valore MINIMUM (TTL) nel SOA

Piero Serini p.serini a ESI.IT
Mer 8 Nov 2000 20:48:14 CET


GiÓ che siamo in tema di DNS, vorrei chiedere come mai il DNS check della RA
continua a verificare che il valore di MINIMUM nel SOA di nomi a dominio
sotto il TLD .IT sia almeno 86400, scambiandolo per il TTL.

Questo controllo dovrebbe essere modificato in quanto il MINIMUM ha
cambiato significato. Quello che la RA vuole verificare Ŕ in realtÓ il
valore
della nuova direttiva $TTL.

RFC 2308 (Marzo 1998) definisce $TTL e MINIMUM come:

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
   negative responses.

   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]







Andrews                     Standards Track                     [Page 8]

RFC 2308                       DNS NCACHE                     March 1998


   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
   field.

Per quanto riguarda il valore, sono assolutamente d'accordo sul fatto
che un buon valore per $TTL sia 86400. Ma ritengo anche che alla
luce del significato del MINIMUM il suo valore debba essere non
superiore all'ora, anche se l'RFC consente valori sino alle tre ore fra
i valori "reasonable".

dal momento poi che il negative caching Ŕ ora obbligatorio:

8 - Changes from RFC 1034

   Negative caching in resolvers is no-longer optional, if a resolver
   caches anything it must also cache negative answers.

(ricordo il significato di "must").

Lo stato di questo RFC Ŕ:

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

e pertanto ritengo si dovrebbe ormai procedere ad aggiornare i
controlli effettuati sui DNS in occasione delle registrazioni.

Piero Serini



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