negative caching time and TTLs

Chris Thompson cet1 at
Tue Apr 14 19:21:36 UTC 2009

On Apr 14 2009, Lena M wrote:

>Which TTL value is supposed to be used for negative caching time?
>-We are running BIND 9.X

That's an awful lot of versions of BIND. Can't you be more specific?

>                         as a caching server. We are seeing that NXDOMAIN
>replies are being cached using $TTL time of a given zone instead of its
>SOA in TTL time.

Post some evidence? AFAIK, even very early versions of BIND 9 had the
RFC2308 negative caching algorithm. Bear in mind the max-ncache-ttl 
setting which defaults to 3 hours.

>-Is $TTL suppose to override SOA's min TTL for the negative caching?
>RFC 2308 is not clear.

How could "$TTL override the SOA's min TTL"? The $TTL value doesn't exist
in the compiled zone, and of course $TTL could occur many times in the input,
with different arguments. Maybe you mean "the TTL value of the SOA record"
or something like that?

Chris Thompson
Email: cet1 at

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