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Internet Engineering Task Force                        Motonori Nakamura
INTERNET-DRAFT                                          Kyoto University
Expires: August 28, 2002                        Jun-ichiro itojun Hagino
                                                 IIJ Research Laboratory
                                                       February 28, 2002

      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 August 28, 2002.


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.

1.  Summary of IPv4 MX operation

For reference purposes, this section outlines how email message delivery
is performed in an IPv4-only environment [Partridge, 1986] .

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In IPv4 SMTP operation, the MX record "example.org." would be registered
as follows:

     example.org.            IN MX   1  mx1.example.org.
                             IN MX   10 mx10.example.org.
     mx1.example.org.        IN A
     mx10.example.org.       IN A

When an MTA wishes to deliver a message to a particular destination
(e.g.  "foo@example.org"), the MTA sends DNS queries in the following

o Lookup MX record for "example.org.".

  o If an MX record is returned, lookup an A record for the right-hand
    side of the MX record.

  o If a CNAME record is returned, try to chase the CNAME chain.
    Eventually an A record will be reached.

    NOTE: RFC2181 [Elz, 1997] prohibits MX records from pointing to
    CNAME records.  However, this was not prohibited in earlier RFCs.
    [Partridge, 1986] CNAME chasing logic is mentioned here just for
    backwards compatibility.  Implementers may want to avoid CNAME
    chasing to better conform with RFC2181.

  o If the MX lookup fails with NO_DATA, it means that there is no MX
    record, but there may be other records (e.g. "example.org.").
    Lookup the A record for "example.org.".

  o If the MX lookup fails with HOST_NOT_FOUND, it means that there is
    no record at all for "example.org.".  This results in a delivery

2.  MX records and dual stack SMTP operation

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

Similar to the way RFC's for IPv6 DNS lookup [Thomson, 1995; Crawford,
2000] use IN class for both IPv4 and IPv6, IN MX records will be used
for both IPv4 and IPv6.

There are several technologies defined for the transition from IPv4 to
IPv6.  This document concentrates on SMTP issues in a dual-stack
environment.  Afterall, there are no special SMTP considerations for
translators; 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 [StJohns, 1993] , however, may
require special consideration when translators are used.

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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.

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.

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.

     ; 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"

(1)  Lookup the MX record for the destination domain.  If a CNAME record
     is returned, go to step (1) with the query's result.  If any MX
     records are returned, go to step (2) with the query's result.  If
     NO_DATA is returned, there is no MX record.  Go to step (3).  If
     HOST_NOT_FOUND is returned, there is no domain.  Raise a permanent
     email delivery failure.  Finish.

     NOTE:  the previous section contains a note about MX records that
     point to CNAME records.

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(2)  There are multiple MX records.  Sort the MX records in ascending
     order based on their preference values, and loop over steps (3) to

(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.

(5)  If there is no A or AAAA record present, try the next MX record (go
     to step (3)).  Sort the query's result based on the
     implementation's preference of A or AAAA records.  If it is
     desirable to encourage the transition from IPv4 SMTP to IPv6 SMTP,
     AAAA records should take precedence.

(6)  For each of the addresses or each part of the list of addresses,
     loop over steps (7) to (8).  If no reachable destination is found,
     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.

(7)  Try to make a TCP connection to the destination.  If unsuccessful,
     try the next available address.  If successful, go to step (8).

(8)  Try an SMTP protocol negotiation.  If the SMTP protocol negotiation
     fails with TEMPFAIL (4xx), try the next MX record (go to step (3)).
     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.  This will help both IPv4 and IPv6
senders to reach the site efficiently.

4.2.  Reachability between the primary and secondary MX

When entering 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
nodes, there is no reachability between the primary MX host and the
other MX hosts.  When email reaches one of the secondary MX hosts, it

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cannot be relayed to the primary MX host.

     ; 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

There are many other ways to ensure that the primary MX host and the
secondary MX hosts can reach one another.  For example, it is possible
to configure the secondary MX hosts to route email statically, i.e.
without considering the DNS MX configuration.  It is also possible to
establish an alternate email routing path (e.g. UUCP or an IPv4/v6
translator) between the secondary MX host and the primary MX host.

5.  Operational experience

Many of the existing IPv6-ready MTA's appear to work in the way
documented in section 3.

>From past experiments and operational experience, it is known that most
of the existing IPv4-only MTA's will not be confused by AAAA records
that are registered for MX hostnames.  No experiments were conducted
with A6 records.

There were, however, cases where IPv6-ready MTA's were confused by
broken DNS servers.  When attempting to canonify a hostname, some broken
name servers return SERVFAIL, 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.

6.  Open issues

o How should scoped addresses in email addresses be interpreted on
  MTA's?  As email is relayed between MTA's, interpretation of scoped
  addresses can be different between MTA's.  Afterall, intermediate
  MTA's may be in different scope zones than the originator.  If a
  scoped IPv6 address is returned as the result of a DNS lookup, how

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  should MTA's behave?

  If scoped addresses in ``route-addr'' specifications  [Crocker, 1982]
  are considered, e.g.


  it gets even trickier.  Luckily, the route-addr form was obsoleted by
  RFC2822 [Resnick, 2001] .

7.  Security considerations

As mentioned in the ``Open issues'' section, 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.  One example of an improper route-addr format is an email from
outside the site border which carries a numeric site-local address in
the route-addr format.


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

Elz, 1997.
R. Elz and R. Bush, "Clarifications to the DNS Specification" in RFC2181
(July 1997). ftp://ftp.isi.edu/in-notes/rfc2181.txt.

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.

Crawford, 2000.
M. Crawford, C. Huitema, and S. Thomson, "DNS Extensions to Support IPv6
Address Aggregation and Renumbering" in RFC2874 (July 2000).

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

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

Crocker, 1982.
D. Crocker, "Standard for the format of ARPA Internet text messages" in
RFC822 (August 1982). ftp://ftp.isi.edu/in-notes/rfc822.txt.

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Resnick, 2001.
P. Resnick, editor, "Internet Message Format" in RFC2822 (April 2001).

Change history

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

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.


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, and JJ Behrens.

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.
     Takebashi Yasuda Bldg.,
     3-13 Kanda Nishiki-cho,
     Chiyoda-ku,Tokyo 101-0054, JAPAN
     Tel: +81-3-5259-6350
     Fax: +81-3-5259-6351
     Email: itojun@iijlab.net

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