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Re: [Asrg] Re: RMX evaluation / Paul Vixie's procedure
2003-05-09 00:54:10
RMX, MAIL-FROM, DSDNSBL all fail for mailing lists and for forwarding no
matter if you compare it to "MAIL FROM" or to envelope "From" header or both.
Envelope "From:" is the worst case since most mailing lists use their own
mailfrom and do not change "From:" (do not assume that what you see in
outlook is what others would see or that its really how mailist messsage
looks like), so when your message arrives and mailfrom-aware recepient
server checks and sees that envelope "From:" is from domain that has
mailfrom record but connecting mailserver is not on list of those domain
outgoing mail servers, then it would reject that email - to deal with this
you have to whitelist maillist to let it through. But if you whitelist
maillist then spammer can use that and forge "mailfrom" to appear that
message is coming out of maillist and then your server will accept the
email eventhough it came from spammer and eventhough he did not have
right to use this envelope "From:".
If you use MAIL FROM header then you are able to accept emails from most
mailing lists but its trivial for spammer to choose some domain for
mail-from that has no mailfrom record but since most users consider
"From:" as sender of the message, this still means spammer can appear to
have sent email from address when he has no right to do so. In addition
there is a bigger problem with forwarders and with maillists that
actually use the original MAIL FROM (how many times have you received
bounces from end-user servers when you send email to mailing list? - by
SMTP specs all bounces go to MAIL FROM). And again a reminder - "MAIL FROM"
is address for sending bounces - it is not the best idea to use as
the only means of verifiyng origin of the email.
Add to this that some mail service providers (such as hotmail) mark as spam
all emails that do not have the same email address for MAIL FROM as for
envelope "From:". This leads to blockage of pretty much any maillist and
forwarder of mailfrom or rmx where to be used.
I do belive that making originator verification linked through DNS is a
good idea and some sort systems which has original in RMX or Mail From
proposals can be used as part of larger solution but not something as
simple as just checking an ip of connecting mail server.
And remember - when evaluating proposals that effect STMP routing (but do
not replace it with new protocol) you have to consider transition situation
as being of the most importance - i.e. when some are aware of your changes
to email system but most are not (this can go on for 5-10 years!). That a
proposal might work if everybody has upgraded can in fact lead to failure of
proposal if its initial impact is too high. Such would be the case with
RMX since if any large domain setup such record (and large domains such as
aol, hotmail, msn, etc are the ones most often forged) and their user's
emails are bouncing, they will soon remove it and not try it again.
This will probably be my only post on current ongoing mailfrom & rmx
discussion, though I'v extensively commented on this before on this
mail list. I'm not against these proposals, but having evaluated them
I do not believe any current ones will work and I'm convinced their
implementation will lead to more harm then good.
On Fri, 9 May 2003, David Walker wrote:
I have to admit that this does seem to be a well laid out and workable method.
(any comments I may have made earlier to the contrary were based on
incomplete
and inaccurate information)
It accomplishes all of the goals of RMX and similar schemes and it is
reasonably close to accepted standards and doesn't abuse them and it does not
require any programming changes on the DNS end.
Due to interactions with internet mailing lists I personally feel that the
comparison should be against the envelope-from instead of mail-from and that
differences between the envelope-from and mail-from should be displayed in
the MUA. I believe Outlook (I know, not a good example but an example)
displays this information via the Sender or X-Sender header if I remember
correctly. The output resembles
"From: asrg(_at_)ietf(_dot_)org on behalf of paul(_at_)vix(_dot_)com"
Are there any/very many servers configured this way currently?
On Thursday 08 May 2003 12:47 pm, Paul Vixie wrote:
I searched for but failed to find Paul Vixie's short mail message
describing the idea. ...
Independent Paul Vixie (Ed.)
Request for Comments: XXXX Category: Experimental
June 6, 2002
Repudiating MAIL FROM
Status of this Memo
This memo describes an experimental procedure for handling received
e-mail. It does not specify an Internet standard of any kind.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
At the time of this writing, more than half of all e-mail received by
the author has a forged return address, due to the total absence of
address authentication in SMTP (see [RFC2821]). We present a simple
and backward compatible method whereby cooperating e-mail senders and
receivers can detect forged source/return addresses in e-mail.
1 - Introduction and Overview
1.1. Internet e-mail return addresses are nonrepudiable by design of the
relevant transport protocols (see [RFC2821]). Simply put, there is no
cause for ANY confidence in the proposition "this e-mail came from where
it says it came from."
1.2. Irresponsible actors who wish to transmit unwanted bulk e-mail
routinely use this designed-in lack of source/return authenticity to
hide their point of origin, which usually involves forging a valid
return address belonging to some highly visible and popular ISP (for
example, HOTMAIL.COM).
1.3. Recipients who wish to reject unwanted bulk e-mail containing
forged source/return addresses are prevented from doing so since the
addresses, as presented, are nonrepudiable by design. Simply put, there
would be too many false positives, and too much valid e-mail rejected,
if one were to program an e-mail relay to "reject all e-mail claiming to
be from HOTMAIL.COM" since, statistically, most e-mail claiming to be
from HOTMAIL.COM is actually from somewhere else. HOTMAIL.COM, in this
example, is a victim of forgery.
Vixie Experimental [Page 1]
RFC XXXX Repudiating MAIL FROM May 26, 2002
1.4. What's needed is a way to guaranty that each received e-mail
message did in fact come from some mail server or relay which can
rightfully originate or relay messages from the purported source/return
address.
1.5. Approaches of the form "use PGP" and "use SSL" are not scalable in
the short term since they depend on end-to-end action and there are just
too many endpoints. An effective solution has to be applicable to mail
relay, not just final delivery.
1.6. Valid ("wanted") e-mail must not be rejected by side effect or
partial adoption of this proposal. Source/return authenticity must be a
confidence effector, as in "we can be sure that this did not come from
where it claims" and simple uncertainty must remain in effect otherwise.
2 - Behaviour
2.1. Domain owners who wish their mail source/return information to be
repudiable will enter stylized MX RR's into their DNS data, whose owner
name is "MAIL-FROM", whose priority is zero, and whose servername
registers an outbound (border) relay for the domain. For example, to
tell the rest of the Internet who they should believe when they receive
mail claiming to be from VIXIE(_at_)ISC(_dot_)ORG, the following DNS MX
RR's should
be entered:
$ORIGIN isc.org.
MAIL-FROM MX 0 rc
MX 0 rc1
In this example, hosts RC.ISC.ORG, and RC1.ISC.ORG are given as
appropriate places to originate mail from @ISC.ORG. Note that this
differs from the normal inbound MX RRset for this example domain:
$ORIGIN isc.org.
@ MX 0 rc
MX 0 isrv4
So, the inbound mail server set partially overlaps with, but differs
from, the example outbound mail server set. This is quite common in the
Internet, and is the reason why the normal inbound mail server set
described by a domain's apex MX RRset cannot be used for repudiation
purposes.
Vixie Experimental [Page 2]
RFC XXXX Repudiating MAIL FROM May 26, 2002
2.2. Second-stage relays such as ISP mail servers are often used and can
be described by adding as many relays as necessary to the MAIL-FROM MX
RRset. In our example, if ISC sometimes used UUNET for outbound mail
services, the DNS data describing this relationship might be as follows:
$ORIGIN isc.org.
MAIL-FROM MX 0 rc
MX 0 rc1
MX 0 uunet.uu.net.
Let it be noted that a domain owner's power to repudiate forged e-mail
is only as strong as the security policy of its registered inbound and
outbound mail relays, and that if such a relay is (for example) open to
third party relay, then no value will be added by registering a domain
MAIL-FROM MX containing that relay, and no inbound MAIL-FROM checking
will be possible on final delivery relays for a domain @ MX containing
that relay. Multistage relays (both inbound and outbound) are a
breeding ground for anonymity unless they are very carefully configured.
2.3. SMTP receivers wishing to attempt repudiation on inbound e-mail
would check the SMTP (see [RFC2821]) MAIL FROM payload at the time of
receipt. The precise method to be used is as follows:
Vixie Experimental [Page 3]
RFC XXXX Repudiating MAIL FROM May 26, 2002
on (MAIL FROM mailfrom) {
switch (repudiated(mailfrom, ipsource))
case tempfail:
smtpreply(450, "temporary dns failure, try again later")
break
case repudiated:
smtpreply(550, "surely you're joking")
dsn("5.7.1", "delivery not authorized")
invalidate() // reject all but QUIT and RSET
break
}
}
repudiated(mailfrom, ipsource) = {
(lhs, rhs) = parse_addr(mailfrom);
// handle "MAIL FROM:<>" somehow
mxset = get_mx("MAIL-FROM" "." rhs);
if (mxset == NULL)
return nonrepudiated;
mxset += configured(perimeter_relays);
foreach mx (mxset) {
aset = get_a(mx.server);
if (ipsource IN aset)
return nonrepudiated;
}
return repudiated;
}
(EDITOR'S NOTE: need to establish a value for 5xx.)
The method amounts to "if there's a MAIL-FROM for the purported domain
and if the IP source isn't on the resulting list, then reject the mail".
Multistage inbound relays are allowed for, by implicitly appending one's
own outer perimeter relay names to every extant MAIL-FROM.
3 - Impact
3.1. This specification is optional, and will only affect cooperating
parties. Any domain owner who does not enter a MAIL-FROM will be
unaffected, and any SMTP receiver who does not look for a MAIL-FROM at
time of receipt will be unaffected. However, both parties working
together CAN work to repudiate forged e-mail return/source information.
3.2. Transport-level e-mail forwarding must be more explicit under this
specification. For example if VIXIE(_at_)NETBSD(_dot_)ORG's account has
a
Vixie Experimental [Page 4]
RFC XXXX Repudiating MAIL FROM May 26, 2002
".forward" file pointing at VIXIE(_at_)ISC(_dot_)ORG, then e-mail sent
to the
former will be received by the latter, but with no change in the payload
of SMTP MAIL FROM. Thus, ISC's inbound relays would have to be
configured to implicitly add NETBSD's outbound mail relays as
"multistage inbound relays." This could scale poorly and may add
pressure toward transport remailing (with a new envelope) rather than
transport forwarding (reusing the old envelope.)
3.3. Roaming hosts such as laptop computers will probably not be able to
be listed in the MAIL-FROM MX RR for their return address domain name,
and may be forced to use an intermediary for outbound e-mail. STARTTLS
or an SSL/SSH tunnel back "home" may become a necessary first hop for
mobile e-mail.
3.4. The likely endgame for this behaviour is to force senders of
unwanted bulk e-mail to stop lying about who they are, which is illegal
in meatspace anyway -- but such laws are unenforceable due to the nature
of the Internet's mail system. Under this proposal, any domain owner
who is the victim of forgery can respond by adding MAIL-FROM data to
their DNS zone, and any relay owner who is the victim of forged unwanted
e-mail can respond by checking for MAIL-FROM data upon receipt of all
incoming e-mail.
3.5. The DNS TTL for MAIL-FROM MX RRsets ought to be shorter than for
the corresponding domain's apex MX RR, since the cost of widely cached
wrong information is much higher for outbound repudiation data than for
inbound delivery data. Consider that an incomplete apex MX RR can cause
mail to be delivered by a suboptimal path, whereas an incomplete MAIL-
FROM MX RR can cause valid mail to be rejected by relays who attempt
repudiation.
4 - Security Considerations
In the continuing absence of widely deployed security for DNS, this
proposal effectively places an access control list for forged
source/return information in a place where it can be attacked. However,
it must be noted that the current senders of forged unwanted bulk e-mail
are typically not technologically capable of attacking the DNS to insert
forged MAIL-FROM data.
5 - Acknowledgements
This idea originated with Jim Miller <jmiller(_at_)jcmco(_dot_)com> in
1998.
Vixie Experimental [Page 5]
RFC XXXX Repudiating MAIL FROM May 26, 2002
5 - Author's Address
Paul Vixie
950 Charter Street
Redwood City, CA 94063
+1 650 779 7000
paul(_at_)vix(_dot_)com
Vixie Experimental [Page 6]
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