With the DNS Caching the result of a successful recursive DNS dissolution a time is kept long in the local Cache, so that further identical DNS Requests can be answered immediately, without having to go through the recursion again. The time interval, which is valid GEC figure eight entry, becomes by its TTL value (English: Time to Live; dt.: Time to live) determines. The TTL value is specified by the Primary name server of the appropriate zone.

Exactly one TTL value is assigned to each DNS entry (resource record). This covers 32 bits and can therefore very large values (several decades) take. The unit is second. A TTL of 3600 corresponds thereby one hour. "“0"” means: This resource records GEC eight may not become.

Negative Caching

Many DNS servers control also negatives the Caching (RFC 2308). It concerns here a obligating function, with which also the fact is noted in the Cache that a name does not admit is. With repeated inquiries can be answered then similarly to the standard Caching immediately.

The question arises naturally, the time interval is how large, in which a negative entry in the Cache is For a missing name no TTL finally exists.

Two cases are to be differentiated: To dissolve in the first case tries an unknown name from a well-known zone. In this case the SOA resource record is provided with the negative answer. This contains the TTL valid for this zone and specifies thereby the Caching duration.

In the second case one tries to dissolve a name from an unknown zone. Since the zone does not admit is, there is also no SOA-RR and thus no zone-specific TTL. The name server uses instead a konfigurierbareren default value (with BIND 9 for these over the option max ncache ttl defined).

Articles in category "DNS Caching"

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