Subject: TXT records, are they alive?

From: "Mohammad Awad" <[email-masked]>
Date: Sat, 12 Apr 2003 08:08:50 +0200

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Hi all,
I'm doing some research that I'll get it dependant on the TXT record in 
RFC1464 of 1993. I've noticed that there was no further drafts, RFCs, nor 
comments about that RR.
So:
1- Is the TXT record still considered a standard and is it really 
implemented in nowadays server?
2- if yes, do the servers really respond to queries that necessitate 
searching inside the RDATA cell of that TXT records?
Thank you all
Moh Awad

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---

From: Derek Atkins <[email-masked]>
Date: 12 Apr 2003 10:40:50 -0400

"Mohammad Awad" <[email-masked]> writes:

> Hi all,
> I'm doing some research that I'll get it dependant on the TXT record
> in RFC1464 of 1993. I've noticed that there was no further drafts,
> RFCs, nor comments about that RR.

Why should there be?  It works fine, and no problems have been found
requiring an update to the draft.

> implemented in nowadays server?

Uh, most definitely.  As far as I can tell just about every DNS Server
and resolver around nowadays supports TXT records.  Hesiod wouldn't
work without it.

> searching inside the RDATA cell of that TXT records?

No -- at least not that I've seen.  The servers only look at the name,
not the rdata, when looking up a response.  This is as it should be in
general.

> Moh Awad

-derek

-- 
       Derek Atkins
       Computer and Internet Security Consultant
       [email-masked]             www.ihtfp.com

---

From: Greg Hudson <[email-masked]>
Date: 12 Apr 2003 10:40:37 -0400

> implemented in nowadays server?

Yup.  They've worked fine in BIND for a while.  (On the server side,
there's been some issue about how much control a zone file has over the
boundaries between the text labels.  I don't know the status of that.)

> searching inside the RDATA cell of that TXT records?

I'm not quite sure what you're asking here.

At MIT, we use them for Hesiod records, which isn't necessarily the
cleanest use of the DNS, but it works well.  Dig for something like
ghudson.passwd.ns.athena.mit.edu txt for an example.

---

From: Peter Koch <[email-masked]>
Date: Sat, 12 Apr 2003 18:14:16 +0200

> RFC1464 of 1993. I've noticed that there was no further drafts, RFCs, nor 

The TXT RR was not introduced by RFC 1464. It's only the idea of internal
structure (tag/value) that was promoted by this document.

> implemented in nowadays server?

Since the TXT RR was part of the initial DNS RFCs it's part of the DNS
standard. RFC 1464 however has "Experimental" status. You can find out the
difference in BCP 9/RFC2026.
Not only do servers implement the TXT RRs, but also resolvers support it.
TXT RRs are not really in widespread use today unless for locally focused
purposes (e.g. hesiod). Within "DE", a TLD with > 6 million names and
more than 4.5 million zones, there are roughly 150000 TXT RRs and only
some in the low hundreds use the RFC 1464 notation, mostly to ``advertise''
ethernet addresses. Some others are used to support the RP RR defined in
RFC 1183.

> searching inside the RDATA cell of that TXT records?

RFC 1464 did not suggest specific server behaviour. Instead, the resolver
or some function between application and resolver would have to extract
the information based on the selected attribute.
There's no other way because there is currently no query type or opcode
which could take into account partially available RDATA (i.e. return
all A RRs for foo.example.com with '10' in the first octet or return all
TXT RRs for bar.example.com with 'location=' at the beginning of RDATA.
And while it's theoretically possible to define such an opcode, it would
be in conflict with both RFC 2181, which mandates that RRSets always
be dealt with as a complete set and DNSSEC, which can only sign complete
RRSets.

-Peter

---

From: Rob Austein <[email-masked]>
Date: Sat, 12 Apr 2003 13:38:33 -0400

> boundaries between the text labels.  I don't know the status of that.)

The original BIND 4.x implementation of TXT RRs treated the entire
RDATA as one big string.  If memory serves, this was fixed circa 1996.

---

From: Paul Vixie <[email-masked]>
Date: Sat, 12 Apr 2003 18:24:31 +0000

> RDATA as one big string.  If memory serves, this was fixed circa 1996.

yes.  both when parsing zone files and when handling it in cache, bind4
before 4.9.3b17 failed to deal with the inner segmentation of txt rdata's.
this meant an incompatibility when it was fixed, since

	FOO	IN	TXT	abc def

used to create

	rdlen	8
	rdata	\07 a b c \020 d e f

and now it creates

	rdlen	8
	rdata	\03 a b c \03 d e f

to get the prior behaviour, quotes are needed (per the standard), i.e.,

	FOO	IN	TXT	"abc def"

---

From: Dean Anderson <[email-masked]>
Date: Sat, 12 Apr 2003 15:10:28 -0400 (EDT)

Supposedly this is a protocol list, not a bind list.  The standards are
not defined by bind behavior.

TXT records certainly are current standard, and should be implemented in a
standards-compliant server. I note that AFS, DCE, CODA, and other
applications use TXT records.

		--Dean

---

From: Greg Hudson <[email-masked]>
Date: 12 Apr 2003 15:48:33 -0400

> not defined by bind behavior.

Of course not, but it's information relevant to the question at hand. 
(In particular, it's useful to know that TXT records passing through an
old BIND cache may have their boundaries rewritten; depending on the
number of such caches out there, that might be an obstacle to using RFC
1464 on a wide scale.)

> applications use TXT records.

This statement doesn't agree with my understanding, although it's tough
to prove a negative.  AFS uses AFSDB records, and does not make any use
of TXT records that I can determine.  Coda does not appear to use either
AFSDB or TXT from a search of the current source code.  I don't know
about DCE, but I imagine it would use the DCE registry for anything you
might consider using TXT records for.

---

From: Dean Anderson <[email-masked]>
Date: Sat, 12 Apr 2003 17:41:13 -0400 (EDT)

> number of such caches out there, that might be an obstacle to using RFC

Sounds like an operational problem to me.  Since these old versions of
bind have pretty severe security problems, it seems unlikely that there
are enough around to be concerned about.

> might consider using TXT records for.

I think OpenAFS has some differences to transarc AFS. Now that you mention
it, TXT was deprecated after the AFSDB record type was approved. Most of
my afs administration was in the old days. I don't have the transarc
source online anymore, due to disk failure on the old ri.osf.org cell. DCE
uses a TXT record to locate other cells. It doesn't have anything similar
to the old CellServDB file. The registry is used by the cell itself to
find RPC endpoints.  I have old sets of DCE and AFS manuals in the office.
I'll be happy to fax you a copy of the relevant pages on monday, if you
want.

		--Dean

---

From: Florian Weimer <[email-masked]>
Date: Sun, 20 Apr 2003 23:19:40 +0200

> Not only do servers implement the TXT RRs, but also resolvers

However, as far as I can tell, stub resolvers lack convenient
interfaces for extracting TXT RRs.  This might explain why a lot of
DNS applications use A RRs to encode things which actually aren't IP
addresses. 8-/

---

From: Derek Atkins <[email-masked]>
Date: 20 Apr 2003 19:00:10 -0400

> DNS applications use A RRs to encode things which actually aren't IP

What applications do something like this?  An A record is extremely
limited in what it can hold (namely, a 4 octet address).  So, I'd be
interested in hearing about "DNS applications" that hold something
else or somehow use A records differently.

-derek

---

From: David Shaw <[email-masked]>
Date: Sun, 20 Apr 2003 22:29:05 -0400

> else or somehow use A records differently.

The various DNS-based email blacklist systems come to mind.  They use
different "addresses" in the loopback net (127.0.0.1, 127.0.0.2, etc)
to indicate different reasons to block a given SMTP connection.

David