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rfc:rfc3974

Network Working Group M. Nakamura Request for Comments: 3974 Kyoto University Category: Informational J. Hagino

                                               IIJ Research Laboratory
                                                          January 2005
     SMTP Operational Experience in Mixed IPv4/v6 Environments

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).

IESG Note:

 The content of this RFC was at one time considered by the IETF, and
 therefore it may resemble a current IETF work in progress or a
 published IETF work.  This RFC is not a candidate for any level of
 Internet Standard.  The IETF disclaims any knowledge of the fitness
 of this RFC for any purpose, and in particular notes that the
 decision to publish is not based on IETF review for such things as
 security, congestion control, or inappropriate interaction with
 deployed protocols.  The RFC Editor has chosen to publish this
 document at its discretion.  Readers of this RFC should exercise
 caution in evaluating its value for implementation and deployment.
 This document contains a specific interpretation of the applicability
 of the MX processing algorithm in RFC 2821, Section 5, to dual-stack
 environments.  Implementors are cautioned that they must reference
 RFC 2821 for the full algorithm; this document is not to be
 considered a full restatement of RFC 2821, and, in case of ambiguity,
 RFC 2821 is authoritative.

Abstract

 This document discusses SMTP operational experiences in IPv4/v6 dual
 stack environments.  As IPv6-capable SMTP servers are deployed, it
 has become apparent that certain configurations of MX records are
 necessary for stable dual-stack (IPv4 and IPv6) SMTP operation.  This
 document clarifies the existing problems in the transition period
 between IPv4 SMTP and IPv6 SMTP.  It also defines operational
 requirements for stable IPv4/v6 SMTP operation.

Nakamura & Hagino Informational [Page 1] RFC 3974 SMTP in Dual Stack Environments January 2005

 This document does not define any new protocol.

1. Introduction

 Delivery of mail messages to the final mail drop is not always done
 by direct IP communication between the submitter and final receiver,
 and there may be some intermediate hosts that relay the messages.  So
 it is difficult to know at message submission (also at receiver side)
 that all intermediate relay hosts are properly configured.  It is not
 easy to configure all systems consistently since the DNS
 configuration used by mail message delivery systems is more complex
 than other Internet services.  During the transition period from IPv4
 to IPv6, more care should be applied to IPv4/v6 interoperability.
 This document talks about SMTP operational experiences in IPv4/v6
 dual stack environments.  As IPv6-capable SMTP servers are deployed,
 it has become apparent that certain configurations of MX records are
 necessary for stable dual-stack (IPv4 and IPv6) SMTP operation.
 This document does not discuss the problems encountered when the
 sending MTA and the receiving MTA have no common protocol (e.g., the
 sending MTA is IPv4-only while the receiving MTA is IPv6-only).  Such
 a situation can be resolved by making either side dual-stack or by
 making either side use a protocol translator (see Appendix A on
 issues with protocol translator).

2. Basic DNS Resource Record Definitions for Mail Routing

 Mail messages on the Internet are typically delivered based on the
 Domain Name System [Mockapetris].  MX RRs are looked up in DNS to
 retrieve the names of hosts running MTAs associated with the domain
 part of the mail address.  DNS lookup uses IN class for both IPv4 and
 IPv6, and similarly IN MX records will be used for mail routing for
 both IPv4 and IPv6.  Hosts which have IPv6 connectivity and also want
 to have the mails delivered using IPv6 must define IPv6 addresses for
 the host name as well as IPv4 addresses [Thomson].
 An MX RR has two parameters, a preference value and the name of
 destination host.  The name of the destination host will be used to
 look up an IP address to initiate an SMTP connection [Partridge].

Nakamura & Hagino Informational [Page 2] RFC 3974 SMTP in Dual Stack Environments January 2005

 For example, an IPv6-only site may have the following DNS
 definitions:
    example.org.            IN MX   1  mx1.example.org.
                            IN MX   10 mx10.example.org.
    mx1.example.org.        IN AAAA 2001:db8:ffff::1
    mx10.example.org.       IN AAAA 2001:db8:ffff::2
 In the transition period from IPv4 to IPv6, there are many IPv4-only
 sites, and such sites will not have mail interoperability with IPv6-
 only sites.  For the transition period, all mail domains should have
 MX records such that MX targets with IPv4 and IPv6 addresses exist,
 e.g.,
    example.org.            IN MX   1  mx1.example.org.
                            IN MX   10 mx10.example.org.
    mx1.example.org.        IN AAAA 2001:db8:ffff::1
                            IN A    192.0.2.1
    mx10.example.org.       IN AAAA 2001:db8:ffff::2
                            IN A    192.0.2.2
 But, not every MX target may support dual-stack operation.  Some host
 entries may have only A RRs or AAAA RRs:
    example.org.            IN MX   1  mx1.example.org.
                            IN MX   10 mx10.example.org.
    mx1.example.org.        IN AAAA 2001:db8:ffff::1
    mx10.example.org.       IN A    192.0.2.1
 The following sections discuss how the sender side should operate
 with IPv4/v6 combined RRs (section 3), and how the receiver should
 define RRs to maintain interoperability between IPv4 and IPv6
 networks (section 4).

3. SMTP Sender Algorithm in a Dual-Stack Environment

 In a dual-stack environment, MX records for a domain resemble the
 following:
    example.org.            IN MX   1  mx1.example.org.
                            IN MX   10 mx10.example.org.
    mx1.example.org.        IN A    192.0.2.1        ; dual-stack
                            IN AAAA 2001:db8:ffff::1
    mx10.example.org.       IN AAAA 2001:db8:ffff::2 ; IPv6-only
 For a single MX record, there are multiple possible final states,
 including: (a) one or more A records for the IPv4 destination, (b)
 one or more AAAA records for the IPv6 destination, (c) a mixture of A

Nakamura & Hagino Informational [Page 3] RFC 3974 SMTP in Dual Stack Environments January 2005

 and AAAA records.  Because multiple MX records may be defined using
 different preference values, multiple addresses must be traversed
 based on multiple MXs.  Domains without MX records and failure
 recovery cases must be handled properly as well.
 The algorithm for a dual-stack SMTP sender is basically the same as
 that for an IPv4-only sender, but it now includes AAAA lookups of MX
 records for SMTP-over-IPv6 delivery.  IPv4/v6 dual stack destinations
 should be treated just like multihomed destinations, as described in
 RFC 2821 [Klensin], section 5.  When there is no destination address
 record found (i.e., the sender MTA is IPv4-only and there are no A
 records available), the case should be treated just like MX records
 without address records, and deliveries should fail.
    ; if the sender MTA is IPv4-only, email delivery to a.example.org
    ; should fail with the same error as deliveries to b.example.org.
    a.example.org.          IN MX   1  mx1.a.example.org.
    mx1.a.example.org.      IN AAAA 2001:db8:ffff::1 ; IPv6-only
    b.example.org.          IN MX   1  mx1.b.example.org. ; no address
 An algorithm for a dual-stack SMTP sender is as follows:
 (1)  Lookup the MX record for the destination domain.  If a CNAME
      record is returned, go to the top of step (1) with replacing the
      destination domain by the query's result.  If any MX records are
      returned, go to step (2) with the query's result (explicit MX).
      If NODATA (i.e., empty answer with NOERROR(0) RCODE) is
      returned, there is no MX record but the name is valid.  Assume
      that there is a record like "name.  IN MX 0 name." (implicit MX)
      and go to step (3).  If HOST_NOT_FOUND (i.e., empty answer with
      NXDOMAIN(3) RCODE) is returned, there is no such domain.  Raise
      a permanent email delivery failure.  Finish.  If SERVFAIL is
      returned, retry after a certain period of time.
 (2)  Compare each host name in MX records with the names of the
      sending host.  If there is match, drop MX records which have an
      equal or larger value than the lowest-preference matching MX
      record (including itself).  If multiple MX records remain, sort
      the MX records in ascending order based on their preference
      values.  Loop over steps (3) to (9) on each host name in MX
      records in a sequence.  If no MX records remain, the sending
      host must be the primary MX host.  Other routing rules should be
      applied.  Finish.
 (3)  If the sending MTA has IPv4 capability, lookup the A records.
      Keep the resulting addresses until step (5).

Nakamura & Hagino Informational [Page 4] RFC 3974 SMTP in Dual Stack Environments January 2005

 (4)  If the sending MTA has IPv6 capability, lookup the AAAA records.
      NOTE: IPv6 addresses for hosts defined by MX records may be
      informed in an additional information section of the DNS
      queries' result as well as IPv4 addresses.  If there is no
      additional address information for the MX hosts, separate
      queries for A or AAAA records should be sent.  There is no way
      to query A and AAAA records at once in current DNS
      implementation.
 (5)  If there is no A and no AAAA record present, try the next MX
      record (go to step (3)).  Note that the next MX record could
      have the same preference.
      NOTE: If one or more address records are found, an
      implementation may sort addresses based on the implementation's
      preference of A or AAAA records.  To encourage the transition
      from IPv4 SMTP to IPv6 SMTP, AAAA records should take
      precedence.  The sorting may only reorder addresses from MX
      records of the same preference.  RFC 2821 section 5 paragraph 4
      suggests randomization of destination addresses.  Randomization
      should only happen among A records, and among AAAA records (do
      not mix A and AAAA records).
 (6)  For each of the addresses, loop over steps (7) to (9).
 (7)  Try to make a TCP connection to the destination's SMTP port
      (25).  The client needs to follow timeouts documented in RFC
      2821 section 4.5.3.2.  If successful, go to step (9).
 (8)  If unsuccessful and there is another available address, try the
      next available address.  Go to step (7).  If all addresses are
      not reachable and if a list of MX records is being traversed,
      try the next MX record (go to step (3)).  If there is no list of
      MX records, or if the end of the list of MX records has been
      reached, raise a temporary email delivery failure.  Finish.
 (9)  Attempt to deliver the email over the connection established, as
      specified in RFC 2821.  If a transient failure condition is
      reported, try the next MX record (go to step (3)).  If an error
      condition is reported, raise a permanent email delivery error,
      and do not try further MX records.  Finish.  If successful, SMTP
      delivery has succeeded.  Finish.

Nakamura & Hagino Informational [Page 5] RFC 3974 SMTP in Dual Stack Environments January 2005

4. MX Configuration in the Recipient Domain

4.1. Ensuring Reachability for Both Protocol Versions

 If a site has dual-stack reachability, the site should configure both
 A and AAAA records for its MX hosts (NOTE: MX hosts can be outside of
 the site).  This will help both IPv4 and IPv6 senders in reaching the
 site efficiently.

4.2. Reachability Between the Primary and Secondary MX

 When registering MX records in a DNS database in a dual-stack
 environment, reachability between MX hosts must be considered
 carefully.  Suppose all inbound email is to be gathered at the
 primary MX host, "mx1.example.org.":
    example.org.    IN MX   1   mx1.example.org.
                    IN MX   10  mx10.example.org.
                    IN MX   100 mx100.example.org.
 If "mx1.example.org" is an IPv6-only node, and the others are IPv4-
 only nodes, there is no reachability between the primary MX host and
 the other MX hosts.  When email reaches one of the lower MX hosts, it
 cannot be relayed to the primary MX host based on MX preferencing
 mechanism.  Therefore, mx1.example.org will not be able to collect
 all the emails (unless there is another transport mechanism(s)
 between lower-preference MX hosts and mx1.example.org).
    ; This configuration is troublesome.
    ; No secondary MX can reach mx1.example.org.
    example.org.    IN MX   1   mx1.example.org.     ; IPv6-only
                    IN MX   10  mx10.example.org.    ; IPv4-only
                    IN MX   100 mx100.example.org.   ; IPv4-only
 The easiest possible configuration is to configure the primary MX
 host as a dual-stack node.  By doing so, secondary MX hosts will have
 no problem reaching the primary MX host.
    ; This configuration works well.
    ; The secondary MX hosts are able to relay email to the primary MX
    ; host without any problems.
    example.org.    IN MX   1   mx1.example.org.     ; dual-stack
                    IN MX   10  mx10.example.org.    ; IPv4-only
                    IN MX   100 mx100.example.org.   ; IPv6-only
 It may not be necessary for the primary MX host and lower MX hosts to
 directly reach one another with IPv4 or IPv6 transport.  For example,
 it is possible to establish a routing path with UUCP or an IPv4/v6

Nakamura & Hagino Informational [Page 6] RFC 3974 SMTP in Dual Stack Environments January 2005

 translator.  It is also possible to drop messages into a single
 mailbox with shared storage using NFS or something else offered by a
 dual-stack server.  It is the receiver site's responsibility that all
 messages delivered to MX hosts arrive at the recipient's mail drop.
 In such cases, a dual-stack MX host may not be listed in the MX list.

5. Operational Experience

 Many of the existing IPv6-ready MTA's appear to work in the way
 documented in section 3.
 There were, however, cases where IPv6-ready MTA's were confused by
 broken DNS servers.  When attempting to obtain a canonical hostname,
 some broken name servers return SERVFAIL (RCODE 2), a temporary
 failure on AAAA record lookups.  Upon this temporary failure, the
 email is queued for a later attempt.  In the interest of IPv4/v6
 interoperability, these broken DNS servers should be fixed.  A
 document by Yasuhiro Morishita [Morishita] has more detail on
 misconfigured/misbehaving DNS servers and their negative side
 effects.

6. Open Issues

 o  How should scoped addresses (i.e., link-local addresses) in email
    addresses be interpreted on MTA's?  We suggest prohibiting the use
    of IPv6 address literals in destination specification.
 o  A future specification of SMTP (revision of RFC 2821) should be
    updated to include IPv6 concerns presented in this memo, such as
    (1) the additional query of AAAA RRs where A RRs and/or MX RRs are
    suggested, and (2) the ordering between IPv6 destination and IPv4
    destination.

7. Security Considerations

 It could be problematic if the route-addr email address format
 [Crocker] (or "obs-route" address format in [Resnick]) is used across
 multiple scope zones.  MTAs would need to reject email with route-
 addr email address formats that cross scope zone borders.

Nakamura & Hagino Informational [Page 7] RFC 3974 SMTP in Dual Stack Environments January 2005

Appendix A. Considerations on Translators

 IPv6-only MTA to IPv4-only MTA cases could use help from IPv6-to-IPv4
 translators such as [Hagino].  Normally there are no special SMTP
 considerations for translators needed.  If there is SMTP traffic from
 an IPv6 MTA to an IPv4 MTA over an IPv6-to-IPv4 translator, the IPv4
 MTA will consider this normal IPv4 SMTP traffic.
 Protocols like IDENT [St.Johns] may require special consideration
 when translators are used.  Also, there are MTAs which perform strict
 checks on the SMTP HELO/EHLO "domain" parameter (perform
 reverse/forward DNS lookups and see if the "domain" really associates
 to the SMTP client's IP address).  In such a case, we need a special
 consideration when translators will be used (for instance, override
 "domain" parameter by translator's FQDN/address).
 Even without a translator, it seems that there are some MTA
 implementations in the wild which send IPv6 address literals in a
 HELO/EHLO message (like "HELO [IPv6:blah]"), even when it is using
 IPv4 transport, or vice versa.  If the SMTP peer is IPv4-only, it
 won't understand the "[IPv6:blah]" syntax and mails won't go out of
 the (broken) MTA.  These implementations have to be corrected.

Normative References

 [Mockapetris] Mockapetris, P., "Domain names - implementation and
               specification", STD 13, RFC 1035, November 1987.
 [Thomson]     Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
               "DNS Extensions to Support IP Version 6", RFC 3596,
               October 2003.
 [Partridge]   Partridge, C., "Mail routing and the domain system",
               STD 10, RFC 974, January 1986.
 [Klensin]     Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
               April 2001.
 [Crocker]     Crocker, D., "Standard for the format of ARPA Internet
               text messages", STD 11, RFC 822, August 1982.
 [Resnick]     Resnick, P., "Internet Message Format", RFC 2822, April
               2001.
 [Hagino]      Hagino, J. and H. Snyder, "IPv6 Multihoming Support at
               Site Exit Routers", RFC 3178, October 2001.

Nakamura & Hagino Informational [Page 8] RFC 3974 SMTP in Dual Stack Environments January 2005

 [St.Johns]    Johns, M. St., "Identification Protocol", RFC 1413,
               February 1993.

Informative References

 [Morishita]   Morishita, Y. and T. Jinmei, "Common Misbehavior
               against DNS Queries for IPv6 Addresses", Work in
               Progress, June 2003.

Acknowledgements

 This document was written based on discussions with Japanese IPv6
 users and help from the WIDE research group.  Here is a (probably
 incomplete) list of people who contributed to the document: Gregory
 Neil Shapiro, Arnt Gulbrandsen, Mohsen Souissi, JJ Behrens, John C
 Klensin, Michael A. Patton, Robert Elz, Dean Strik, Pekka Savola, and
 Rob Austein.

Authors' Addresses

 Motonori NAKAMURA
 Academic Center for Computing and Media Studies, Kyoto University
 Yoshida-honmachi, Sakyo, Kyoto 606-8501, JAPAN
 Fax:   +81-75-753-7450
 EMail: motonori@media.kyoto-u.ac.jp
 Jun-ichiro itojun HAGINO
 Research Laboratory, Internet Initiative Japan Inc.
 1-105, Kanda Jinbo-cho,
 Chiyoda-ku,Tokyo 101-0051, JAPAN
 Phone: +81-3-5205-6464
 Fax:   +81-3-5205-6466
 EMail: itojun@iijlab.net

Nakamura & Hagino Informational [Page 9] RFC 3974 SMTP in Dual Stack Environments January 2005

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 Copyright (C) The Internet Society (2005).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and at www.rfc-editor.org, and except as set
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Acknowledgement

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 Internet Society.

Nakamura & Hagino Informational [Page 10]

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