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Concise Guide to DNS and BIND, The by Nicolai Langfeldt

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Caching and TTLs

The $TTL zone directive and the TTL of the SOA record are also important parameters. Besides old zone data on slave servers, a source of old data is caches. This can't be avoided. The caches are out of your reach; you can't troubleshoot them. All you can do is set the TTLs correctly. The $TTL directive sets the default TTL for the zone. If your BIND version does not support the $TTL directive, the default TTL is taken from the minimum TTL field of the SOA. In modern caches, the SOA minimum TTL is used as the TTL for negative caching. Negative caching is described in RFC 2308. If a name is looked up and not found, the fact that it does not exist is cached for the time set by the minimum TTL. Older versions of BIND and DNS software ...

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