[Top] [All Lists]

Re: RFC 5321bis / 2821ter

2009-01-24 10:01:53

--On Saturday, January 24, 2009 6:23 -0700 David MacQuigg
<david_macquigg(_at_)yahoo(_dot_)com> wrote:

At 05:53 PM 1/23/2009 -0500, John C Klensin wrote:
--On Friday, January 23, 2009 14:32 -0700 David MacQuigg

I have yet to see any sensible use-case that would be
complicated by a requirement that the HELO/EHLO name end in a
registered, verifiable domain name.


Today, under 5321, EHLO clearly requires a fully-qualified
domain name that can be resolved unless the host doesn't know
its name and has to use an IP address, a situation that the
spec doesn't encourage.  My reading of 5321 (but not
necessarily 821) says that HELO does too.

Perhaps it is just a matter of emphasis.  As I read the spec
it *does* seem to encourage using HELO/EHLO names that are
useless for authentication.  For example, the language in
4.1.4 "if the verification fails, the server MUST NOT refuse
to accept a message" puts the burden on the receiver to deal
with mis-configured transmitters.  Receivers are expected to
accept mail from transmitters that say "HELO this is Jupiter".

Sigh.  Read more carefully...

(1) It says, quite deliberately, "MUST NOT refuse to accept a
message on that basis".  That is _very_ different from the
excerpt you quote.

(2) The place where the names get "used", i.e., the SMTP client,
is covered by the strongest type of statement that we can make
in a standard:

        "The SMTP client MUST, if possible, ensure that the
        domain parameter to the EHLO command is a primary host
        name as specified for this command in Section 2.3.5."

The key case that justifies "if possible" is mentioned in the
next sentence, but there are others.  2.3.5 is very explicit,
arguably to the point of tedium -- the string must be
fully-qualified (no local names or incomplete strings permitted)
and it must be resolvable (by any plausible reading "in the
public DNS" -- and I wrote myself a note in December to ask
where things had gotten weird enough that we had to add that

(3) "HELO this is Jupiter" fails the syntax rule

   helo = "HELO" SP Domain CRLF

in several ways.  If you receive such a string, you are
perfectly justified, conformant, and in line with the spec to
reject it with a 501 code.   Indeed, you would be outside the
spec if you did not do so.  You wouldn't even have to figure out
whether "this" was a resolvable FQDN or not.

My imagination doesn't stretch nearly far enough to get from
those three points to "seems to encourage using HELO/EHLO names
that are useless for authentication".

That prohibition in 4.1.4 goes back to RFC 1123, which predates
2821 by nearly a dozen years.  There are a number of reasons for
it.  Some of them are subtle, some of them involve cases that we
would now prefer to see handled via submission servers and prior
agreements.  But let me give you a contemporary example that
might make the situation clear.   Suppose there is a laptop with
a full service, conforming, SMTP client on it.  It sends mail,
manages queues, etc. -- no submission server involved.  It has a
permanent name, say  However, it is not
connected at a fixed point to the public Internet.  It roams,
and may have different addresses on different networks on
different days.  If its owner is being really careful she even
uses dynamic DNS to be sure that, at any time the host is
online, its domain names points to its actual address.   Or, if
she knows that the host is used at only a half-dozen locations
and has a usual address at each, she might set up static DNS
entries that list each of them as its address.     Now suppose a
message is sent to you from that host, after which it is shut
down.  When it reappears on the network a few hours later, it is
at a different address.  If you resolve the domain name at that
time, you get a valid address for the host, but it isn't the
same address the host had when it initiated sending the message.
Do you really want to reject the message?  If you do, please
don't justify the decision on the basis that that you have done
a careful and adequate job of authentication -- _nothing_ you
can do with HELO/EHLO as they are defined in 5321 is going to
get you to a point at which that is possible.

The purpose of changes should be to avoid lost mail, as
opposed to rejected mail, which doesn't have any damaging
effects on reliability.  SMTP REJECT should be encouraged as
the proper response to doubtful requests.  (Sorry, we don't
accept mail from unidentified transmitters.)

2821/5321 were carefully written to give you the option of
making that decision... or the decision to not accept mail from
even-numbered IP addresses for that matter.  I personally cannot
recommend it unless you have a much better notion of
"identified" than anything you can get out of HELO/EHLO.  I'd
also suggest to you that the typical user who is waiting for a
message to arrive is not likely to distinguish between
"rejected" and "lost".  Unless the former causes the sender to
get on the phone, the only issue is "didn't arrive".  Your users
and experience may differ.

So what are you asking for that isn't there already?
Elimination of the IP address literal option?  Some criteria
for "verifiable" that would presumably lie well outside the
scope of SMTP?

If you can deprecate HELO, surely you can do the same for
address literals.

Independent of some practical issues and the fact that address
literals can sometimes actually provide useful information, that
would accomplish what?    You can already make a policy decision
to reject any message that starts with EHLO and an address
literal; doing so would be consistent with your position that
false rejects are ok as long as one doesn't drop the message.

I can't recommend that we try to dig into the question in the
standard, but consider whether you would prefer:

   EHLO [#.#.#.#]

where "#.#.#.#" is a valid, public, IP address that might even
be static and stable and have a legitimate reverse mapping to a
host name,     and


where "" is a valid, registered, resolvable
FQDN but the only information at its DNS node is

     IN TXT This is a mobile host and I can't really tell you
what or where it is without compromising my privacy.

You don't have much "authentication" information either way; you
might be able to extract as much or more contact information
from the first.

Do keep in mind that we know from experience that the bad guys
have absolutely no trouble obtaining perfectly valid domain

I agree that reputation requirements should be outside the
scope of 5321.  Same for the details of any particular
authentication method.  What I am suggesting is not that level
of detail.

I know you can't turn back the clock and make radical changes
to SMTP, but in revising the spec, we should do everything we
can to make up for the original deficiency in the HELO command
and the lack of attention to security issues.

I think that, had you made this point at the time (others did,
more or less), the DRUMS WG would have told you that the changes
from the language of 821 about that field to the requirement for
a resolvable FQDN _and_ the addition of the IP address literal
option for SMTP clients that didn't reliably know their own
names, was "everything we can to make up for...".  It seems to
me that you are looking for something else entirely, either to
push the Domain name that is expected to appear in that field
beyond its plausible limits (unless you are willing to start
prohibiting some legitimate cases) or to replace the Domain name
with an authentication token or identifier.  And that is why I
suggested in an earlier note that, if you or others really think
that would be useful, you start writing an extension spec for an
authenticator that would supply the information you want, rather
than trying to overload it onto EHLO.  Of course, you'd probably
then have to adopt a policy of rejecting (or assigning a very
low score to) any message that came from a source that didn't
adopt and support your extension, but that is how it goes with
twenty-odd years of installed base.


<Prev in Thread] Current Thread [Next in Thread>