Hi Doug,
At 13:07 23-08-2013, Douglas Otis wrote:
The SPFbis document improperly conflates DNS terminology with
identical terms invented by this document. Examples are terms used
to describe mechanisms having the same identifier differentiated
between mechanisms and DNS resource records by using lower and upper
case respectively. References to SPF records are differentiated by
a textual prefix and not by TYPE as defined by DNS.
Could you provide some examples of the above as I would like to
clearly understand the argument?
In addition, the MARID WG NEVER reached consensus. A follow-on
group operating outside of the IETF required a promise of support to
subscribe to their mailing list. When one looks at how SPF is
commonly used, the pre-existing APL resource record offered an
effective alternative, but was oppose by a particular vendor
unwilling to fully implement DNS. Currently this vendor is seldom
used to directly interface with MTAs on the Internet and no longer
justifies the use of the TXT records. As such, the SPF Resource
Record should not have been deprecated.
There are other messages on the Last Call about the SPF Resource
Record. I'll take up the above together with the other comments.
This draft should be made Informational and not Standards Track.
I suggest providing arguments for that.
Section 4.6.4 fails to offer a sufficiently clear warning about
potential magnitudes of DNS transactions initiated by a single SPF
evaluation where two are recommended to occur one for the separate
identifiers. In fact, this section appears to make assurances no
more that 10 DNS queries will result and is widely misunderstood.
There is a discussion about Section 4.6.4 at
http://www.ietf.org/mail-archive/web/spfbis/current/msg03305.html
4.6.4. DNS Lookup Limits
Was:
,--
SPF implementations MUST limit the total number of mechanisms and
modifiers ("terms") that cause any DNS query to 10 during SPF
evaluation.
'--
Change to:
,---
SPF evaluation must limit the number of mechanisms, and the modifier
term 'redirect' to occur in no more than10 instances within the
evaluation process. The mechanisms 'ip4', ip6', and 'all' are
excluded from this instance limitation. Each mechanism is permitted
to resolve subsequent resource record sets (RRsets) that MUST not
contain more than 10 resource records to complete a match check.
When the number of instances exceeds 10, or when subsequent
resolutions exceeds 10, check_host() MUST produce a "permerror"
result.
The maximum number of DNS transactions initiated by an SPF
evaluation is therefore 1 for the initial SPF resource record, 10 for
each mechanism times 10 transactions needed to complete a matching
process for a total of 111 DNS transactions. This number excludes
those required by DNS to fulfill a request and those required by an
EXP modifier.
I'll list the above as not addressed yet.
The recommended 20 second evaluation timeout imposes a maximum
network distance of less than 27,000 kilometers or a little more than
half the circumference of the Earth. Even a 60 millisecond delay can
result in more than a 6.6 seconds consumed by the round trip
overhead needed for each SPF evaluation.
During the WGLC Murray Kucherawy asked a question about the 20 second
evaluation timeout (
http://www.ietf.org/mail-archive/web/spfbis/current/msg03700.html
). The answer was that the timeout is already in RFC 4408.
The overall resulting maximum number of DNS transactions for
both "HELO" and "MAIL FROM" is 222 DNS transactions. Since
check_host() introduces macros and name expansions that
combine mechanisms and modifiers in a manner not directly
supported by DNS, the entire set and sequence of SPF based
DNS transactions is required for each evaluation. While SPF now
has a 2 limit of void lookups, use of synthetic domains has become
a popular technique for tracking traffic which can be used by
malefactors to overcome this SPF void lookup limit.
The draft agenda for the IETF 83 session was posted to the SPFBIS
mailing list on March 14, 2012. One of the items on that agenda was
"DNS amplification attacks" (
http://www.ietf.org/mail-archive/web/spfbis/current/msg01021.html
). It would have been good more discussion of the issue (Issue #24)
from a DNS perspective. As there has been numerous comments about
the DNS angle during this Last Call, maybe an IETF participant will
be able to provide some input about the above.
Once DNSsec becomes a requirement, SPF will have created an
inordinate number of transactions and overall traffic per message
exchange that will impose a SIGNIFICANT reflected amplification risk.
draft-ietf-spfbis-4408bis-19 does not have any DNSSEC requirement.
'---
Related information:
<random-string>cdr-ds.metric.gstatic.com
<random-string>.g.vm.akamaistream.net
are domains on sizable networks that act as a wildcards. In the
second instance, the wildcard points to a CNAME that then resolves
an A record. Such use compared against
http://tools.ietf.org/html/draft-otis-spf-dos-exploit-01 produces a
much larger amplification than the x320 gain estimated. Use of
DNSsec further increases this gain estimate. Although details
differ from case to case, the synthetic domain technique is seeing
greater use. One of the initial reasons for using TXT records
without any prefix was to permit use of wildcards. This dangerous
feature means SPF allows malefactors to leverage any large TXT
resource record that otherwise would not have been
problematic. Again this risk increases when DNSsec becomes required
and represents another reason for not deprecating the SPF resource record type.
I welcome comments from other IETF participants about the above.
Remove this misleading paragraph within section 4.6.4:
,---
When evaluating the "mx" mechanism, the number of "MX" resource
records queried is included in the overall limit of 10 mechanisms/
modifiers that cause DNS lookups described above. The evaluation of
each "MX" record MUST NOT result in querying more than 10 address
records, either "A" or "AAAA" resource records. If this limit is
exceeded, the "mx" mechanism MUST produce a "permerror" result.
'---
MX resource records do not contain IP addresses.
Per RFC1035 the structure for an MX record is roughly as follows:
A 16 bit PREFERENCE (distance) value followed by an MTA hostname.
A DNS MX reply may offer A records for the hostnames in the
Additional Section however per:
Per RFC2181 Section 5.4:
,---
Servers must never merge RRs from a response with RRs in their
cache to form an RRSet. If a response contains data that would form
an RRSet with data in a server's cache the server must either ignore
the RRs in the response, or discard the entire RRSet currently in the
cache, as appropriate.
'---
This will not prove ideal for SPF with respect to effective use of DNS.
Also hostnames contained within MX resource record might be within
any domain, and not necessarily the same domain as that of the base SPF record.
Per RFC2181 5.4.1. Ranking data
An authoritative answer from a reply should replace cached data that
had been obtained from additional information in an earlier reply.
However additional information from a reply will be ignored if the
cache contains data from an authoritative answer or a zone file.
I agree that MX resource records do not contain IP addresses. There
is a thread at
http://www.ietf.org/mail-archive/web/spfbis/current/msg03305.html
about reasonable DNS error limits. I could not spot the problem in
the paragraph that is described as misleading. An example to
illustrate the problem might make it clearer.
Mistake.
Section 3.4
Second paragraph:
Similarly, the sizes for replies to all queries related to SPF have
to be evaluated to fit in a single 512 octet UDP packet.
s/UDP packet/DNS message/
My understanding of the working group discussion is that the intent
is to have the DNS reply fit within a UDP packet.
Per RFC1035
Section 2.3.4. Size limits
UDP messages 512 octets or less
Section 4.2.1. UDP usage
Messages carried by UDP are restricted to 512 bytes (not counting
the IP or UDP headers).
The DNS message size limitation is not the same as a UDP packet
limit as suggested in Section 3.4.
I am not sure whether the above refers to the "450 octets". I'll
note a comment from Andrew Sullivan (
http://www.ietf.org/mail-archive/web/spfbis/current/msg03103.html )
so that I don't have to search for it again.
The SPFbis WG charter permitted removal of unused protocol elements
where the "ptr" mechanism was deprecated and the SPF resource type
was removed. Use of SPF's very dangerous macro feature currently has
less than 0.053% of the domains making use of these macros and
clearly falls below the WG removal threshold. We have also been
told by a few very large providers that SPF records containing any
macro reference are ignored for reasons related to both efficiency
and security. Marcos also inhibit the primary use by large
providers which is to compile the domain's IP address list.
There was an appeal about the meaning of the word "unused" (
http://www.ietf.org/mail-archive/web/spfbis/current/msg02167.html
). I'll highlight a comment from Pete Resnick:
"There's also the comment from Doug Otis about the local-part
macro. I'm inclined to declare that out of scope, maybe after a
recharter. That's not documenting current practice, but is an overt
change."
There are some threads about local part macros:
http://www.ietf.org/mail-archive/web/spfbis/current/msg00911.html
http://www.ietf.org/mail-archive/web/spfbis/current/msg01870.html
There is a short thread and a few other threads about the "ptr"
mechanism (
http://www.ietf.org/mail-archive/web/spfbis/current/msg00808.html
http://www.ietf.org/mail-archive/web/spfbis/current/msg00811.html
http://www.ietf.org/mail-archive/web/spfbis/current/msg03184.html ).
There is the following text in the write-up:
"There was some controversy about whether the use of macros was a
security risk and whether to deprecate the PTR feature. There was a
formal appeal of the SPFBIS WG chair' interpretation of the charter,
specifically regarding the removal of "unused" features. The two
features in particular which drove the appeal were the PTR feature
and the local-part macro feature. These features were not removed
from the document given that the appeal was denied by the Responsible
Area Director."
I hope that the above explains the decision.
The WG should have been told to focus on security and at better
insuring interchange to achieve a safer stance moving forward.
The working group was reminded about BCP 72.
Regards,
S. Moonesamy (as document shepherd)