Max. Number of char in a TXT Record

Vincent Rivellino vince at cuz.cx
Fri Mar 27 00:11:52 UTC 2009


On Thu, March 26, 2009 19:19, Kevin Darcy wrote:
>> 2) What is the maximum number of includes inside a SFP field?
>>
> I assume you mean SPF (Sender Policy Framework), the anti-SPAM(ming)
> mechanism (?)
>
> I'm no SPF expert but in glancing at the documentation it looks like
> there is no arbitrary limit on the number of "include"s. Since SPF
> "records" are stuffed into (one or more?) TXT records, the above limits
> in the answer to question #1 would apply.

Kevin, it looks like you are correct.  From the RFC:

--------
3.1.4.  Record Size

   The published SPF record for a given domain name SHOULD remain small
   enough that the results of a query for it will fit within 512 octets.
   This will keep even older DNS implementations from falling over to
   TCP.  Since the answer size is dependent on many things outside the
   scope of this document, it is only possible to give this guideline:
   If the combined length of the DNS name and the text of all the
   records of a given type (TXT or SPF) is under 450 characters, then
   DNS answers should fit in UDP packets.  Note that when computing the
   sizes for queries of the TXT format, one must take into account any
   other TXT records published at the domain name.  Records that are too
   long to fit in a single UDP packet MAY be silently ignored by SPF
   clients.
--------

So, it appears that you can as many include's in a SPF record as you wish,
but it would be wise to keep the size of the SPF record below ~450
characters.

There is lots of good SPF information at www.openspf.org


-Vince




More information about the bind-users mailing list