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Internet Engineering Task Force                        Motonori Nakamura
INTERNET-DRAFT                                          Kyoto University
Expires: April 24, 2002                         Jun-ichiro itojun Hagino
                                                 IIJ Research Laboratory
                                                        October 24, 2001

                   IPv6 SMTP operational requirements

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 April 24, 2002.


The memo lists operational requirements for IPv6 SMTP, and IPv6-capable
MX DNS records.  As we deploy IPv6 SMTP servers, it became apparent that
we need certain configuration in IPv6-capable MX DNS record, for stable
dual-stack (IPv4 and IPv6) SMTP operations.  The document tries to
clarify the problems we have in transition period between IPv4 SMTP and
IPv6 SMTP, and operational requirements for stable IPv4/v6 SMTP

The document does not try to define any new protocol.

1.  Summary of IPv4 MX operation

For reference purpose, the section outlines how mail message delivery is
performed in IPv4-only environment [Partridge, 1986] .

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In IPv4 SMTP operation, we register MX records like below, for
"example.org."  domain:

     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 delivers a message to a particular destination (say it is to
foo@example.org), the MTA would send DNS queries to lookup DNS database
in the following order:

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

  o If an MX record is returned, try to lookup A record on the righthand
    side of the MX record.

  o If a CNAME record is returned, try to chase the CNAME chain.
    Eventually we will reach some A record.

    NOTE: from RFC2181 MX records must not point CNAME records [Elz,
    1997] .  However, it has been permitted in older RFCs [Partridge,
    1986] .  We mention CNAME chasing logic here just for backward
    compatibility.  Implementers may want to avoid CNAME chasing to be
    more conformant to RFC2181.

  o If MX lookup failed with NO_DATA, it means that there is no MX
    record but there can be other record for "example.org.".  Lookup A
    record for "example.org.".

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

2.  MX records and IPv6 SMTP operation

The following sections talk about how to make IPv4 SMTP and IPv6 SMTP
coexist, under dual-stack environment during the transition period
between IPv4 to IPv6.  In the future, when we have completely migrated
to IPv6-only network, we can forget about IPv4/v6 SMTP interaction.

As IPv6 DNS lookup RFCs [Thomson, 1995; Crawford, 2000] use IN class for
both IPv4 and IPv6, we will use IN MX records for both IPv4 and IPv6.

For simplicity, the document lists DNS records for IPv6 address as AAAA
records, not as A6 records [Crawford, 2000] .  In reality, we can use a
chain of A6 records, instead of AAAA records.

There are couple of technologies defined for IPv4 and IPv6 transition.
The document concentrates on issues with dual stack environment.
Translators do not need special consideration from SMTP point of view;
If we have SMTP traffic from IPv6 MTA to IPv4 MTA over an IPv6-to-IPv4

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translator, the traffic will be considered as a normal IPv4 SMTP
traffic, from the IPv4 MTA point of view.  We may, however, need some
consideration on translators for protocols like IDENT [StJohns, 1993] .

3.  SMTP sender algorithm in dual stack environment

When we lookup MX records for the domain in IPv4/v6 dual stack
environment, we will see records like below:

     example.org.            IN MX   1  mx1.example.org.
                             IN MX   10 mx10.example.org.
     mx1.example.org.        IN A          ; IPv4/v6 dual stack
                             IN AAAA 3ffe:501:ffff::1
     mx10.example.org.       IN AAAA 3ffe:501:ffff::2 ; IPv6 only

For single MX record, we have many possibility for the final lookup
result, including: (a) single, or multiple A records for IPv4
destination, (b) single, or multiple AAAA records for IPv6 destination,
(c) mixture of A and AAAA records.  As we can define multiple MX records
with different preference value, we also need to go through multiple
addresses based on multiple MXes.  We need to cope with domains without
MX records, and failure recovery cases too.

The algorithm for a SMTP sender would be like this.

(1)  Lookup MX record for the destination domain.  If a CNAME record is
     returned, go back to step (1) with the queried result.  If MX
     records are returned, go to step (2) with the result.  If NO_DATA
     is returned, go to step (3) as there is no MX record.  If
     HOST_NOT_FOUND is returned, there is no domain, raise permanent
     email delivery failure (finish).

     NOTE: regarding to MX records pointing CNAME records, see a note in
     the previous section.

(2)  We have multiple MX records with us.  Loop steps from (3) to (8),
     based on MX preference values, in ascending order.

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

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

(5)  Reorder queried result based on implementation-dependent preference
     between A and AAAA records.  If you would like to encourage the
     transition from IPv4 SMTP to IPv6 SMTP, AAAAs should take

(6)  Loop steps from (7) to (8), for all the addresses (or part of the
     list of addresses) we have.  If no reachable destination is found,
     and if we are going through a list of MX records, go back to (3)

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     and try the next MX record.  If we do not have a list of MX
     records, or we have reached the end of the list of MX records,
     raise temporary delivery failure (finish).

(7)  Try to make a TCP connection to the destination.  If it fails, try
     the next address we have.  If it succeeds, go to step (8).

(8)  Try a SMTP protocol negotiation.  If SMTP protocol negotiation
     fails with TEMPFAIL (4xx), go back to (3) and try the next MX
     record.  If it succeeds, SMTP delivery was successful (finish).

4.  MX configuration in receipient domain

4.1.  Ensuring reachability for both protocol versions

If a site has IPv4/v6 dual stack reachability, the site SHOULD configure
both A and AAAA records onto its MX hosts.  It will help both IPv4 and
IPv6 senders to reach the site efficienlty.

4.2.  Reachability between primary and secondary MX

When we configure MX records onto DNS database in dual-stack
environment, we need to be careful about reachability between MX hosts.
Suppose we try to gather all inbound email to 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 rest are IPv4 only node,
we have no reachability between primary MX host and the rest.  Once an
email reaches one of secondary MX host, the email will never reach the
primary MX.

     ; the 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 an IPv4/v6 dual stack node.  By doing so, secondaries will have no
problem reaching the primary MX host.

     ; the configuration works just fine.
     ; emails reaches from secondary MX to primary with no trouble.
     example.org.    IN MX   1   mx1.example.org.     ; IPv4/v6 dual stack
                     IN MX   10  mx10.example.org.    ; IPv4 only
                     IN MX   100 mx100.example.org.   ; IPv6 only

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There are many other ways to ensure the reachability between secondary
MX and primary MX.  For example, we could configure secondary MX to
route emails statically, without considering DNS MX configuration.  Or
we could estalish alternative email routing path (i.e. UUCP, or via
IPv4/v6 translator) between secondary MX and the primary MX.

5.  Operational experiences

Many of the existing IPv6-ready MTAs seem to work in the way documented
in section 3.

>From past experiments and operational experiences, it is known that most
of the existing IPv4-only MTAs will not get confused by AAAA records,
registered for MX hostnames.  No experiments were conducted with A6

There were, however, cases where IPv6-ready MTAs get confused by broken
DNS servers.  When attempting to canonify a hostname, some broken name
servers will return SERVFAIL (a temporary failure) on AAAA record
lookups.  Upon this temporary failure, the mail is queued up for a later
attempt.  These broken DNS servers should get fixed, for this issue as
well as for other IPv6 operations.

6.  Open issues

o How to interpret scoped address in email addresses, on MTAs.  As we
  relay emails between MTAs, interpretation of scoped address can be
  different between MTAs, as intermediate MTAs may be in different scope
  zone as the originator.  If we get scoped IPv6 address as a result of
  DNS lookups, how MTAs should behave?

  If we consider scoped address in ``route-addr'' specification
  [Crocker, 1982] like


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

7.  Security consideration

As presented in ``Open issues'' section, it could be problematical if
route-addr email address format is used across multiple scope zones.
MTAs would need to reject emails with improper route-addr email address
formats.  A possible example of improper route-addr format would be like
this: an email from outside of the site border, which carries numeric
site-local address in route-addr format.

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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).

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.

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

Change history

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

01 -> 02
     Refer RFC2822, not 822.  Use example.org, not sample.org.  Based on
     comments from Arnt Gulbrandse.  Add Operational experiences
     section.  Clarify MX-points-to-CNAME, based on comments from Mohsen

02 -> 03
     Some cases, IPv6-ready MTAs gets troubled by wrong DNS server
     responses for AAAA queries.  From Gregory Neil Shapiro.

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The draft was written based on discussions with Japanese IPv6 users, and
help from WIDE research group.  Here are a (probably incomplete) list of
people contributed to the draft: Gregory Neil Shapiro, Arnt Gulbrandse,
and Mohsen Souissi.

Author's address

     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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