Local DNS Caching

Local DNS Caching
Once the local DNS receives the reply, it will cache that information for a specified time, known as time to live (TTL). TTL is specified by the authoritative DNS as part of its reply. That means, the local DNS will simply reply to all subsequent requests with the information it has from the earlier DNS reply until the TTL expires. Once the TTL expires, the next request to the local DNS will trigger a request to the authoritative DNS again. Caching helps ensure faster response time for the same name to address resolution queries from subsequent clients. At the same time, TTL helps ensure that the local DNS captures any updates or changes from the authoritative DNS. Changing the TTL to a lower value causes the local DNS to query the authoritative DNS more often. Changing the TTL to a higher value puts the local DNS at the risk of having
stale information for increased durations.
If the local DNS receives multiple IP addresses as part of the DNS reply, it may give one IP address to each of its clients in a round−robin manner. In addition to the local DNS caching the DNS responses, the client browser also caches the DNS response. Unfortunately, popular client browsers currently ignore the TTL set by the authoritative DNS. Versions 3.x of Microsoft Internet Explorer, for example, cache the DNS response for 24 hours. Unless the browser
application is terminated and restarted, it does not query the DNS again for 24 hours for a given domain.
Versions 4.x and later cache the DNS response for 30 minutes. Microsoft provides a note on the support section of its Web site on how to change the cache time−out value for Internet Explorer by modifying certain entries in the registry. (Search for keywords ie cache dns timeout in the support section of Microsoft’s Web site.)

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