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Versions: (draft-kucherawy-greylisting-bcp) 00 01 02 03 04 05 06 07 08 09 RFC 6647

Individual submission                                       M. Kucherawy
Internet-Draft                                                 Cloudmark
Intended status: Standards Track                              D. Crocker
Expires: September 26, 2012                  Brandenburg InternetWorking
                                                          March 25, 2012

         Email Greylisting: An Applicability Statement for SMTP


   This memo describes the art of email greylisting, the practice of
   providing temporarily degraded service to unknown email clients as an
   anti-abuse mechanism.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 26, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Background . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Types of Greylisting . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Connection-Level Greylisting . . . . . . . . . . . . . . .  4
     2.2.  SMTP HELO/EHLO Greylisting . . . . . . . . . . . . . . . .  5
     2.3.  SMTP MAIL Greylisting  . . . . . . . . . . . . . . . . . .  5
     2.4.  SMTP RCPT Greylisting  . . . . . . . . . . . . . . . . . .  5
     2.5.  SMTP DATA Greylisting  . . . . . . . . . . . . . . . . . .  6
     2.6.  Additional Heuristics  . . . . . . . . . . . . . . . . . .  7
     2.7.  Exceptions . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Benefits and Costs . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Unintended Consequences  . . . . . . . . . . . . . . . . . . .  8
     4.1.  Unintended Mail Delivery Failures  . . . . . . . . . . . .  9
     4.2.  Unintended SMTP Client Failures  . . . . . . . . . . . . . 10
     4.3.  Address Space Saturation . . . . . . . . . . . . . . . . . 11
   5.  Recommendations  . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  Measuring Effectiveness  . . . . . . . . . . . . . . . . . . . 13
   7.  IPv6 Applicability . . . . . . . . . . . . . . . . . . . . . . 13
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
     9.1.  Tradeoffs  . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.2.  Database . . . . . . . . . . . . . . . . . . . . . . . . . 14
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     10.2. Informative References . . . . . . . . . . . . . . . . . . 15
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16

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1.  Introduction

   Preferred techniques for handling email abuse explicitly identify
   good actors and bad actors, giving each significantly differential
   service.  In some cases an actor does not have a known reputation;
   this can justify providing degraded service, until there is a basis
   for provider better service.  This latter approach is known as
   "greylisting".  Broadly, the term refers to any degradation of
   service for an unknown or suspect source, over a period of time.  The
   narrow use of the term refers to generation of an SMTP temporary
   failure reply code for traffic from such sources.  There are diverse
   implementations of this basic concept, and, predictably therefore,
   some blurred terminology.

   This memo documents common greylisting techniques and discusses their
   benefits and costs.  It also defines terminology to enable clear
   distinction and discussion of these techniques.

   There is some confusion in industry that conflates greylisting with
   an SMTP temporary failure for any reason.  The purpose of this memo
   is also to dispel such confusion.

1.1.  Background

   For many years, large amounts of spam have been sent through purpose-
   built software, or "spamware", that supports only a constrained
   version of SMTP.  In particular, such software does not perform
   retransmission attempts after receiving an SMTP temporary failure.
   That is, if the spamware cannot deliver a message, it just goes on to
   the next address in its list since, in spamming, volume counts for
   far more than reliability.  Greylisting exploits this by rejecting
   mail from unfamiliar sources with a "transient (soft) fail" (4xx)
   [SMTP] error code.  Another application of greylisting is to delay
   mail from newly seen IP addresses on the theory that, if it's a spam
   source, then by the time it retries, it will appear in a list of
   sources to be filtered, and the mail will not be accepted.

   Early references for greylisting descriptions and implementations can
   be found at [SAUCE] and [PUREMAGIC].

1.2.  Definitions

1.2.1.  Keywords

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [KEYWORDS].

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1.2.2.  E-Mail Architecture Terminology

   Readers need to be familiar with the material and terminology
   discussed in [MAIL] and [EMAIL-ARCH].

2.  Types of Greylisting

   Greylisting is primarily performed at some phase during an SMTP
   session.  A set of attributes about the client-side SMTP server are
   used for assessing whether to perform greylisting.  At its simplest,
   the attribute is the IP address of the client and the assessment is
   whether it has previously connected, recently.  More elaborate
   attribute combinations and more sophisticated assessment, can be
   performed.  The following discussion covers the most common

2.1.  Connection-Level Greylisting

   Connection-level greylisting decides whether to accept the (TCP)
   connection from a "new" [SMTP] client.  At this point in the
   communication between the client and the server, the only information
   known to the receiving server is the incoming IP address.  This, of
   course, is often (but not always) translatable into a host name.

   The typical application of greylisting here is to keep a record of
   SMTP client IP addresses and/or host names (collectively, "sources")
   that have been seen.  Such a database acts as a cache of known
   senders and might or might not expire records after some period.  If
   the source is not in the database, or the record of the source has
   not reached some required minimum age -- such as 30 minutes since the
   initial connection attempt -- the server does one of the following,
   inviting a later retry:

   o  returns a 421 SMTP reply, and closes the connection;

   o  returns a different 4yz SMTP reply to all further commands in this
      SMTP session

   A useful variant of the basic known/unknown policy is to limit
   greylisting to those addresses that are on some list of IP addresses
   known to be affiliated with bad actors.  Whereas the simpler policy
   affects all new connections, including those from good actors, the
   constrained policy applies greylisting actions only to sites that
   already have a negative reputation.

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2.2.  SMTP HELO/EHLO Greylisting

   HELO/EHLO greylisting refers to the first [SMTP] command verb in an
   SMTP session.  It includes a single, required parameter that is
   supposed to contain the client's fully-qualified host name or its
   literal IP address.

   Greylisting implemented at this phase retains a record of sources
   coupled with HELO/EHLO parameters.  It returns 4yz SMTP replies to
   all commands until the end of the SMTP session if that tuple has not
   previously been recorded or if the record exists but has not reached
   some configured minimum age.

2.3.  SMTP MAIL Greylisting

   MAIL command greylisting refers to the [SMTP] command verb in an SMTP
   session that initiates a new transaction.  It includes at least one
   required parameter that indicates the return email address
   (RFC5321.MailFrom) of the message being relayed from the client to
   the server.

   Greylisting implemented at this phase retains a record of sources
   coupled with return email addresses.  It returns 4yz SMTP replies to
   all commands for the remainder of the SMTP session if that tuple has
   not previously been recorded or if the record exists but has not met
   some configured minimum age.

2.4.  SMTP RCPT Greylisting

   RCPT greylisting refers to the [SMTP] command verb in an SMTP session
   that specifies intended recipients of an email transaction.  It
   includes at least one required parameter that indicates the email
   address of an intended recipient of the message being relayed from
   the client to the server.

   Greylisting implemented at this phase retains a record of tuples that
   combines the provided recipient address with any combination of the

   o  the source, as described above;

   o  the return email address;

   o  the other recipient addresses of the message (if any)

   If the selected tuple is not found in the database, or if the record
   is present but has not reached some configured minimum age, the
   greylisting MTA returns 4yz SMTP replies to all commands for the

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   remainder of the SMTP session.

   Note that often a match on a tuple involving the first valid RCPT is
   sufficient to identify a retry correctly, and further checks can be

2.5.  SMTP DATA Greylisting

   DATA greylisting refers to the [SMTP] command verb in an SMTP session
   that transmits the actual message content, as opposed to its envelope
   details (see [MAIL]).

   This type of greylisting can be performed at two places in the SMTP

   1.  on receipt of the DATA command, because at that point the entire
       envelope has been received (i.e., all MAIL and RCPT commands have
       been issued);

   2.  on completion of the DATA command, i.e., after the "." that
       terminates transmission of the message body, since at that point
       a digest or other analysis of the message could be performed.

   Some implementations do filtering here because there are clients that
   don't bother checking SMTP reply codes to commands other than DATA.

   Numerous greylisting policies are possible at this point.  All of
   them retain a record of tuples that combine the various parts of the
   SMTP transaction in some combination, including:

   o  the source, as described above;

   o  the return email address;

   o  the recipients of the message, as a set or individually;

   o  identifiers in the message, such as the contents of the
      RFC5322.From or RFC5322.To fields;

   o  other prominent parts of the content, such as the RFC5322.Subject

   o  a digest of some or all of the message content, as a test for

   o  analysis of arbitrary portions of the message body.

   (The last four items in that list are only possible at the end of

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   DATA, not on receipt of the DATA command.)

   If the selected tuple is not found in the database, or if the record
   exists but has not reached some configured minimum age, the
   greylisting MTA returns 4yz SMTP replies to all commands for the
   remainder of the SMTP session.

2.6.  Additional Heuristics

   Since greylisting seeks to target spamware, it follows that being
   able to identify spamware within the SMTP context beyond the simple
   notion of "not seen before" would be desirable.  A more targeted
   approach might also include in its selection such heuristics as:

   o  if a [DNSBL] lists an IP address but the implementer wishes to be
      cautious with mitigation actions rather than blocking traffic from
      the IP address outright, then subject it to greylisting;

   o  if the value found in a PTR record follows common naming patterns
      for dynamic IP addresses, then subject it to greylisting.

2.7.  Exceptions

   Most greylisting systems provide for an exception mechanism, allowing
   one to specify IP addresses, IP address [CIDR] blocks, hostnames or
   domain names that are exempt from greylisting checks and thus whose
   SMTP client sessions are not subject to such interference.

   Likely candidates to be excepted from greylisting include those known
   not to retry according to a pattern that will be observed as
   legitimate, and those that send so rarely that they will age out of
   the database.  In both cases the excepted source is known not to be
   an abusive one by the site implementing greylisting.  Otherwise,
   typical non-abusive senders will enter the exception list on the
   first proper retry, and remain there permanently.

   One could also use a [DNSBL] that lists known good hosts as a
   greylisting exception set.

3.  Benefits and Costs

   The most obvious benefit with any of the above techniques is that
   spamware generally does not retry, and is therefore less likely to
   succeed, absent a record of a previous delivery attempts.

   The most obvious detriment to implementing greylisting is the
   imposition of delay on legitimate mail.  Some popular MTAs do not

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   retry failed delivery attempts for an hour or more, which can cause
   expensive delays when delivery of mail is timely.  Worse, some
   legitimate MTAs do not retry at all(!)  The counterargument to this
   "false positive" problem is that email has always been a "best-
   effort" mechanism, and thus this cost is ultimately low in comparison
   to the cost of dealing with high volumes of unwanted mail.  Still,
   the actual effect of such delays can be significant, such as altering
   the tone of a multi-participant discussion to a mailing list.

   The cache of information stored about SMTP client history does not
   benefit legitimate clients that are already listed for acceptance,
   when the clients are subjected to any kind of reconfiguration,
   especially such as network renumbering.  To the greylisting
   implementation such clients are once again unknown, and they will
   once again be subjected to the delay.

   Another obvious cost is for the required database.  It has to be
   large enough to keep the necessary history, and fast enough to avoid
   excessive inefficiencies in the server's operations.  The primary
   consideration is the maximum age of records in the database.  If
   records age out too soon, then hosts that do retry per [SMTP] will be
   periodically subjected to greylisting even though they are well-
   behaved; if records age out after too long a period, then eventually
   spamware that launches a new campaign will not be identified as
   "unknown" in this manner, and will not be required to retry.

   Presuming that known friendly senders will be manually configured as
   exceptions to the greylisting check, a steady state will eventually
   be reached wherein the only mail that is delayed is mail from an IP
   address that has never sent mail before.  Experience suggests that
   the, the vast majority of mail comes from places on such a developed
   exception list, so after a training period, only a small proportion
   of mail is actually affected.  The training period could be replaced
   by processing a history of email traffic and adding the IP addresses
   from which most traffic arrives to the exception list.

   Applying greylisting based on actual message content (i.e., post-
   DATA) is substantially more expensive than any of the other
   alternatives both in terms of the resources required to accept and
   temporarily store a complete message body (which can be quite
   substantial) and any processing that is done on that content.  As a
   consequence, such methods incur more cost during the session and thus
   is not a typical practice.

4.  Unintended Consequences

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4.1.  Unintended Mail Delivery Failures

   There are a few failure modes of greylisting that are worth
   considering.  For example, consider an email message intended for
   user@example.com.  The example.com domain is served by two receiving
   mail servers, one called mail1.example.com and one called
   mail2.example.com.  On the first delivery attempt, mail1.example.com
   greylists the client, and thus the client places the message in its
   outgoing queue for later retry.  Later, when a retry is attempted,
   mail2.example.com is selected for the delivery, either because
   mail1.example.com is unavailable or because a round-robin [DNS]
   evaluation produces that result.  However, the two example.com hosts
   do not share greylisting databases, so the second host again denies
   the attempt.  Thus, although example.com has sought to improve its
   email throughput by having two servers, it has in fact amplified the
   problem of legitimate mail delay introduced by greylisting.

   Similarly, consider a site with multiple outbound MTAs that share a
   common queue.  On a first outbound delivery attempt to example.com,
   the attempt is grey listed.  On a later retry, a different outbound
   MTA is selected, which means example.com sees a different source, and
   once again greylisting occurs on the same message.

   For systems that do DATA-level greylisting, if any part of the
   message has changed since the first attempt, the tuple constructed
   might be different than the one for the first attempt, and the
   delivery is again greylisted.  Some MTAs do reformulate portions of
   the message at submission time and this can produce visible
   differences for each attempt.

   A host that sends mail to a particular destination infrequently might
   not remain "known" in the receiving server's database and will
   therefore be greylisted for a high percentage of mail despite
   possibly being a legitimate sender.

   All of these and other similar cases can cause greylisting to be
   applied improperly to legitimate MTAs multiple times, leading to long
   delays in delivery or ultimately the return of the message to its
   sender.  Other side effects include out-of-order delivery of related,
   sequenced messages.

   Address translation technologies such as [NAT] cause distinct MTAs to
   appear to come from a common IP address.  This can cause greylisting
   to be applied only to the first connection attempt from the shared IP
   address, meaning future MTAs connecting for the first time will be
   exempted from the protection greylisting provides.

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4.2.  Unintended SMTP Client Failures

   Atypical SMTP client behaviours also need to be considered when
   deploying greylisting.

   Some clients do not retry messages for very long periods.  Popular
   open source MTAs implement increasing backoff times when messages
   receive temporary failure messages, and/or degrade queue priority for
   very large messages.  This means greylisting introduces even more
   delay for MTAs implementing such schemes, and the delay can become
   large enough to become a nuisance to users.

   Some clients do not retry messages at all.  This means greylisting
   will cause outright delivery failure right away for sources,
   envelopes, or messages that it has not seen before, regardless of the
   client attempting the delivery, essentially treating legitimate mail
   and spam the same.

   If a greylisting scheme requires a database record to have reached a
   certain age rather than merely testing for the presence of the record
   in the database, and the client has a retry schedule that is too
   aggressive, the client could be subjected to rate limiting by the MTA
   independent of the restrictions imposed by greylisting.

   Some SMTP implementations make the error of treating all error codes
   as fatal; that is, a 4yz [SMTP] response is treated as if it were a
   5yz response, and the message is returned to the sender as
   undeliverable.  This can result in such things as inadvertent removal
   from mailing lists in response to the perceived rejections.

   Some clients encode message-specific details in the address parameter
   to the [SMTP] MAIL command.  If doing so causes the parameter to
   change between retry attempts, a greylisting implementation could see
   it as a new delivery rather than a retry, and disallow the delivery.
   In such cases, the mail will never be delivered, and will be returned
   to the sender after the retry timeout expires.

   A client subjected to greylisting might move to the next host found
   in the ordered [DNS] MX record set for the destination domain and re-
   attempt delivery.  This has several considerations of its own:

   o  An increase in traffic to those alternate servers merely as a
      result of greylisting.

   o  Alternate (MX) servers SHOULD share the same greylisting database.
      When they do not -- as is often true when the servers occupy
      different Administrative Management Domains (ADMDs) -- SMTP
      clients can see variable treatment if they try to send to

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      different MX hosts.

   o  When alternate MX servers relay mail back to the "primary" MX
      server, the latter SHOULD be configured to permit the other
      servers to relay mail without being subjected to greylisting.

   There are some applications that connect to an SMTP server and
   simulate a transaction up to the point of sending the RCPT command in
   an attempt to confirm that an address is valid.  Some of these are
   legitimate applications (e.g., mailing list servers) and others are
   automated programs that attempt to ascertain valid addresses to which
   to send spam (a "directory harvesting" attack).  Greylisting can
   interfere with both instances, with harmful effects on the former.

4.3.  Address Space Saturation

   Greylisting is obviously not a fool-proof solution to avoiding
   abusive traffic.  Bad actors that send mail with just enough
   frequency to avoid having their records expire will never be caught
   by this mechanism after the first instance.

   Where this is a concern, combining greylisting with some form of
   reputation service that estimates the likely behaviour for IP
   addresses that are not intercepted by the greylisting function would
   be a good choice.

5.  Recommendations

   The following practices are RECOMMENDED based on collected

   1.  Implement greylisting based a tuple consisting of (IP address,
       RFC5321.MailFrom, and the first RFC5321.RcptTo).  It has shown
       sufficient to use only the first RFC5321.RcptTo as legitimate
       MTAs appear not to reorder recipients between retries.  Including
       RFC5321.MailFrom improves accuracy where the IP address is being
       matched in clusters (e.g., CIDR blocks) rather than precisely
       (see below).  After a successful retry, allow all further [SMTP]
       traffic from the IP address in that tuple regardless of envelope

   2.  Include a time window within which a retry from a greylisted host
       is considered, and ignored otherwise.  The default window SHOULD
       range from one minute to 24 hours.  Retries during the period of
       this window are permitted and satisfy the greylisting test, and
       thus the client is no longer likely to be spamware; retries after
       the end of the window SHOULD be considered to be a new message

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       for the purposes of greylisting evaluation (i.e., reset the
       "first seen" timestamp for that IP address).  Some sites use a
       higher time value for the low end of the window time to match
       common legitimate MTA retry timeouts, but additional benefit from
       doing so appears unlikely.

   3.  Include a timeout for database entries, after which records for
       IP addresses that have generated no recent traffic are deleted.
       This step is intended to re-enable greylisting for an IP address
       in the event that it has changed "owners", and will subject the
       client to another round of greylisting.  The default SHOULD be at
       least one week.

   4.  For an Administrative Domain (ADMD) all inbound border MTAs
       listed in the [DNS] SHOULD share a common greylisting database
       and common greylisting policies.  This handles sequences in which
       a client's retry goes to a different server after the first 4yz
       reply, and it lets all servers share the list of hosts that did
       retry successfully.

   5.  To accommodate those senders that have clusters of outgoing mail
       servers, greylisting servers MAY track CIDR blocks of a size of
       its own choosing, such as /24, rather than the full IPv4 address.
       (Note, however, that this heuristic will not work for clusters
       having machines on different networks.)  A similar grouping
       capability MAY be established based on the domain name of the
       mail server if one can be determined.

   6.  Include a manual override capability for adding specific IP
       addresses or network blocks that always bypass checks.  There are
       legitimate senders that simply don't respond well to greylisting
       for a variety of reasons, most of which do not conflict with
       [SMTP].  There are also some highly visible online entities such
       as email service providers that will be certain to retry, and
       thus those that are known SHOULD be allowed to bypass the filter.

   7.  Greylisting SHOULD NOT be applied by an ADMD's submission service
       (see [SUBMISSION]) for authetnicated client hosts.
       Authentication can include whatever mechanisms are deemed
       appropriate for the ADMD, such as known internal IP addresses,
       protocol-level client authentication, or the like.

   There is no specific recommendation as to the specific choice of 4yz
   code to be returned as a result of a greylisting delay.  Per [SMTP],
   however, the only two reasonable choices are 421 if the
   implementation wishes to terminate the connection immediately, and
   450 otherwise.  It is possible that some clients treat different 4yz
   codes differently, but no data are available on whether using 421

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   versus some other 4yz code is particularly advantageous.

   There is also no specific recommendation as to the choice of text to
   include in the SMTP reply, if any.  Some implementers argue that
   indicating that greylisting is in effect can give spamware a hint as
   to when to try again for successful delivery, while others suspect
   that it won't matter to spamware and thus the more likely audience is
   legitimate senders seeking to understand why their mail is being

6.  Measuring Effectiveness

   A few techniques are common when measuring the effectiveness of
   greylisting in a particular installation:

   o  Arrange to log the spam vs. legitimate determinations of messages
      and what the greylisting decision would have been if enabled; then
      determine whether there is a correlation (and, of course, whether
      too much legitimate email would also be affected);

   o  Continuing from the previous point, query the set of IP addresses
      subjected to greylisting in any popular [DNSBL] to see if there is
      a strong correlation.

7.  IPv6 Applicability

   The descriptions and recommendations presented in this memo are based
   on many years of experience with greylisting in the IPv4 Internet
   environment, and so they clearly pertain to IPv4 deployments only.

   The greater size of an IPv6 address seems likely to permit
   differences in behaviours by bad actors, and this could well mean
   needing to alter the details for applying greylisting; it might even
   negate any benefits in using greylisting at all.  At a minimum, it is
   likely to call for different specific choices for any greylisting
   algorithm variables.

   In addition, an obvious consideration is that the size of the
   database required to store records of all of the IP addresses seen
   will likely be substantially larger in the IPv6 environment.

8.  IANA Considerations

   No actions are requested of IANA in this memo.

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   [RFC Editor: Please remove this section prior to publication.]

9.  Security Considerations

   This section discusses potential security issues related to

9.1.  Tradeoffs

   The discussion above highlights the fact that, although greylisting
   provides some obvious and valuable defenses, it can introduce
   unintentional and detrimental consequences for delivery of legitimate
   mail.  Where timely delivery of email is essential, especially for
   security-related applications, the possible consequences of such
   systems need to be carefully considered.

   Specific sources can be exempted from greylisting, but of course that
   means they have elevated privilege in terms of access to the
   mailboxes on the greylisting system, and malefactors can seek to
   exploit this.

9.2.  Database

   The database that has to be maintained as part of any greylisting
   system will grow as the diversity of its SMTP clients hosts grows,
   and of course is larger in general depending on the nature of the
   tuple stored about each delivery attempt.  Even with a record aging
   policy in place, such a database could grow large enough to interfere
   with the system hosting it, or at least to a point at which
   greylisting service is degraded.  Moreover, an attacker knowing which
   greylisting scheme is in use could rotate parameters of SMTP clients
   under its control, in an attempt to inflate the database to the point
   of denial-of-service.

   Implementers could consider configuring an appropriate failure policy
   so that something locally acceptable happens when the database is
   attacked or otherwise unavailable.

   In practice, this has not appeared as a serious concern, because any
   reasonable aging policy successfully moderates database growth.  It
   is nevertheless identified here as a consideration as there may be
   implementations in some environments where this is indeed an issue.

10.  References

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10.1.  Normative References

              Crocker, D., "Internet Mail Architecture", RFC 5598,
              October 2008.

              Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

              Gellens, R. and J. Klensin, "Message Submission for Mail",
              RFC 6409, November 2011.

10.2.  Informative References

   [CIDR]     Fuller, V. and T. Li, "Classless Inter-domain Routing
              (CIDR): The Internet Address Assignment and Aggregation
              Plan", RFC 4632, August 2006.

   [DNS]      Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [DNSBL]    Levine, J., "DNS Blacklists and Whitelists", RFC 5782,
              February 2010.

   [MAIL]     Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [NAT]      Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              January 2001.

              Harris, E., "The Next Step in the Spam Control War:
              Greylisting", August 2003, <http://projects.puremagic.com/

   [SAUCE]    Jackson, I., "GNU SAUCE", 2001,

Appendix A.  Acknowledgments

   The author wishes to acknowledge Mike Adkins, Steve Atkins, Mihai

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   Costea, Dave Crocker, Derek Diget, Peter J. Holzer, John Levine,
   Chris Lewis, Jose-Marcio Martins da Cruz, John Klensin, S. Moonesamy,
   Suresh Ramasubramanian, Mark Risher, Jordan Rosenwald, Gregory
   Shapiro, Joe Sniderman, Roland Turner, and Michael Wise for their
   contributions to this memo.  The various participants of the MAAWG
   Open Sessions about greylisting were also valued contributors.

Authors' Addresses

   Murray S. Kucherawy
   128 King St., 2nd Floor
   San Francisco, CA  94107

   Phone: +1 415 946 3800
   Email: msk@cloudmark.com

   D. Crocker
   Brandenburg InternetWorking
   675 Spruce Dr.
   Sunnyvale  94086

   Phone: +1.408.246.8253
   Email: dcrocker@bbiw.net
   URI:   http://bbiw.net

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