Negative Caching TTL
Kevin Darcy
kcd at daimlerchrysler.com
Tue Dec 16 22:28:34 UTC 2003
Jim McAtee wrote:
>Can someone explain to me how to best use negative caching TTL. It's not
>clear to me if setting this value in the SOA record affects how our BIND
>9.2.3 servers answer queries, or if the value is open to interpretation by
>the receiving DNS client.
>
>$TTL 1d
>@ IN SOA ns1.modyssey.net. admin.modyssey.net. (
> 2003101701 ; serial
> 4h ; refresh
> 30m ; retry
> 14d ; expire
> 15m ) ; negative ttl
>
>With the above, will older BIND servers see the default TTL for records as 1
>day or 15 minutes?
>
The *positive* caching TTL (what's in the $TTL directive if not
explicitly overridden on a record-by-record basis) determines how long a
caching nameserver will remember the value(s) of a particular RRset (set
of records) that it received from an authoritative server or a forwarder.
The *negative* caching TTL (the value of the last field of the SOA RR)
determines how long a caching nameserver will remember that a particular
RRset *does*not*exist*, when told by an authoritative server or a
forwarder. Note that there are 2 different variations of negative
caching: NXDOMAIN = name doesn't own any records at all, NODATA (a
pseudo-response-code) = name owns records, but not of the type requested
(see RFC 2308 for more details).
To put it more simply, the positive caching TTL governs the persistence
of records that *do* exist in your zone; the negative caching TTL
governs the persistence of negative responses, i.e. the persistence of
record sets that could but *don't* exist in your zone, so to speak.
- Kevin
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