Back in 1997, that would have been one correct SOA record interpretation of several. :) Things were a little more ambiguous back then.
RFC 2308 reclassified the last field of the SOA record as the negative caching interval, otherwise known as the
NCACHE field. RFC2308 §4 is the most applicable here. It not only redefines this as the
NCACHE field, but also explains why coding the default TTL into the SOA record would have been misguided to begin with. (emphasis for the latter is in bold)
4 - SOA Minimum Field
The SOA minimum field has been overloaded in the past to have three
different meanings, the minimum TTL value of all RRs in a zone, the
default TTL of RRs which did not contain a TTL value and the TTL of
Despite being the original defined meaning, the first of these, the
minimum TTL value of all RRs in a zone, has never in practice been
used and is hereby deprecated.
The second, the default TTL of RRs which contain no explicit TTL in
the master zone file, is relevant only at the primary server. After
a zone transfer all RRs have explicit TTLs and it is impossible to
determine whether the TTL for a record was explicitly set or derived
from the default after a zone transfer. Where a server does not
require RRs to include the TTL value explicitly, it should provide a
mechanism, not being the value of the MINIMUM field of the SOA
record, from which the missing TTL values are obtained. How this is
done is implementation dependent.
The Master File format [RFC 1035 Section 5] is extended to include
the following directive:
$TTL <TTL> [comment]
All resource records appearing after the directive, and which do not
explicitly include a TTL value, have their TTL set to the TTL given
in the $TTL directive. SIG records without a explicit TTL get their
TTL from the "original TTL" of the SIG record [RFC 2065 Section 4.5].
The remaining of the current meanings, of being the TTL to be used
for negative responses, is the new defined meaning of the SOA minimum
Breaking this down:
- The last field of the SOA record (
NCACHE) specifies how long remote nameservers should cache a negative response if a query results in NXDOMAIN.
- The default TTL is defined either by a default in the software configuration, or something equivalent to the
$TTL directive for text based zone files.
- As a consequence of this implementation, there is no way to query a remote nameserver via the DNS protocol to determine the default TTL for a zone.
$TTL and family are not records, therefore you cannot query for them and they will not survive a zone transfer. (instead, all missing TTLs will be replaced with this value at the time of zone transfer)
As Andy says, the default TTL is moot if the individual records specify their own TTLs.