is required to speed up the resolution process.
DNS Poisoning
DNS poisoning is another term used to refer to a computer security vulnerability exploited over the past decade, where the DNS resolution for either a client machine or DNS host will be attacked. The result of the exploit is that a malicious website is entered as the resolved website for valid sites the user normally visits such as CNN, Google, and ESPN. When the user enters the URL or selects a bookmark in the web browser, they will be redirected to a malicious website.
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