Internet Engineering Task Force Motonori Nakamura INTERNET-DRAFT Kyoto University Expires: December 28, 2002 Jun-ichiro itojun Hagino IIJ Research Laboratory June 28, 2002 SMTP operational experience in mixed IPv4/IPv6 environements draft-ietf-ngtrans-ipv6-smtp-requirement-06.txt 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 http://www.ietf.org/shadow.html. Distribution of this memo is unlimited. The internet-draft will expire in 6 months. The date of expiration will be December 28, 2002. Abstract 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. 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 NAKAMURA, HAGINO Expires: December 28, 2002 [Page 1] DRAFT SMTP in dual stack environments June 2002 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. 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 [Johns, 1993] , however, 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. 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, every IPv6 sites should have both transport for each domain part of mail addresses, e.g, for example: NAKAMURA, HAGINO Expires: December 28, 2002 [Page 2] DRAFT SMTP in dual stack environments June 2002 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 192.0.2.1 mx10.example.org. IN AAAA 3ffe:501:ffff::2 IN A 192.0.2.2 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 192.0.2.1 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 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 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, and deliveries never fail because of no known address if other addresses are available related to other MX records. NAKAMURA, HAGINO Expires: December 28, 2002 [Page 3] DRAFT SMTP in dual stack environments June 2002 ; 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 SMTP sender in a dual-stack environment 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 (Implicit MX). If NO_DATA (i.e. empty answer with NOERROR(0) RCODE) is returned, there is no MX record but other records (e.g. SOA, NS or A etc.) may be found. 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. NOTE: Some guard mechanism must required to break circular CNAME references. (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. (5) If there is no A or AAAA record present, try the next MX record (go to step (3)). 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. NAKAMURA, HAGINO Expires: December 28, 2002 [Page 4] DRAFT SMTP in dual stack environments June 2002 (6) For each of the addresses, loop over steps (7) to (9). (7) Try to make a TCP connection to the destination. 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) Try an SMTP protocol negotiation according to 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. This will help both IPv4 and IPv6 senders to reach 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. ; 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 NAKAMURA, HAGINO Expires: December 28, 2002 [Page 5] DRAFT SMTP in dual stack environments June 2002 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. >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 (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. 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 should MTA's behave? If scoped addresses in ``route-addr'' specifications [Crocker, 1982] are considered, e.g. <@kame.net,@[fec0::1]:itojun@itojun.org> it gets even trickier. Luckily, the route-addr form was obsoleted by NAKAMURA, HAGINO Expires: December 28, 2002 [Page 6] DRAFT SMTP in dual stack environments June 2002 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. References Johns, 1993. M. St. Johns, "Identification Protocol" in RFC1413 (January 1993). ftp://ftp.isi.edu/in-notes/rfc1413.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. 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. 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). ftp://ftp.isi.edu/in-notes/rfc2822.txt. 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. NAKAMURA, HAGINO Expires: December 28, 2002 [Page 7] DRAFT SMTP in dual stack environments June 2002 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 cases. 04 -> 05 Change title, suggested by Alain Durand. 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. Acknowledgements 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 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 NAKAMURA, HAGINO Expires: December 28, 2002 [Page 8]