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Network Working Group                                        M. NakamuraRequest for Comments: 3974                              Kyoto UniversityCategory: Informational                                        J. Hagino                                                 IIJ Research Laboratory                                                            January 2005SMTP Operational Experience in Mixed IPv4/v6 EnvironmentsStatus 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 inRFC 2821, Section 5, to dual-stack   environments.  Implementors are cautioned that they must referenceRFC 2821 for the full algorithm; this document is not to be   considered a full restatement ofRFC 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 (seeAppendix 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 ANakamura & 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 inRFC 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 inRFC2821section 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 inRFC 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 20054.  MX Configuration in the Recipient Domain4.1.  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/v6Nakamura & 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 insection 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 ofRFC 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 2005Appendix 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.netNakamura & Hagino            Informational                      [Page 9]

RFC 3974            SMTP in Dual Stack Environments         January 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and at www.rfc-editor.org, and except as set   forth therein, the authors retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the ISOC's procedures with respect to rights in ISOC Documents can   be found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Nakamura & Hagino            Informational                     [Page 10]