Network Working Group F. Martin, Ed. Internet-Draft LinkedIn Updates: 5321 (if approved) A. Peterson, Ed. Intended status: Standards Track Message Systems Expires: November 16, 2014 May 15, 2014 SMTP IPv6 to IPv4 Fallback: An Applicability Statement draft-martin-smtp-ipv6-to-ipv4-fallback-01 Abstract This Applicability Statement describes how Mail Transfer Agents (MTAs) can be encouraged to fall back to IPv4 when a message is refused over IPv6. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on November 16, 2014. Copyright Notice Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Martin & Peterson Expires November 16, 2014 [Page 1] Internet-Draft SMTP IPv6 to IPv4 Fallback May 2014 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Moving from IP Reputation to Domain Based Reputation . . 2 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3 3. Indicating the sender-SMTP server to fall back to IPv4 . . . 3 3.1. Using 421 SMTP Error code . . . . . . . . . . . . . . . . 3 4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 5.1. SMTP Enhanced Status Codes Registry update . . . . . . . 4 6. Security Considerations . . . . . . . . . . . . . . . . . . . 5 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 7.1. Normative References . . . . . . . . . . . . . . . . . . 5 7.2. Informative References . . . . . . . . . . . . . . . . . 6 Appendix A. Examples and research . . . . . . . . . . . . . . . 6 A.1. SMTP fall back from IPv6 to IPv4 . . . . . . . . . . . . 6 A.2. Common Open Source MTA Design . . . . . . . . . . . . . . 7 A.2.1. MX RR configuration . . . . . . . . . . . . . . . . . 8 A.2.2. Rejecting Messages for Immediate Retry on IPv4 . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 1. Introduction The Simple Mail Transfer Protocol (SMTP) is defined in [RFC5321]. Section 5 of that document describes the process of host selection. SMTP clients in well known Mail Transfer Agent (MTA) software will retry a message using a different Mail Exchanger (MX) or network address if the message is temporarily rejected. This document describes under which circumstances well known and widely deployed open source MTAs (and others) can be made to retry over IPv4 when an initial connection to IPv6 results in a temporary rejection. This behavior could be useful in, for instance, enforcing higher requirements for Simple Mail Transfer Protocol (SMTP) sessions over IPv6 than what exists on IPv4 without simply rejecting the message outright. 1.1. Moving from IP Reputation to Domain Based Reputation IPv6 brings more IP addresses, which means building an IP-based reputation system using IPv6 addresses could be difficult to achieve. Moving from an IP based reputation system to a domain based reputation system is expected to be easier. However, it requires that all SMTP servers participate. IPv4 address space is well known and many tools have been built to handle unwanted emails from certain IPv4 addresses. There is not yet such expertise on IPv6 nor tools. However, labels, like domain names are more stable, unlike the more dynamic nature of IP address Martin & Peterson Expires November 16, 2014 [Page 2] Internet-Draft SMTP IPv6 to IPv4 Fallback May 2014 allocation, and provide a relatively better chain to associate an email to its author, provided such labels can be authentified. Such labels allow better reporting of unwanted emails to the system administrators of mail servers in these domains. As IPv6 is still relatively nascent, there is a chance to mandate use of the Sender Policy Framework (SPF, [RFC4408]) or DomainKeys Identified Mail (DKIM, [RFC6376]) or similar domain-based authentication mechanisms for messages sent over IPv6 and, if these fail, to do retries over IPv4. 2. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. 3. Indicating the sender-SMTP server to fall back to IPv4 To move from IP based reputation to a domain based reputation system for email, a receiver-SMTP could, for example, require that messages pass SPF [RFC6652] or DKIM [RFC6376]. This section does not discuss the merit of such policy but proposes mechanisms for any policy to get the sender-SMTP to fall back to IPv4 from an IPv6 connection. 3.1. Using 421 SMTP Error code The receiver-SMTP server MUST have at least one MX RR of the highest priority pointing to a single stack hostname with only an A RR record. For example: example.org. IN MX 1 mx1.example.org. IN MX 1 mx2.example.org mx1.example.org. IN A 192.0.2.1 mx2.example.org. IN AAAA 2001:db8:ffff::2 mx2.example.org. IN A 192.0.2.2 Martin & Peterson Expires November 16, 2014 [Page 3] Internet-Draft SMTP IPv6 to IPv4 Fallback May 2014 If the sender-SMTP decides to select first the MX with an hostname having an AAAA RR, and the receiver-SMTP makes a policy decision not to accept a message over IPv6, the receiver-SMTP MUST reject the message with a 451 code if done at connection time and 421 code if done later and MUST terminates the connection immediately after so the sender-SMTP requeues the message for immediate retry by selecting the next receiver in priority order, according to Section 5 of [RFC5321]. Because the highest priority MX points to an hostname with only an A RR, the next connection will be over IPv4... As per Section 4.2.3 of [RFC5321] the 421 error code means "Service not available, closing transmission channel". As such the sender- SMTP usually immediately retries the message on next available MX. If the receiver-SMTP implements enhanced status codes [RFC5248], The SMTP enhanced status code SHOULD be 4.4.8. The rejection at connection time MAY be issued only when it has been demonstrated that a majority of messages would not pass the policy. The rejection at connection time SHOULD be for a limited time to give a chance for later messages to be re-evaluted against the policy. Research presented in annexes has shown that common MTA exhibit this behavior. 4. Acknowledgements Thanks to Murray Kucheraway for guidance in getting this draft out. 5. IANA Considerations This section describes actions requested of IANA. 5.1. SMTP Enhanced Status Codes Registry update IANA is requested to add the following to the Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes Registry: o Enumerated Status Codes o Code: X.4.8 o Summary: retry on IPv4 o Associated basic status code: 421,451 Martin & Peterson Expires November 16, 2014 [Page 4] Internet-Draft SMTP IPv6 to IPv4 Fallback May 2014 o Description: the mail system will not accept this message over IPv6 because it lacks some requirments described in the full text of the rejection, however the sending mail system can retry immediately to submit the message over IPv4 only. 6. Security Considerations SMTP clients might not not fall back to IPv4 when requested (by not implementing this proposal) and keep retrying on IPv6. MTA administrators ought to monitor for such servers, and could whitelist them to accept messages over IPv6 or take other action as appropriate. Messages may start to queue on the sender-SMTP side and the mail administrator may not notice them or take appropriate action in time. If the policy is not explained clearly the mail administrator may not know what is required to pass the policy. If the policy is to cumbersome and is not based on widely adopted standards and recommendations, the mail administrator may decide not to jump through hoops to get the email delivered. Some SMTP clients may fail completely when the MX points to an hostname with only an AAAA RR, therefore it is preferred that any hostname with an AAAA record be dual stacked, ie. have an A RR too. 7. References 7.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1", RFC 4408, April 2006. [RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced Mail System Status Codes", BCP 138, RFC 5248, June 2008. [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, October 2008. [RFC6376] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, September 2011. Martin & Peterson Expires November 16, 2014 [Page 5] Internet-Draft SMTP IPv6 to IPv4 Fallback May 2014 [RFC6652] Kitterman, S., "Sender Policy Framework (SPF) Authentication Failure Reporting Using the Abuse Reporting Format", RFC 6652, June 2012. 7.2. Informative References [RFC3974] Nakamura, M. and J. Hagino, "SMTP Operational Experience in Mixed IPv4/v6 Environments", RFC 3974, January 2005. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. Appendix A. Examples and research A.1. SMTP fall back from IPv6 to IPv4 The sender-SMTP server opens a connection to the receiver-SMTP example.org over IPv6, the message is refused because no domain authentication can be performed therefore the sender-SMTP retries immediately using IPv4. "S" indicates text sent by a server, and "C" indicates text sent by a client. Line terminations are omitted in this illustration. Martin & Peterson Expires November 16, 2014 [Page 6] Internet-Draft SMTP IPv6 to IPv4 Fallback May 2014 S: 220 ipv6.example.com Simple Mail Transfer Service Ready C: EHLO example.org S: 250-ipv6.example.com greets example.org over IPv6 S: 250-8BITMIME S: 250-SIZE S: 250-DSN S: 250 HELP C: MAIL FROM: S: 250 OK C: RCPT TO: S: 250 OK C: DATA S: 354 Start mail input; end with . C: [message body] C: . S: 421 4.4.8 message with no SPF or DKIM and over IPv6 S: S: 220 ipv4.example.com Simple Mail Transfer Service Ready C: EHLO example.org S: 250-ipv4.example.com greets example.org over IPv4 S: 250-8BITMIME S: 250-SIZE S: 250-DSN S: 250 HELP C: MAIL FROM: S: 250 OK C: RCPT TO: S: 250 OK C: DATA S: 354 Start mail input; end with . C: [message body] C: . S: 250 OK C: QUIT S: 221 foo.com Service closing transmission channel A.2. Common Open Source MTA Design This secton discusses current implementations in common open source MTAs. SMTP clients only interpret SMTP status codes, but the use of SMTP enhanced status codes can be used to better monitor and act on the reason of the temporary failure. Martin & Peterson Expires November 16, 2014 [Page 7] Internet-Draft SMTP IPv6 to IPv4 Fallback May 2014 For instance, at the end of DATA, if the message was sent over IPv6, the SMTP server can evalute whether the message passes SPF or DKIM and reject the message using 421 if neither pass. If one passes, then an authenticated domain name is available and domain reputation rules can be applied. The IPv6 address of the SMTP client can be noted, and futher connections over IPv6 can be temporarily failed using the 451 status code for some period of time period so as to minimize resources to evaluate each message after the end of DATA. On the other hand, if a message coming from an IPv6 address does not pass SPF or DKIM, it is unlikely that this state would quickly change for the next message coming from the same network address. For proprietary mail systems or large mailbox providers, they all do a form of domain authentication, either SPF or DKIM, so the above may not apply to them. Not all are yet enabled to send email over IPv6. Some observations: o When presented with a permanent failure code (5yz) during connection establishment, MTA clients will declare the message non deliverable and will not retry it on a different MX. Some clients do retry however. o When an SMTP server rejects during the connection phase using the code 451 and disconnects, the SMTP client will typically retry the message immediately on a different MX. o When an SMTP server rejects after the DATA phase using a 421 SMTP reply code followed by a disconnect, the SMTP client will retry the message immediately on a different MX. MX RR configuration and the proper rejection need both to be properly defined. A.2.1. MX RR configuration When a message is temporarily refused, using a 400-series SMTP error code, the strategy for the SMTP client is sometimes to retry immediately to a different MTA, as defined by the target selection process defined in [RFC5321]. When the new MX refers to a dual-stacked machine (see [RFC3974]), some MTA software will not pick up all of the A or AAAA RRs (Resource Records), but will instead select only the RRs matching the address family preferred by the local TCP/IP implementation. Thus, MX RRs MUST have at least one record of the highest priority pointing to a single-stack machine with an A RR only. Martin & Peterson Expires November 16, 2014 [Page 8] Internet-Draft SMTP IPv6 to IPv4 Fallback May 2014 For instance, the following configuration is desired: example.org. IN MX 10 mx1.example.org. IN MX 10 mx2.example.org. mx1.example.org. IN A 192.0.2.1 mx2.example.org. IN A 192.0.2.2 mx2.example.org. IN AAAA 2001:db8:ffff::2 In this configuration, the SMTP client see two MXes at the same preference, and will automatically pick one and try the other one if the message is properly temp-failed. Therefore, in the above example, if the first connection was over IPv6 and the message temporarly refused and the session disconnected, the next connection will be over IPv4. A.2.2. Rejecting Messages for Immediate Retry on IPv4 Here is an example of an initial message sent over IPv6. The SMTP client then retries immediately on the next MX record, here an IPv4 address. "S" indicates text sent by a server, and "C" indicates text sent by a client. Line terminations are omitted in this illustration. C: S: 220 example.net Simple Mail Transfer Service Ready C: EHLO example.com S: 250-example.net greets example.com over IPv6 S: 250-8BITMIME S: 250-SIZE S: 250-DSN S: 250 HELP C: MAIL FROM: S: 250 OK C: RCPT TO: S: 250 OK C: DATA S: 354 Start mail input; end with . C: C: . S: 421 4.4.8 SPF and/or DKIM required on IPv6; try elsewhere C: QUIT S: 221 foo.com Service closing transmission channel Where the client is known to not use DKIM and/or SPF, the server may terminate the connection immediately: Martin & Peterson Expires November 16, 2014 [Page 9] Internet-Draft SMTP IPv6 to IPv4 Fallback May 2014 C: S: 451 4.4.8 Come back on IPv4 as I told you before Authors' Addresses Franck Martin (editor) LinkedIn Mountain View, CA US Email: fmartin@linkedin.com Alec Peterson (editor) Message Systems Columbia, MD US Email: alec@messagesystems.com Martin & Peterson Expires November 16, 2014 [Page 10]