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Internet Engineering Task Force                        Motonori Nakamura
INTERNET-DRAFT                                          Kyoto University
Expires: July 15, 2004                          Jun-ichiro itojun Hagino
                                                 IIJ Research Laboratory
                                                        January 15, 2004

      SMTP operational experience in mixed IPv4/IPv6 environements

Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as ``work in progress.''

To view the list Internet-Draft Shadow Directories, see

Distribution of this memo is unlimited.

The internet-draft will expire in 6 months.  The date of expiration will
be July 15, 2004.


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 are necessary in
IPv6-capable MX DNS records for stable dual-stack (IPv4 and IPv6) SMTP
operation.  This document clarifies the problems that exist in the
transition period between IPv4 SMTP and IPv6 SMTP.  It also defines
operational requirements for stable IPv4/v6 SMTP operation.

This document does not define any new protocol.

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1.  Introduction

Deliveries of mail messages to the final mail drop is not always done by
direct IP communication with submiter and final receiver, and there may
be some intermediate hosts to 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 so easy to configure all
the system with consistency since mail message delivery system is rather
complex on DNS setting than other Internet services.  For the transition
state from IPv4 to IPv6, both IPv4 and IPv6 interoperability should be
kept more carefully.

There are several technologies defined for the transition from IPv4 to
IPv6.  This document concentrates on SMTP issues in a dual-stack
environment, e.g. delivery of emails from IPv6-only MTA to IPv4-only MTA
is outside of the scope of the document.

IPv6-only MTA to IPv4-only MTA case could use help from IPv6-to-IPv4
translators such as [Hagino, 2001] , however, 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 as a normal IPv4 SMTP traffic.  Protocols like IDENT
[St.Johns, 1993] as well as SMTP HELO/EHLO may require special
consideration when translators are used.

The following sections explain how to make IPv4 SMTP and IPv6 SMTP
coexist in a dual-stack environment.

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
should be resolved by making either side dual-stack or by making either
side use a protocol translator.

2.  Basic DNS resource record definitions for mail routing

Mail messages on the Internet are delivered based on domain name system
generally.  MX RRs are looked up to know destination hosts associated
with domain part of a mail addresse.  Similar to the way RFC's for IPv6
DNS lookup [Thomson, 1995] use IN class for both IPv4 and IPv6, IN MX
records will be used for both IPv4 and IPv6 on mail message routing,
hosts which have IPv6 transport and want to be delivered with the IPv6
transport must define IPv6 IP addresses for the host name as well as
IPv4 IP addresses.

A MX RR have two data, a preference value and the name of destination
host.  IP addresses for the destination host are also looked up to make
SMTP transport [Partridge, 1986] .  In IPv4 environment, IPv4 IP
addresses are defined with A RRs.

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For example, 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 3ffe:501:ffff::1
     mx10.example.org.       IN AAAA 3ffe:501:ffff::2

In transition period from IPv4 to IPv6, there are many IPv4 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, for example:

     example.org.            IN MX   1  mx1.example.org.
                             IN MX   10 mx10.example.org.
     mx1.example.org.        IN AAAA 3ffe:501:ffff::1
                             IN A
     mx10.example.org.       IN AAAA 3ffe:501:ffff::2
                             IN A

But, every host may not support dual stack operation, some host entries
may have only IPv4 or IPv6 RRs:

     example.org.            IN MX   1  mx1.example.org.
                             IN MX   10 mx10.example.org.
     mx1.example.org.        IN AAAA 3ffe:501:ffff::1
     mx10.example.org.       IN A

In the following sections, how sender side operates with IPv4/IPv6
combined RR definitions (section 3), and how receiver side should define
RRs to keep interoperability with both IPv4 and IPv6 Internet (section
4) are considerd.

3.  SMTP sender algorithm in a dual-stack environment

In a dual-stack environment MX records for a domain resemble the

     example.org.            IN MX   1  mx1.example.org.
                             IN MX   10 mx10.example.org.
     mx1.example.org.        IN A        ; dual-stack
                             IN AAAA 3ffe:501:ffff::1
     mx10.example.org.       IN AAAA 3ffe:501:ffff::2 ; IPv6 only

For a single MX record there are many 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 and AAAA records.
Because multiple MX records may be defined using different preference
values, multiple addresses based on multiple MX's must be traversed.
Domains without MX records and failure recovery cases must be handled
properly as well.

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The algorithm for an 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 RFC2821
[Klensin, 2001] section 5.  When there is no reachable destionation
address record found (for example, 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 never fail
because of no known address if other addresses are available related to
other MX records.

     ; 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  3ffe:501:ffff::1 ; IPv6 only
     b.example.org.          IN MX    1  mx1.b.example.org.
     mx1.b.example.org.      IN HINFO "NO ADDRESS RECORDS"

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
     NO_DATA (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.

(2)  Compare each host name in MX records with the name of sending host.
     If there is a record which has the same name, drop MX records which
     have equal to or larger than preference value of the matched 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 rule should be applied.  Finish.

(3)  If the sending MTA has IPv4 capability, lookup the A record.  Keep
     the resulting address until step (5).

(4)  If the sending MTA has IPv6 capability, lookup the AAAA record.

     NOTE: IPv6 addresses for hosts defined by MX records may be
     informed in additional information section of DNS querie's 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.

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(5)  If there is no A or AAAA record present, try the next MX record (go
     to step (3)).  Note that the next MX record could have the same

     NOTE: If one or more address records are found, some MTA
     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.  But
     this type of sorting is optional.

(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 RFC2821 section  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 e-mail over the connection established, as
     specified in RFC2821 [Klensin, 2001] .  If a transient failure
     condision reported, try the next MX record (go to step (3)).  If an
     error condition reported, raise a permanent email delivery error,
     and further MX records are not tried.  Finish.  If successful, SMTP
     delivery has succeeded.  Finish.

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 to reach the site

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,

     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

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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 needed that the primary MX host and lower MX hosts reach
directly one another with IPv4 or IPv6 transport.  For example, it is
possible to establish a routing path with UUCP or an IPv4/v6 translator.
It is also possible to drop messages into single mailbox with shared
storage using NFS or something else offered by a dual-stack server.  It
is receiver site's matter that all messages delivered to each MX hosts
must be reached to recipient's mail drop.  In such cases, 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 draft by
Yasuhiro Morishita [Morishita, 2003] has more detail on
misconfigured/misbehaving DNS servers and their bad sideeffects.

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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 source routes (Scoped addresses should not
  appear on the global DNS database).

o The document should really be integrated into RFC2821 [Klensin, 2001]
  (i.e. RFC2821 should talk about IPv6 cases).

7.  Security considerations

It could be problematic if the route-addr email address format is used
across multiple scope zones.  MTA's would need to reject email with
improper route-addr email address formats.


Hagino, 2001.
Jun-ichiro Hagino and Hal Snyder, "IPv6 multihoming support at site exit
routers" in RFC3178 (October 2001). ftp://ftp.isi.edu/in-

St.Johns, 1993.
M. St.Johns, "Identification Protocol" in RFC1413 (January 1993).

Thomson, 1995.
S. Thomson and C. Huitema, "DNS Extensions to support IP version 6" in
RFC1886 (December 1995). ftp://ftp.isi.edu/in-notes/rfc1886.txt.

Partridge, 1986.
C. Partridge, "Mail routing and the domain system" in RFC974 (January
1986). ftp://ftp.isi.edu/in-notes/rfc974.txt.

Klensin, 2001.
J. Klensin, Editor, "Simple Mail Transfer Protocol" in RFC2821 (April
2001). ftp://ftp.isi.edu/in-notes/rfc2821.txt.

Morishita, 2003.
Y. Morishita and T. Jinmei, "Common Misbehavior against DNS Queries for
IPv6 Addresses" in draft-morishita-dnsop-misbehavior-against-aaaa-00.txt
(June 2003). work in progress material.

Change history

00 -> 01
     Corrected the email address notation for source-routed emails,
     based on a comment from Gregory Neil Shapiro.

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01 -> 02
     Change a reference to refer to RFC2822, not 822.  Used
     "example.org", not "sample.org".  These changes were based on
     comments from Arnt Gulbrandsen.  Added an ``Operational
     experiences'' section.  Clarified the case where an MX record
     points to a CNAME record, based on comments from Mohsen Souissi.

02 -> 03
     In some cases, IPv6-ready MTA's are troubled by incorrect DNS
     server responses for AAAA queries.  This change was based on
     comments from Gregory Neil Shapiro.

03 -> 04
     Grammar cleanups by JJ Behrens.  More text on the delivery error

04 -> 05
     Change title, suggested by Alain Durand.  Limit the scope of the
     document to dual stack environment (interoperation of IPv6-only
     cloud and IPv4-only cloud is out of scope).

05 -> 06
     Section on summary of IPv4 MX operation is deleted (Replaced by
     Introduction).  Clarify on CNAME chain. Cleanups on sender's
     algorithm.  Suggested by Patrik Faltstrom.

06 -> 07
     Site local address is being obsoleted in IPv6 wg, so remove
     reference to site-locals.  Reflect comments from John C Klensin:
     fixes to sending rules, correct route-addr issues.  Reflect
     comments from Michael A. Patton: HELO on connection via translator.
     Reflect comments from Robert Elz.

07 -> 08
     Refer a draft by Yasuhiro Morishita.


This draft 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 draft: Gregory Neil Shapiro, Arnt
Gulbrandsen, Mohsen Souissi, JJ Behrens, John C Klensin, Michael A.
Patton, and Robert Elz.

Author's address

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     Motonori NAKAMURA
     Center for Information and Multimedia Studies, Kyoto University
     Yoshida-nihonmatsu-cho, Sakyo, Kyoto 606-8501, JAPAN
     Tel: +81-75-753-9063
     Fax: +81-75-753-9056
     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
     Tel: +81-3-5205-6464
     Fax: +81-3-5205-6466
     Email: itojun@iijlab.net

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