On Sep 7, 2013, at 6:31 AM, Pete Resnick
<presnick(_at_)qti(_dot_)qualcomm(_dot_)com> wrote:
Below is the list of issues brought up during Last Call of
draft-ietf-spfbis-4408bis. I have tried to collect together the common issues
and tease out the ones that are slightly different. Below each issue, I've
given what I take to be the answer to the issue (either the change that needs
to be made, or the explanation of why no change is necessary). I've not put
names to the objections or the answers, simply because multiple people gave
different versions of the same objections or the same answers, and sorting
that out seemed useless.
Issues:
1. Overloading of the TXT RR for this use is bad.
- As far as I can tell, there is nobody that disagrees with this
statement. However, it is also not in-and-of-itself an objection to the
document: Nobody seems to have argued that this document should forbid use of
the TXT RR completely in SPF. The only question is whether the document
should provide a transition mechanism to the SPF RR or whether it is
reasonable to go forward with this protocol using only the TXT RR. There is
the "precedent"objection I will discuss in 2 below, but the specific
technical objections seem mostly *not* to be about forbidding TXT RR use from
the get-go. Some of the specific objections could be taken as arguments
against the document going forward if it has no transition mechanism, but I
believe all of those have either been addressed or can be addressed with
clarifying language in the document:
- The only complete solution to many of the problems that fall under this
category is if *all* misuse of TXT RR went away. There has been no convincing
argument put forward that this is plausible in the foreseeable future.
1a. TXT RR can cause large RDATA.
- In theory, that's certainly true. But I have not seen an argument
that SPF is causing a major problem to date, nor that there is an expectation
that it will in the future.
- There were some suggested text clarifications for section 3.4 to
make it clear which size limitations relate to DNS response size, UDP payload
size, or MTU size. They seem reasonable and were not objected to. I'll work
with the editor and others to clarify that text.
1b. Use of TXT RR can cause collisions with other applications.
- Again, there is no indication that this has caused a problem for SPF
(or others) to date, and SPF requires rejection of TXT RR data that does not
conform to the spec, so I see no evidence of the existing harm, nor of a
solid reason to believe there will be future harm. (Again, I'm leaving aside
the "precedent" arguments until 2.)
- There appears to be an effort underway to document (via an IANA
registry) such uses to minimize the potential for these sorts of collision
problems in the future.
1c. Use of TXT RR for multiple purposes makes it impossible to do access
control based on type of data (i.e., to allow delegation of the management
for TXT RRs that are solely for SPF use).
- Many organizations have been managing these records already; no
reasoning was given that this fine-grained management is necessary.
- Delegation is possible by pointing a TXT RR of (e.g.) example.com to
_spf.example.com, delegating the latter.
- All of the above was discussed extensively in the WG and taken into
consideration. Given the charter limitations, a reasonable choice was made.
2. Use of the TXT RR sets a bad precedent for future use.
- Several people responded that some additional text in 3.1 or elsewhere
(either by way of some sort of applicability guidance or an overt IESG
Statement) would address this issue. I think an IESG Statement is unnecessary
since I have not heard significant objection to including some guidance, and
I think something reasonable along these lines can be crafted. I'll work with
the editor and others to get such text in the document.
- The impediments that caused SPF to use TXT RR in the first place are
mostly gone. New protocols are unlikely to face the same challenges.
3. Removing SPF RR support is a charter violation.
- Because the original spec has a non-interoperable mechanism for use,
this constituted an "error" in the spec that was to be corrected.
4. A new transition mechanism from TXT RR to SPF RR should be put into the
spec.
- This was extensively considered by the WG.
- Backward compatibility would require support of TXT RR for the
forseeable future anyway.
- The proposed transition mechanisms have technical issues: Doubling
request traffic (e.g., doing queries in parallel), introducing delays (e.g.,
querying SPF RR first and running into firewalls, etc).
- This would be a new, unchartered requirement for the WG.
- The widely held conclusion was that such a transition plan would not be
undertaken by implementers. (See also 5.)
5. That there will be a lack of adoption of SPF RR was based on RFC 6686,
which does not support the conclusion.
5a. There is some current use of the SPF RR; it will increase if we put in
a new transition mechanism.
- 6686 showed only minimal use, and reports of those in the industry
shows the use dropping (i.e., the momentum is in the other direction).
- Bigger sites are conservative and will not transition for fear of
breaking current usage.
- No solid case was made for why to believe that the transition would
occur.
5b. Use of SPF RR is on the increase now.
- Claims that use is increasing are only anecdotal, and disagree with
the experience of those in the industry.
5c. Things may have changed since 6686. We should do more data collection.
- There's no reason to believe that the small amounts of recently
presented data are representative.
- Nobody presented any basis to doubt the folks working in the
industry.
- There has been ample opportunity (and motivation) for folks outside
of the WG to do more data collection; none has been presented.
- It is an unreasonable burden to place on the WG at this point.
There were a few smaller issues:
6. The term "SPF records" is confusing because it could refer to SPF RR.
- Because the document no longer uses SPF RR, this shouldn't be a
problem in practice.
7. Clarifications are needed regarding the number of lookups to do in
4.6.4.
- This will be reviewed prior to publication.
8. There should be a limit on PTR lookups.
- This was already considered by the WG and rejected.
The only looming issue large issue is the architectural one:
9. Using TXT RR for this purpose violates the architecture of the DNS.
I list this separately because this is not about the immediate technical
implications (like objection 1) or a claim about precedence setting (like
objection 2), but appears to be a claim that violating the architecture is in
and of itself a reason to not put something on the standards track. The
problem I have with this is, short of actual technical harm, I'm not sure how
to judge this. Our processes judge whether standards should be adopted on the
basis of interoperability, deployment, and solving a useful problem. We have
many examples of protocols that violate architectural principles, but
standardize them nonetheless. (We can all name our most hated.) We have found
it more useful to document how to interoperate with protocols that may not be
ideal but are widely deployed rather than reject them on principle. I can't
see how to treat this protocol differently.
[There were a number of "straw men": There were responses to objections that
never got brought up during Last Call, and a number of followups to responses
to objections where the response was not one offered. Examples of these
included:
- A response to the objection that we haven't let the Experiment run long
enough. (Nobody ever argued that during Last Call.)
- A followup denying the contention that there is limited server support
for SPF RRs. (Nobody ever said that the reason to use TXT RR now was because
of limited server support.)
I have ignored these threads.]
So, my conclusions in summary are:
- The document needs to make a statement in the document clarifying why the
SPF RR is no longer used in the spec and making it clear that no precedent
should be created by this protocol's continued use of TXT RR.
- A few clarifications are required in the text (size limitations, perhaps
number of lookup limitations).
- The remainder of the objections were fully considered and understood by the
WG, and were addressed to a reasonable extent, and therefore that there is
rough consensus to go forward with this document.
Dear Pete,
You missed two concerns raised in the last call:
10. As DNSSEC becomes more broadly used with email, SPF is likely to introduce
problems neither foreseen nor properly considered by the WG. "Not a problem
for SPF" does not mean SPF will not impose problems for DNS which may include
DNSSEC. It should be obvious which protocol SPF or DNS should receive greater
consideration The SPF protocol also ignores the number of PTR Resource Records
returned by a query made by a recipient on behalf of unknown senders against
various third-party domains. These domains can be constructed and modulated by
message elements via macro expansion greatly increasing the reflected
amplification this protocol enables. The same issue could occur with a series
of otherwise innocent TXT resource records that the SPF protocol also permits.
This "feature" could prove highly problematic and extremely difficult if not
impossible to defend against. Even limits on NX domains can be easily
sidestepped by malefactors leveraging the practice of using sy!
nthetic domains to track users without the use of cookies.
11. The continued specification of SPF macros inhibits interchange. Because
it is common for SPF records not to produce a pass, this issue is not likely to
have been given adequate attention. When SPF macros are not implemented by
receivers for any number of very valid reasons, such as ensuring effective
caching of DNS, their required use can and will lead to interchange issues.
SPF may impose complex macro handling over multiple DNS responses determined by
a sequence of queries that can not be directly handled by DNS itself. Even the
operation of SPF macros represents security concerns threatening the integrity
of the associated SMTP and DNS servers. Since the publication of SPF macros
is well below the level used to justify removal of the SPF RR record type, the
same consideration should have been given SPF macros. Use of SPF macros also
interferes with forensic efforts at handling interchange problems. As such, it
is not surprising to find extremely few domains p!
ublish SPF records using macros and large providers not processing SPF macros.
The lack of consideration given DNS by the SPF protocol offers overwhelming
justification not to consider this protocol suitable for endorsing as a
standard. Going from experimental to informational should not represent any
hardship, but would serve as a warning protocols should pay attention to their
impact on underlying infrastructure. There are limits on making it easy to
send messages since the Internet is not suffering from a message scarcity.
Regards,
Douglas Otis