Network Working Group M. Nakamura
Request for Comments: 3974 Kyoto University
Category: Informational J. Hagino
IIJ Research Laboratory
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 (C) The Internet Society (2005).
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.
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.
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