is $TTL and the soa minimum ttl the same thing.

[Alexander Ottl]

Mark Mentovai wrote:
> 
> proside at home.com wrote:
> >i currently set my $TTL and the minimum ttl value to the same thing.
> >i am wondering if they ARE the same thing (if so why bother..)
> >
> >one thing im thinking is that the $TTL is like a max ttl and the soa ttl
> >is the min ttl.
> 
> They're not the same thing.
> 
> $TTL is a master file directive which defines the TTL of all RRs in the
> zone which haven't been supplied with an explicit TTL.  As a master file
> directive, it is not made directly available through the DNS, it is only
> used to affect the interpretation of the RRs in the master file.
> 
> The SOA "minimum TTL" field is currently interpreted as the negative
> cache TTL for the zone, that is, the time that a negative response to a
> DNS query should be cached.  The SOA record is available through the DNS.
> 
> The name "minimum TTL" comes from the original intended use for the
> field, the minimum of all TTL values in the zone.  In practice, this
> field was never put to use in this way.  Many nameservers treated it as
> the default TTL for the zone (much like the $TTL directive.)  RFC 2308
> defined the current behavior.
> 
> There's no such thing as a "maximum TTL," and like "minimum TTL," there
> would not be much use for it.
> 
> Mark

But there is a maximum TTL in the context of BINDs cache configuration.
To quote the docu:

       max-ncache-ttl
         To reduce network traffic and increase performance the server
store
         negative answers.  max-ncache-ttl is used to set a maximum
retention
         time for these answers in the server is seconds.  The default
max-
         ncache-ttl is 10800 seconds (3 hours).  max-ncache-ttl cannot
exceed
         the maximum retention time for ordinary (positive) answers (7
days)
         and will be silently truncated to 7 days if set to a value
which is
         greater that 7 days.

-- 
Alexander Ottl
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