Mark Delany wrote on 2020-03-14 01:00:
...
First off, why on earth does TXT even have substrings? What value do
they add and which applications are taking advantage of such value?
Was there some external limitation such as zone-file parsing that
introduced substrings in the first place? I mean, why do substrings
even exist?
the <character-string> construct was needed for HINFO, so reused for TXT.
Second off, why an 8-bit length field? Why not a 16-bit or even a
32bit length field and eliminate substrings? Alternatively why not
some sort of end-of-RR sentinal or implicit length as occurs with all
other RRs? Are we all floundering around TXT processing due to a 1980s
premature optimization around a byte or two of payload?
yes. this code had to run on a PDP-11.
The biggest problem with an 8-bit length is that you have no clue
whether the input string was split due to length limitations or
semantic requirements. This is where a lot of the hand-wringing seems
to come from.
Methinks the safest outcome is that TXT substrings should be treated
as an artifact of the limitations of TXT RRs or a side-effect of
serialization and should have no semantic value whatsoever.
that's differentially reasonable. in the oldest zone file parser known,
the following were equivalent on the wire (two <character-string>'s):
foo IN TXT bar baz
foo IN TXT ( bar baz )
foo IN TXT ( bar
baz )
whereas this would be a single <character-string>:
foo IN TXT "bar baz"
in my TXT-consuming work i have always treated each <character-string>
as a word. in MIT HESIOD only one <character-string> was produced for
each /etc/group or /etc/passwd entry thus encoded, so no argument arose.
Maybe a one-sentence RFC is in order?
if it would take that little space, there would be need for it. BIND 4
got this wrong (concatenating the zone file input into as few
<character-string>'s as could accept the "rdata"). microsoft inherited
this in NT3 and then copied it thereafter. djbdns copied it but used
non-maximum-sized segments. BIND 4.9, 8 and 9 went back to 1035 style.
as far as i know knot, nsd, and powerdns do what BIND has (since 1993) done.
DKIM is being liberal in what it accepts. SPF is not.
--
P Vixie
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