Internet DRAFT - draft-martin-smtp-ipv6-to-ipv4-fallback
draft-martin-smtp-ipv6-to-ipv4-fallback
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
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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
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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
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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
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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.
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[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.
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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:<Smith@example.org>
S: 250 OK
C: RCPT TO:<Jones@example.com>
S: 250 OK
C: DATA
S: 354 Start mail input; end with <CRLF>.<CRLF>
C: [message body]
C: .
S: 421 4.4.8 message with no SPF or DKIM and over IPv6
S: <disconnect>
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:<Smith@bar.com>
S: 250 OK
C: RCPT TO:<Jones@example.com>
S: 250 OK
C: DATA
S: 354 Start mail input; end with <CRLF>.<CRLF>
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.
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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.
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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: <connection establishment>
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:<Smith@example.com>
S: 250 OK
C: RCPT TO:<Jones@example.net>
S: 250 OK
C: DATA
S: 354 Start mail input; end with <CRLF>.<CRLF>
C: <message content>
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:
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C: <connection establishment>
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
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