BIND 9 and TTLs

Kevin Darcy kcd at daimlerchrysler.com
Wed Sep 20 22:53:10 UTC 2000


The infamous "Brian" wrote:

> Thanks John:
>
> John Horne wrote in message ...
> >I only run a local (on my pc) caching server, but with BIND 9
> >and redhat 6.1 linux, this worked okay. I changed the localhost
>
> >A record to a '1D' TTL with no problems.
>
> >From what I've read that's been a pretty standard practice for years
> although sometimes the logic of it all escapes me.
>
> Why now require yet another TTL directive when one exists in the SOA? Is it
> not true that only one TTL directive is true for the zone?

You still haven't read RFC 2308, apparently. The last field of the SOA is
*not* the default or minimum TTL to use for all records in the zone, the
"TTL directive", as you call it: it's been redefined to mean the negative
caching TTL, which only applies to the *non-existence* of RR's, not to actual
RR's (Jean-Paul Sartre, please forgive me!). So in the absence of a $TTL
directive or an explicit TTL on the first RR in the zonefile, how is
"named" to know what TTL that record should have? There is no redundancy here.
"named" _needs_ this information. Would you prefer that it just pick a
TTL value at random?

> Is it not true
> that the individual records are not subject to updates, that the whole zone
> is updated?

A zone is basically a collection of records. When an individual record is
updated, then yes, in one sense the whole zone has been updated, just as a
change to a grain of sand is a change to the whole beach, or a change to a
drop of water is a change to the whole ocean in which it is contained. But I
fail to see the relevance of this question to TTLs. Each record has its own
TTL value. The $TTL directive, or, failing that, the explicit TTL on the first
record on the zone, establishes a *default* TTL value to use for RR's. But you
can always override that default for a given RR in the zone.


- Kevin

P.S. I would recommend subscribing to the bind9-users list. A lot of what we
are discussing here is specific to BIND 9, and we risk confusing the heck out
of the BIND 8 folks, where the rules about default TTL's are a little
different.




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