Re: [dnsext] loads of TXT records for fun and profit

[David Miller <[e-mail]>]

On 5/3/2013 9:48 PM, John Levine wrote:
>>> No, that section is all about having a single TXT RR whose complete
>>> content doesn't fit in 255 characters.  It illustrates how to achieve
>>> this within a single RR in zone file format.
>>
>> ... which doesn't prevent people from splitting them across multiple 
>> records.
> 
> Section 3.1.2 of RFC 4408 forbids an SPF checker from looking at
> multiple records at the same name.  Section 3.2 of the current draft
> has the same language.  SPF has to forbid multiple records at the same
> name, since their semantics are obviously impossible to define.
> 
> A single TXT record can, of course, contain any number of strings and
> can be arbitrarily long.  See RFC 1035 sec 3.3.14.

TXT records don't have a specified maximum length, but they cannot be
arbitrarily long.  They have to fit into an RR.  RDLENGTH, which
specifies the length of the RDATA field, is an unsigned 16 bit integer
(max 65535).  See RFC 1035 sec 3.2.1.

64k is very long, but not arbitrarily long.

> 
> Is there some reason it's preferable to guess rather than reading the
> spec?
> 
> R's,
> John

-DMM