Some of the principles which guided the design of this UPDATE specification are as follows. Note that these are not part of the formal specification and any disagreement between this section and any other section of this document should be resolved in favour of the other section.
7.1. Using metavalues for CLASS is possible only because all RRs in the packet are assumed to be in the same zone, and CLASS is an attribute of a zone rather than of an RRset. (It is for this reason that the Zone Section is not optional.)
7.2. Since there are no data-present or data-absent errors possible from processing the Update Section, any necessary data-present and data- absent dependencies should be specified in the Prerequisite Section.
7.3. The Additional Data Section can be used to supply a server with out of zone glue that will be needed in referrals. For example, if adding a new NS RR to HOME.VIX.COM specifying a nameserver called NS.AU.OZ, the A RR for NS.AU.OZ can be included in the Additional Data Section. Servers can use this information or ignore it, at the discretion of the implementor. We discourage caching this information for use in subsequent DNS responses.
7.4. The Additional Data Section might be used if some of the RRs later needed for Secure DNS Update are not actually zone updates, but rather ancillary keys or signatures not intended to be stored in the zone (as an update would be), yet necessary for validating the update operation.
7.5. It is expected that in the absence of Secure DNS Update, a server will only accept updates if they come from a source address that has been statically configured in the server's description of a primary master zone. DHCP servers would be likely candidates for inclusion in this statically configured list.
7.6. It is not possible to create a zone using this protocol, since there is no provision for a slave server to be told who its master servers are. It is expected that this protocol will be extended in the future to cover this case. Therefore, at this time, the addition of SOA RRs is unsupported. For similar reasons, deletion of SOA RRs is also unsupported.
7.7. The prerequisite for specifying that a name own at least one RR differs semantically from QUERY, in that QUERY would return <NOERROR,ANCOUNT=0> rather than NXDOMAIN if queried for an RRset at this name, while UPDATE's prerequisite condition [Section 2.4.4] would NOT be satisfied.
7.8. It is possible for a UDP response to be lost in transit and for a request to be retried due to a timeout condition. In this case an UPDATE that was successful the first time it was received by the primary master might ultimately appear to have failed when the response to a duplicate request is finally received by the requestor. (This is because the original prerequisites may no longer be satisfied after the update has been applied.) For this reason, requestors who require an accurate response code must use TCP.
7.9. Because a requestor who requires an accurate response code will initiate their UPDATE transaction using TCP, a forwarder who receives a request via TCP must forward it using TCP.
7.10. Deferral of SOA SERIAL autoincrements is made possible so that serial numbers can be conserved and wraparound at 2**32 can be made an infrequent occurance. Visible (to DNS clients) SOA SERIALs need to differ if the zone differs. Note that the Authority Section SOA in a QUERY response is a form of visibility, for the purposes of this prerequisite.
7.11. A zone's SOA SERIAL should never be set to zero (0) due to interoperability problems with some older but widely installed implementations of DNS. When incrementing an SOA SERIAL, if the result of the increment is zero (0) (as will be true when wrapping around 2**32), it is necessary to increment it again or set it to one (1). See [RFC1982] for more detail on this subject.
7.12. Due to the TTL minimalization necessary when caching an RRset, it is recommended that all TTLs in an RRset be set to the same value. While the DNS Message Format permits variant TTLs to exist in the same RRset, and this variance can exist inside a zone, such variance will have counterintuitive results and its use is discouraged.
7.13. Zone cut management presents some obscure corner cases to the add and delete operations in the Update Section. It is possible to delete an NS RR as long as it is not the last NS RR at the root of a zone. If deleting all RRs from a name, SOA and NS RRs at the root of a zone are unaffected. If deleting RRsets, it is not possible to delete either SOA or NS RRsets at the top of a zone. An attempt to add an SOA will be treated as a replace operation if an SOA already exists, or as a no-op if the SOA would be new.
7.14. No semantic checking is required in the primary master server when adding new RRs. Therefore a requestor can cause CNAME or NS or any other kind of RR to be added even if their target name does not exist or does not have the proper RRsets to make the original RR useful. Primary master servers that DO implement this kind of checking should take great care to avoid out-of-zone dependencies (whose veracity cannot be authoritatively checked) and should implement all such checking during the prescan phase.
7.15. Nonterminal or wildcard CNAMEs are not well specified by [RFC1035] and their use will probably lead to unpredictable results. Their use is discouraged.
7.16. Empty nonterminals (nodes with children but no RRs of their own) will cause <NOERROR,ANCOUNT=0> responses to be sent in response to a query of any type for that name. There is no provision for empty terminal nodes -- so if all RRs of a terminal node are deleted, the name is no longer in use, and queries of any type for that name will result in an NXDOMAIN response.
7.17. In a deep AXFR dependency graph, it has not historically been an error for slaves to depend mutually upon each other. This configuration has been used to enable a zone to flow from the primary master to all slaves even though not all slaves have continuous connectivity to the primary master. UPDATE's use of the AXFR dependency graph for forwarding prohibits this kind of dependency loop, since UPDATE forwarding has no loop detection analagous to the SOA SERIAL pretest used by AXFR.
7.18. Previously existing names which are occluded by a new zone cut are still considered part of the parent zone, for the purposes of zone transfers, even though queries for such names will be referred to the new subzone's servers. If a zone cut is removed, all parent zone names that were occluded by it will again become visible to queries. (This is a clarification of [RFC1034].)
7.19. If a server is authoritative for both a zone and its child, then queries for names at the zone cut between them will be answered authoritatively using only data from the child zone. (This is a clarification of [RFC1034].)
7.20. Update ordering using the SOA RR is problematic since there is no way to know which of a zone's NS RRs represents the primary master, and the zone slaves can be out of date if their SOA.REFRESH timers have not elapsed since the last time the zone was changed on the primary master. We recommend that a zone needing ordered updates use only servers which implement NOTIFY (see [RFC1996]) and IXFR (see [RFC1995]), and that a client receiving a prerequisite error while attempting an ordered update simply retry after a random delay period to allow the zone to settle.