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Diffstat (limited to 'doc/rdata-class.rst')
| -rw-r--r-- | doc/rdata-class.rst | 30 |
1 files changed, 28 insertions, 2 deletions
diff --git a/doc/rdata-class.rst b/doc/rdata-class.rst index 723c706..a7308fa 100644 --- a/doc/rdata-class.rst +++ b/doc/rdata-class.rst @@ -1,7 +1,8 @@ .. _rdata-class: -DNS Rdata Base Class -==================== +====================== + DNS Rdata Base Class +====================== All Rdata objects are instances of some subclass of ``dns.rdata.Rdata``, and are immutable. The Rdata factory functions @@ -11,5 +12,30 @@ an instance of the ``dns.rdtypes.IN.AAAA`` class, but a record of TYPE12345, which we don't know anything specific about, will be an instance of ``dns.rdata.GenericRdata``. +Rdata of the same type and class are ordered. For rdata that do not +contain domain names, or which contain absolute domain names, the +order is the same as the DNSSEC ordering. For rdata containing at +least one relative name, that rdata will sort before any rdata with an +absolute name. This makes comparison well defined (compared to +earlier versions of dnspython), but is a stop-gap measure for backwards +compatibility. We want to disallow this type of comparison because it easily +leads to bugs. Consider this rdataset:: + + $ORIGIN example. + name 300 IN NS a ; 1 + NS a. ; 2 + +In this case the record marked "2" sorts before the one marked "1" +when all the names are made absolute and the DNSSEC ordering is used. +But when relative comparisons are allowed, "1" sorts before "2". This +isn't merely cosmetic, as code making a DNSSEC signature or computing +a zone checksum would get different answers for the same content +if it failed to make all names absolute before sorting. + +Comparing relative rdata with absolute is thus deprecated and will be +removed in a future version of dnspython. Setting +``dns.rdata._allow_relative_comparisons`` to ``True`` will allow the +future behavior to be tested with existing code. + .. autoclass:: dns.rdata.Rdata :members: |