Re: MX and TXT

Geoff and I discussed this thread a bit offlist.  I believe we both mostly 
agree with the following.
1. Name servers normally send additional records other than the ones 
requested, for example, NS requests get back NS +A, MX requests get back MX 
+A.  These are called "Additional records" and the section of the reply is 
called "Additional Records section" or AR section.  (I don't know if these 
AR responses are correctly called "glue" or if glue refers only to A 
records that extend into a subdomain defined with NS... I have heard "glue" 
used in both contexts).
2. It's possible to modify the servers to send back TXT additional records 
in response to an MX query, but modifying all servers out there to do this 
is hard... the additional records behavior is not part of the data or 
config, but hard-coded into the server for servers we know of.  The benefit 
of adding TXT responses piggy-backed on an MX query is debateable, since MX 
records are looked up when sending and SPF data is needed when receiving.
3. A better idea (and this has probably already been thought of, so we're 
not claiming it's original) is to send back MX and A in response to an SPF 
query.  That saves another lookup for folks using the MX and A methods. 
However, TXT is already hardcoded into various servers and changing the 
reply behavior.
4. Whenever an SPF data type becomes available, that would be a great time 
to set the AR behavior as well, so that while the server is being patched 
to support SPF queries it can send the appropriate additionals.  Probably 
the easiest thing to do would be to always return MX and A records for the 
domain, and A records for those MX, but it's also possible for the DNS 
server to look at the SPF record and see if MX or A methods are used. As 
with any Additional Records in response, if there isn't enough room for 
them in the packet they can be dropped and the client can do a second 
lookup.
--
Greg Connor <gconnor(_at_)nekodojo(_dot_)org>

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