[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Issues raised in comments by Steve Belovin from January 2004
> Steve Bellovin is security area advisor at IETF and part of IESG. While
> we're at it please look through his message and compare if we solved other
> issues mentioned there too.
Actually, I'm just going to reforward his message (its important as we're
on last call) and go through the issues mentioned. Again original is at
http://www.interesting-people.org/archives/interesting-people/200401/msg00037.html
My comments are listed in (--- ... ---).
------------------------------------------------------------------------
Delivered-To: dfarber+@xxxxxxxxxxxxxxxxxx
Date: Sun, 04 Jan 2004 18:23:55 -0500
From: "Steven M. Bellovin" <bellovin@xxxxxxx>
Subject: Re: [IP] MTA packagers please consider SPF
To: dave@xxxxxxxxxx
>From: mengwong@xxxxxxxxxxxxxxx (Meng Weng Wong)
>Subject: MTA packagers please consider SPF
>To: chip@xxxxxxxxxxxxxxx
>Cc: dave@xxxxxxxxxx
>
>SUMMARY
>
> This message goes to some of the maintainers of MTA packages for major
> Linux distributions. I ask that they consider adding SPF support to
> their packages in helping to solve spam. I founded pobox.com, which
> produced the MySQL extensions for Postfix. I authored the SPF
> standard and am now trying to get it widely adopted.
Dave, I hope that people don't jump too rapidly on the SPF "standard".
From my persepctive, it's very far from ready for prime time.
(Note that although I'm a member of the IESG, I'm speaking as an
individual. I'm not even saying how I'd vote if this document were
to come before the IESG today -- IESG evaluations are a deliberative
process, and I could very easily be talked out of some or all of
my points.)
There are several major problems with the document as written,
including its semantic model, the uptake model, and the "specsmanship".
The latter is easiest to fix, but it will render current implementations
useless if the eventual spec is different. Running code is a great
way to test a concept; too much deployment of bad running code is
a tremendous obstacle to a decent standard. I'm not even talking
of the perfect being the enemy of the good.
To make it easier to read, I've indented the major sections of this
note:
Specsmanship:
The version number definition is problematic -- it only
has major version numbers. I suspect that we need minor
version numbers as well, for operational debugging.
(----
This is solved with new protocol SPF2 records that have minor version.
I'm not certain how important this is though... (it could be introduced
to spf1 as neutral "comment" modifier to be ignored)
----)
The most glaring problem with SPF is the use of TXT records.
TXT records are supposed to be free-form text, with no
semantics attached. The use of TXT for test purposes is
understandable (though regrettable -- an experimental record
type code would be better); the use of TXT records for
textual error messages is not. The document itself notes
the problem of ordering of multi-record messages.
(----
Current drafts do not allow for "multi-record messages". I note that I'm
personally for allowing it but we will need modifier specifying ordering
then and it may get more complicated.
----)
Beyond that, there are problems with internationalization:
what language should the error message be in, and in what
character set is it encoded? A simple URI would be a better
solution; at the least, it should point to an SPFERR record.
(----
I remember discussions about this at spf-discuss. I don't remember
if there was any resolution...
----)
(Record subtyping in the DNS causes problems; see RFC 3445
for some details on why.)
The use of TXT-like records is problematic because it
requires parsing an ASCII string in a DNS resolver. (Yes,
I know that NAPTR records require the same sort of parse.
I don't much like that, either.) The more complex the
parse, the harder it is to get right, both for the author
and the receiver of such records.
(----
As Yakov pointed out this should be listed as an issue in security section.
----)
A TLV-based structure
permits parsing by the author's DNS server, and is easier
to interpret on the receiving end.
The Received-SPF header line is badly specified. It doesn't
follow the the standards for other RFC 822/2822 headers
(i.e., it requires exactly one space in certain places
where an arbitrary amount of white space (including none)
is permitted in other headers); it has some things as
comments (receiving host) that should be parseable; and it
doesn't mandate that Received-SPF lines from outside of
the domain MUST be deleted. (The actual requirements here
are more complex; I won't go into details in this note.)
Yes, the line as specified is a bit easier to parse, but
any spam filter is going to have to deal with many other
headers, and hence will have to have a full-fledged 822/2822
parser.
(----
Open issue. We need new draft on reporting results of spf verification
in either special new header or as part of received header.
----)
Too many cases can result in an "unknown" return value.
That makes debugging hard. There needs to be a "none"
value, for cases where there is no SPF record; there needs
to be a type code for "unknown", to distinguish among the
many error cases. Beyond that, the set of type codes needs
to be enumerated -- as is, we'll see an operational nightmare.
(----
I think this is now listed on page 16 of protocol draft.
Mark, do you want to comment if this is now resolved?
----)
Section 5 speaks of using Received: lines. Such lines have
been forged by spammers for many years. While they can be
used, great care must be taken. This document needs to
define the necessary steps appropriately.
(----
Open issue as has been seen in latest MUA related discussions. I'm in
favor of specifying that they should not be used beyond the very top
(last added one) in the email message.
----)
5.1 speaks of cidr-lengths, but 5.2 et seq. speak of
dual-cidr-length. That looks like something where the
editing hasn't caught up yet. But having a CIDR length on
an MX record is a bad idea, since there may be multiple MX
records with different appropriate lengths.
The macro language scares me -- it's very complex. Note
that DNS records are limited to 512 bytes unless EDNS0 is
used; EDNS0 is not widely used today, and may incur other
costs. But the real problem is that the functionality
permitted may be far too rich -- arbitrary semantics are
not a good idea, since they can lead to random breakage
when some site implements a test that another site isn't
prepared to meet. In Postel's language, a sender can't be
conservative in what it emits if it doesn't know what the
requirements are.
(----
I'm not certain if these are resolved or not.
Mark may want to comment on cidr-lenghts and macro.
----)
8.4 ruins much of the effectiveness of the scheme -- it
provides ways to avoid processing. For example, a spam
engine could send email with a local-seeming HELO, MAIL
FROM, and From: entries, in which case (per Example 3) SPF
isn't to be used. Spam from abuse@ or postmaster@ can also
bypass checks.
(----
I think this issue does not come up with SenderID/PRA drafts...
Somebody please do comment if it still exist with Classic-SPF.
----)
The suggestion that this scheme become default in April,
2004 (Section 9.4) is preposterous. Even if the IESG were
to approve this document today -- and very few documents
are passed on first try -- it would take far longer than
four months to build, test, and deploy production-grade
clients and servers.
(----
The "April 2004" date is no longer of consequence...
----)
The security considerations section mentions IP address
spoofing, though the FAQ claims that they aren't real. I
agree that classical spoofing, per Morris' 1984 memo, is
probably not a major threat here. But spammers are using
BGP to steal entire address blocks -- that's a bigger
threat. (The FAQ also points to RFC 2761 when it should
be 2671.)
(----
As suggested by Jim Fenton, IP hijacking is now listed as a threat
in the appropriate section of core draft.
----)
Uptake Model:
As Rick Adams has pointed out, there is no consensus yet
that this is the right way to go. The major ISPs on the
net -- AOL, Yahoo, MSN, etc. -- have not bought into this
scheme. Unless and until they do, it doesn't help much,
either for their customers (who make up a substantial
proportion of the user population) or for everyone else
(since their addresses could be forged).
(----
AOL now published SPF records and commented on this list regarding
SenderID and I suspect MSN supports this too as its part of Microsoft.
----)
Semantic Model:
In a strong sense, the part that requires the most debate
is the semantic model. SPF strongly binds a sender to some
DNS records. But that isn't always a good idea. People
who use portable email addresses will now be constrained
to use the domain owner's SMTP sender, which may not even
exist. (A more interesting model would permit delegation
of individual user names to particular sending machines.
But that would probably require too much public key
cryptography to be affordable.)
(----
I believe how individual users can be permitted to do have separate
email signatures is part of Jim Fenton's Identified Mail proposal.
Similarly this is also supported by my MTA Signatures proposal
as individual public keys could be used by users that could be
signed by entity's main public key. Both of these proposals are to be
discussed as part of http://www.imc.org/ietf-mailsig/index.html
SPF protocol by means of macros also allows to have individual user
dns records but it might not be very popular way to do it.
----)
The net effect will be to bind users more strongly to their
ISPs and/or their employers. While big ISPs may like that,
it flies in the face of current (American) public policy
-- witness local telephone number portability. Ironically,
it will also discourage a current anti-spam strategy used
by many: throw-away email addresses for particular purposes.
It will also make life harder for people who regularly use
multiple sending email addresses. For reasons of privacy,
my children generally use email address that are not readily
tied to their real names. But for certain very important
kinds of communication -- sending email to teachers, for
example -- they use a family-linked email address.
It isn't always clear to people what SMTP server they're
actually using. Over the last few years, I've noticed that
one major hotel chain intercepts outbound SMTP connections.
I don't know if they're trying to defend against check-in
spammers or if they're trying to help travelers whose
laptops are hard-wired to point to their company's or home
ISP's SMTP servers. Granted, people should use VPNs or
SMTP/SASL for such things; too may don't and perhaps can't
-- if your ISP doesn't support it, for example, you can't
use it, and switching ISPs carries a non-trivial cost,
especially if you have only one choice of broadband ISP.
I've underscored some of my points here by using a portable
email address of my own, rather than my usual email address.
(----
I think all these issues have been raised before and people
decided that certain limitations are worth it for better email
security. Possibly draft needs to talk about this more...
----)
Conclusion:
The basic concept may or may not be a good idea. The
authors themselves admit that it's only part of a total
anti-spam solution, and I'm not convinced that it's worth
the deployment effort. Its strongest in dealing with "joe
jobs" -- spammers (and worms) impersonating real email
addresses -- but that's the part that most runs afoul of
my semantic concerns.
--------------------------------------------------------------------
---
William Leibzon, Elan Networks:
mailto: william@xxxxxxxx
Anti-Spam Research Worksite:
http://www.elan.net/~william/asrg/