Max TXT Size Entry in DNS

[Kevin Darcy]

At the risk of sounding snotty, do you really think DNS is suited to this kind
of thing? If you want a near-ubiquitous protocol which can be used for storing
and retrieving more-or-less arbitrary data in a hierarchical, distributed
database, I think LDAP would probably be a much better choice. Granted, it's
still not quite as mature as DNS, but then if custom hacks to the DNS code are
required to make your application work, maturity would appear to be a moot
point anyway...

If you're determined to get this working within DNS, though, and you have to
hack on the code *anyway*, maybe you should consider just implementing the
experimental SINK (Kitchen Sink) record type. See
http://www.ietf.org/internet-drafts/draft-ietf-dnsind-kitchen-sink-02.txt


- Kevin

Tristan Austin wrote:

> I'm writing the code on NT with Bind source version 4.9.7, is this flag for
> 8.2.2P5 on the server or the client. (I'm assuming server). Either way, I've
> been getting larger responses back than 255.
>
> In the header of the "answer", I've found that when the data exceeds 255, a
> flag is set and two u_chars are added together to indicate the end of the
> data:
>
> eg.
> flag  endData    endBlock    startdata
> 0     138        137            d........
> 1     25         255            d........
>
> In the first instance the end of the data is at 137 from the start of the
> data, the second it is at 280 (25 + 255) from the start of the data. As you
> said though, this means you can either have one string of max length 500, or
> more with max length 255 (and only going up to 255 from the start of the
> first data). Even with this it's still not enough though.
>
> I've stepped through the code as it's making the request and I'm pretty sure
> it's going via TCP.
>
> I'm about to upgrade to v8.2.5P5 on the server to see if that's any better.
> Thanks for your help.
>
> As a last resort we'll have to split it over a number of RRs.
>
> > -----Original Message-----
> > From: Jim Reid [mailto:jim at rfc1035.com]
> > Sent: Thursday, 3 February 2000 6:10
> > To: Tristan Austin
> > Cc: comp-protocols-dns-bind at moderators.isc.org
> > Subject: Re: Max TXT Size Entry in DNS
> >
> >
> > >>>>> "Tristan" == Tristan Austin <tristan.austin at authentic8.com> writes:
> >
> >     Tristan> Is there a maximum size a TXT entry can be in DNS (using
> >     Tristan> bind). I'm attempting to read back a large piece of
> >     Tristan> information and it's begin truncated.
> >
> >     Tristan> If there is a maximum size, is it configurable?
> >
> > The maxiumum length of a string in a  TXT record appears to be 255
> > characters. From db_defs.h:
> >
> >         /* max length of data in a TXT RR segment */
> >       #define MAXCHARSTRING 255
> >
> > However there's an #ifdef in db_load.c (ALLOW_LONG_TXT_RDATA) which is
> > switched on in 8.2.2P5. This allows for TXT records of up to MAXDATA -
> > approx. 2Kbytes - to be loaded, though these have to be composed of
> > several strings, none of which is more than 255 characters long.
> >
> > If TXT records are that big, the DNS answers will be truncated because
> > the default payload in a UDP datagram is 512 bytes. [EDNS allows for
> > bigger payloads in UDP replies.] When a resolver gets a truncated
> > response like this, it should make a TCP connection to the name server
> > and repeat the query to get the full response. Maybe something's not
> > handling truncated responses from the name server?
> >
> > BTW if one of your TXT records is too big, you should see complaints
> > like "total RDATA too long" or "RDATA field too long" in the name
> > server logs.
> >