Re: [spfbis] Question about SPF checks based on RFC 7208

Kurt Andersen <> Mon, 02 May 2016 15:56 UTC

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Date: Mon, 2 May 2016 08:56:17 -0700
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From: Kurt Andersen <>
To: S Moonesamy <>
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Cc: "" <>, Frank Bulk <>
Subject: Re: [spfbis] Question about SPF checks based on RFC 7208
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On Mon, May 2, 2016 at 12:58 AM, S Moonesamy <>; wrote:

> Hi Frank,
> At 17:45 30-04-2016, Frank Bulk wrote:
>> . . . tries to work through the MX records of, but for
>> some reason it specifically queries for an AAAA for each MX record, of
>> which
>> we don't have one (because our inbound email gateway does not yet support
>> IPv6).  Since we have four MX records, it work through two of them (which
>> both fail) and then the tool hard fails saying that,
>>         "The maximum void DNS lookup limit of 2 has been exceeded during
>> the
>> evaluation (see RFC7208 Section 4.6.4.)."
> It does not make sense to query for an "A" record when the client address
> is an IPv6 address.  The issue in your case is that your outbound is over
> IPv6 while you do not have inbound IPv6.  I took a quick look and I did not
> find a discussion about that in relation to the void DNS lookup limit in
> the mailing list archive.
> . . . does RFC 7208 need some clarification on this matter?  My guess
> is that "it performs an address lookup on each MX name" is not equivalent
>> to
>> "check for an AAAA on each MX name and check for an A on each MX name",
>> but
>> I'm not sure.
> Your question is about whether there is a "bug" in the technical
> specification.  The second sentence is about how to get around a "void DNS
> lookup".  At the moment I am not sure about what would be the appropriate
> answer.

My suggestion is to clarify exactly what constitutes a "void DNS lookup" in
the case of an MX mechanism. I suggest that we define a void MX lookup to
be one that either returns no records or returns the "null MX record"
(RFC7505). Could this be done as an erratum item?

I think we also need to highlight the importance of putting "lookup
dependent mechanisms", and especially 2nd degree dependent mechanisms (such
as mx) *after* any explicit IP specifications to publishers of SPF records.
The "traditional"/historical suggestion that has been provided by many
record creation "wizards" is flawed by putting "a mx" at the beginning of
their recommendations.

--Kurt Andersen