On Sep 14, 2006, at 6:45 AM, Peter Koch wrote:
Tony Finch:
The "no service here" meaning of this kind of "." is explicitly
specified
for SRV records.
Yes, RFC 2782 says
Target
The domain name of the target host. There MUST be one or more
address records for this name, the name MUST NOT be an
alias (in
the sense of RFC 1034 or RFC 2181). Implementors are
urged, but
not required, to return the address record(s) in the
Additional
Data section. Unless and until permitted by future standards
action, name compression is not to be used for this field.
A Target of "." means that the service is decidedly not
available at this domain.
Again, it's in the original spec (here and RFC 2052), "." additional
processing is to be avoided (although these paragraphs have to be
polished
for 2782bis) and SRV predates RFC 2606.
This text defining a null target does not prevent misuse of the ".",
even when clearly defined initially in the RR type.
Joe Abley:
I've seen many people use "." as part of MX RDATA to indicate that a
host should not receive mail (e.g. "PRINTER.EXAMPLE.COM. 3600 IN MX
0 ."). I seem to remember that meaning is documented informally in
Cricket Liu's book, but I could be wrong about that (I don't have a
There was an I-D promoting this, as well. The problem here, as with
RNAME,
is that it retroactively applies a special meaning to the "." domain.
For MX RRs thats bad due to the additional section processing, which
should not occur for the SOA RR as per RFC 1035 (although it is done
for MNAMEs)
While perhaps not utilized at a high rate, there are many instances
where a null target is desired. Using root "." as being analogous to
"null" has produced undesired traffic elsewhere, specifically in the
case of SRV records where text defining this name as null should have
prevented the root traffic. As this draft is attempting to
eliminate extraneous traffic, it seems also avoiding this construct
here and elsewhere in the future could set precedent. Can there be a
name established for this construct? Perhaps something that will not
be accepted in a DNS query.
-Doug
.
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