Re: Last Call: draft-klensin-rfc2821bis: closing the implicit MX issue

[Douglas Otis <[e-mail]>]

On Apr 14, 2008, at 7:33 PM, Tony Hansen wrote:

> The SMTP implementations that have made the transition to support  
> IPv6 appear to already have done it in a way that supports AAAA  
> records for the implicit MX case. In some cases they are following  
> RFC 3974, and other cases they are just using getaddrinfo() and  
> letting it do the rest. Note that RFC 3974 itself was supposedly  
> "based on experience in deploying IPv6". At least one of these MTAs  
> is in common use around the network in the IPv4 world.
>
> In essence, these implementations are following the RFC 821 and RFC  
> 974 (sans WKS) rules of looking for address records. They've ignored  
> the A-only wording of RFC 2821 and are acting like we specify  
> currently in 2821bis-09.
>
> In my queries I haven't yet found any IPv6-capable SMTP server that  
> doesn't do it.
>
> I've seen examples of sites that are in regular use that mail would  
> break in an IPv6-only world if implicit MX to AAAA did not work.
>
> From this viewpoint, running code wins.

Abusive running code wins as well.  Indeed, many bad-actors will  
appreciate not being limited to A or MX records, as specifically  
specified in RFC 2821 discovery mechanisms.  Whether the specifics of  
this specification were in error is a separate issue, where endorsing  
AAAA records as a valid discovery mechanism represents a substantial  
change, and one likely to prove unsuccessful when more widely adopted  
and then abused.  The A resource record enabled a transition to MX  
resource records.  The necessity of the implied MX record is no longer  
justifiable.  Permitting A records as a means to discover SMTP servers  
often generates a steady amount of abuse by bad-actors that use A  
record only host-names as spoofed domains in their email-address.

To overcome what is destine to become an undesirable MX fall-back  
mode, some have suggested bogus MX records could be published  
instead.  Bogus MX records would then become perhaps the _only_ means  
to protect hosts not involved in the public reception of SMTP  
messages.  Much of the undesired traffic does not directly emanate  
from clients controlled by bad-actors.  Often, undesired traffic  
results from attempts to validate oft spoofed email sources.  The  
level of this undesired traffic can be substantial.

> I'm also swayed by the principle of "least surprise". Some of the  
> responses I've gotten have been along the lines of "Why's this a  
> question? Of course you do AAAA lookup". One person who had a site  
> set up with an IPv6-only connection and no MX record told me "I  
> wanted to forward my e-mail to an account on that machine. It worked  
> the first time, so I didn't see a need to change it." As mentioned  
> above, at least one of the IPv6-capable MTAs is in common use around  
> the network in the IPv4 world, and turning on IPv6 on those boxes  
> should not cause surprises.

Those wanting inbound SMTP on their IPv6 only hosts can routinely  
include MX records in addition to their AAAA records.  Keeping a  
requirement to publish either A or MX records would alleviate the rest  
of the world from seeking protection by publishing bogus MX records  
for all hosts where inbound SMTP is not desired.  High levels of abuse  
often require public inbound receivers to specialize in defending  
against the abusive traffic.  The percentage of public SMTP servers  
represent a shrinking minority of hosts that might benefit from a  
convenience of not needing to publish MX records.  However, to improve  
both the performance of SMTP servers in general, and acceptance rates  
of IPv6 only hosts, publishing MX records for an email domain is  
likely to become increasingly critical.  "Least surprise" is assured  
by discovery methods that work on a large scale while also limiting  
avenues for abuse.  Having AAAA records imply MX resource records on a  
large scale without rampant abuse would be astonishing.

> Last of all, I'm swayed by the discussions around RFC 974 and the  
> DRUMS archive search around the question of what led to the wording  
> change in 2821bis saying explicitly to do A lookups. These indicate  
> that the intent of adding the A record description was to be  
> descriptive, not prescriptive nor proscriptive.

SMTP represents a great achievement.  Much of this achievement has  
occurred by minimizing administrative efforts needed to establish SMTP  
services.  These minimal administrative efforts now play a significant  
role in fostering current levels of abuse afflicting SMTP.   
Standardizing on AAAA as an MX fall-back moves SMTP and IPv6 in the  
wrong direction.  Sensitivity to abuse adopts a practice of "opt-in"  
rather than "opt-out".   AAAA as an MX fall-back may necessitate SMTP  
IPv6 hosts the need to publish bogus MX records for the specific  
purpose of "opting out".

> So the bottom line is that I see sufficient support for including  
> AAAA lookups when implicit MX comes into play.

It seems the specification should attempt to differentiate what might  
be achievable within private versus public server configurations.

> It's been suggested that 2821bis revert back to either the implicit  
> MX description found in RFC 821 or RFC 974, although Glen Anderson  
> had some suggested improvements to that latter's description that do  
> make it clearer. Any of these three would satisfy this decision, and  
> I'll let John choose the wording he prefers.

While ensuring that the 'S' in SMTP upholds the concept of Simple,  
avoiding greater levels of abuse will help further that goal.  While  
those that live and breath SMTP don't want to see any administrative  
dependencies added, this costs hosts running unrelated protocols that  
must then terminate a seemingly endless stream of SMTP related abuse.   
The success of SMTP requires greater administrative care be taken.  To  
take this to the logical conclusion, the transition to IPv6 with a  
reduction in SMTP related abuse would be further aided by deprecating  
use of A records as an MX fall-back mechanism published in a follow-on  
draft, where the current draft could warn of this possible change.

-Doug



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