< draft-ietf-dane-smtp-with-dane-16.txt   draft-ietf-dane-smtp-with-dane-17.txt >
DANE V. Dukhovni DANE V. Dukhovni
Internet-Draft Two Sigma Internet-Draft Two Sigma
Intended status: Standards Track W. Hardaker Intended status: Standards Track W. Hardaker
Expires: October 24, 2015 Parsons Expires: November 17, 2015 Parsons
April 22, 2015 May 16, 2015
SMTP security via opportunistic DANE TLS SMTP security via opportunistic DANE TLS
draft-ietf-dane-smtp-with-dane-16 draft-ietf-dane-smtp-with-dane-17
Abstract Abstract
This memo describes a downgrade-resistant protocol for SMTP transport This memo describes a downgrade-resistant protocol for SMTP transport
security between Mail Transfer Agents (MTAs) based on the DNS-Based security between Mail Transfer Agents (MTAs) based on the DNS-Based
Authentication of Named Entities (DANE) TLSA DNS record. Adoption of Authentication of Named Entities (DANE) TLSA DNS record. Adoption of
this protocol enables an incremental transition of the Internet email this protocol enables an incremental transition of the Internet email
backbone to one using encrypted and authenticated Transport Layer backbone to one using encrypted and authenticated Transport Layer
Security (TLS). Security (TLS).
skipping to change at page 1, line 36 skipping to change at page 1, line 36
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 24, 2015. This Internet-Draft will expire on November 17, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 18 skipping to change at page 2, line 18
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Background . . . . . . . . . . . . . . . . . . . . . . . 5 1.2. Background . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. SMTP channel security . . . . . . . . . . . . . . . . . . 6 1.3. SMTP channel security . . . . . . . . . . . . . . . . . . 6
1.3.1. STARTTLS downgrade attack . . . . . . . . . . . . . . 6 1.3.1. STARTTLS downgrade attack . . . . . . . . . . . . . . 6
1.3.2. Insecure server name without DNSSEC . . . . . . . . . 7 1.3.2. Insecure server name without DNSSEC . . . . . . . . . 7
1.3.3. Sender policy does not scale . . . . . . . . . . . . 8 1.3.3. Sender policy does not scale . . . . . . . . . . . . 8
1.3.4. Too many certification authorities . . . . . . . . . 8 1.3.4. Too many certification authorities . . . . . . . . . 8
2. Identifying applicable TLSA records . . . . . . . . . . . . . 8 2. Identifying applicable TLSA records . . . . . . . . . . . . . 8
2.1. DNS considerations . . . . . . . . . . . . . . . . . . . 8 2.1. DNS considerations . . . . . . . . . . . . . . . . . . . 9
2.1.1. DNS errors, bogus and indeterminate responses . . . . 8 2.1.1. DNS errors, bogus and indeterminate responses . . . . 9
2.1.2. DNS error handling . . . . . . . . . . . . . . . . . 11 2.1.2. DNS error handling . . . . . . . . . . . . . . . . . 11
2.1.3. Stub resolver considerations . . . . . . . . . . . . 11 2.1.3. Stub resolver considerations . . . . . . . . . . . . 11
2.2. TLS discovery . . . . . . . . . . . . . . . . . . . . . . 12 2.2. TLS discovery . . . . . . . . . . . . . . . . . . . . . . 13
2.2.1. MX resolution . . . . . . . . . . . . . . . . . . . . 14 2.2.1. MX resolution . . . . . . . . . . . . . . . . . . . . 14
2.2.2. Non-MX destinations . . . . . . . . . . . . . . . . . 15 2.2.2. Non-MX destinations . . . . . . . . . . . . . . . . . 15
2.2.3. TLSA record lookup . . . . . . . . . . . . . . . . . 17 2.2.3. TLSA record lookup . . . . . . . . . . . . . . . . . 17
3. DANE authentication . . . . . . . . . . . . . . . . . . . . . 19 3. DANE authentication . . . . . . . . . . . . . . . . . . . . . 19
3.1. TLSA certificate usages . . . . . . . . . . . . . . . . . 19 3.1. TLSA certificate usages . . . . . . . . . . . . . . . . . 19
3.1.1. Certificate usage DANE-EE(3) . . . . . . . . . . . . 20 3.1.1. Certificate usage DANE-EE(3) . . . . . . . . . . . . 20
3.1.2. Certificate usage DANE-TA(2) . . . . . . . . . . . . 20 3.1.2. Certificate usage DANE-TA(2) . . . . . . . . . . . . 21
3.1.3. Certificate usages PKIX-TA(0) and PKIX-EE(1) . . . . 22 3.1.3. Certificate usages PKIX-TA(0) and PKIX-EE(1) . . . . 22
3.2. Certificate matching . . . . . . . . . . . . . . . . . . 23 3.2. Certificate matching . . . . . . . . . . . . . . . . . . 23
3.2.1. DANE-EE(3) name checks . . . . . . . . . . . . . . . 23 3.2.1. DANE-EE(3) name checks . . . . . . . . . . . . . . . 23
3.2.2. DANE-TA(2) name checks . . . . . . . . . . . . . . . 23 3.2.2. DANE-TA(2) name checks . . . . . . . . . . . . . . . 23
3.2.3. Reference identifier matching . . . . . . . . . . . . 24 3.2.3. Reference identifier matching . . . . . . . . . . . . 24
4. Server key management . . . . . . . . . . . . . . . . . . . . 25 4. Server key management . . . . . . . . . . . . . . . . . . . . 25
5. Digest algorithm agility . . . . . . . . . . . . . . . . . . 26 5. Digest algorithm agility . . . . . . . . . . . . . . . . . . 26
6. Mandatory TLS Security . . . . . . . . . . . . . . . . . . . 26 6. Mandatory TLS Security . . . . . . . . . . . . . . . . . . . 26
7. Note on DANE for Message User Agents . . . . . . . . . . . . 26 7. Note on DANE for Message User Agents . . . . . . . . . . . . 27
8. Interoperability considerations . . . . . . . . . . . . . . . 27 8. Interoperability considerations . . . . . . . . . . . . . . . 27
8.1. SNI support . . . . . . . . . . . . . . . . . . . . . . . 27 8.1. SNI support . . . . . . . . . . . . . . . . . . . . . . . 27
8.2. Anonymous TLS cipher suites . . . . . . . . . . . . . . . 27 8.2. Anonymous TLS cipher suites . . . . . . . . . . . . . . . 28
9. Operational Considerations . . . . . . . . . . . . . . . . . 28 9. Operational Considerations . . . . . . . . . . . . . . . . . 28
9.1. Client Operational Considerations . . . . . . . . . . . . 28 9.1. Client Operational Considerations . . . . . . . . . . . . 28
9.2. Publisher Operational Considerations . . . . . . . . . . 28 9.2. Publisher Operational Considerations . . . . . . . . . . 29
10. Security Considerations . . . . . . . . . . . . . . . . . . . 29 10. Security Considerations . . . . . . . . . . . . . . . . . . . 29
11. IANA considerations . . . . . . . . . . . . . . . . . . . . . 30 11. IANA considerations . . . . . . . . . . . . . . . . . . . . . 30
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 30 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
13.1. Normative References . . . . . . . . . . . . . . . . . . 30 13.1. Normative References . . . . . . . . . . . . . . . . . . 30
13.2. Informative References . . . . . . . . . . . . . . . . . 31 13.2. Informative References . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
1. Introduction 1. Introduction
This memo specifies a new connection security model for Message This memo specifies a new connection security model for Message
Transfer Agents (MTAs). This model is motivated by key features of Transfer Agents (MTAs). This model is motivated by key features of
inter-domain SMTP delivery, in particular the fact that the inter-domain SMTP delivery, in particular the fact that the
destination server is selected indirectly via DNS Mail Exchange (MX) destination server is selected indirectly via DNS Mail Exchange (MX)
records and that neither email addresses nor MX hostnames signal a records and that neither email addresses nor MX hostnames signal a
requirement for either secure or cleartext transport. Therefore, requirement for either secure or cleartext transport. Therefore,
skipping to change at page 4, line 5 skipping to change at page 4, line 5
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in "OPTIONAL" in this document are to be interpreted as described in
[RFC2119]. [RFC2119].
The following terms or concepts are used through the document: The following terms or concepts are used through the document:
Man-in-the-middle or MITM attack: Active modification of network Man-in-the-middle or MITM attack: Active modification of network
traffic by an adversary able to thereby compromise the traffic by an adversary able to thereby compromise the
confidentiality or integrity of the data. confidentiality or integrity of the data.
Downgrade attack: (From [RFC4949]). A type of man-in-the-middle
attack in which the attacker can cause two parties, at the time
they negotiate a security association, to agree on a lower level
of protection than the highest level that could have been
supported by both of them.
Downgrade-resistant: A protocol is "downgrade-resistant" if it
employs effective counter-measures against downgrade attacks.
secure, bogus, insecure, indeterminate: DNSSEC validation results, secure, bogus, insecure, indeterminate: DNSSEC validation results,
as defined in Section 4.3 of [RFC4035]. as defined in Section 4.3 of [RFC4035].
Validating Security-Aware Stub Resolver and Non-Validating Validating Security-Aware Stub Resolver and Non-Validating
Security-Aware Stub Resolver: Security-Aware Stub Resolver:
Capabilities of the stub resolver in use as defined in [RFC4033]; Capabilities of the stub resolver in use as defined in [RFC4033];
note that this specification requires the use of a Security-Aware note that this specification requires the use of a Security-Aware
Stub Resolver. Stub Resolver.
(pre-DANE) opportunistic TLS: Best-effort use of TLS that is (pre-DANE) opportunistic TLS: Best-effort use of TLS that is
skipping to change at page 15, line 17 skipping to change at page 15, line 23
tamper-evident when SMTP servers found via "insecure" MX records are tamper-evident when SMTP servers found via "insecure" MX records are
recorded as the next-hop relay in the MTA delivery logs in their recorded as the next-hop relay in the MTA delivery logs in their
original (rather than CNAME expanded) form. Sending MTAs SHOULD log original (rather than CNAME expanded) form. Sending MTAs SHOULD log
unexpanded MX hostnames when these result from insecure MX lookups. unexpanded MX hostnames when these result from insecure MX lookups.
Any successful authentication via an insecurely determined MX host Any successful authentication via an insecurely determined MX host
MUST NOT be misrepresented in the mail logs as secure delivery to the MUST NOT be misrepresented in the mail logs as secure delivery to the
intended next-hop domain. When DANE TLS is mandatory (Section 6) for intended next-hop domain. When DANE TLS is mandatory (Section 6) for
a given destination, delivery MUST be delayed when the MX RRSet is a given destination, delivery MUST be delayed when the MX RRSet is
not "secure". not "secure".
If the MX RRset is not "insecure", then -- assuming no DNS lookup In the absence of DNS lookup errors (Section 2.1.1), if the MX RRset
errors (Section 2.1.1) -- the MX RRSet is "secure", and the SMTP is not "insecure" then it is "secure", and the SMTP client MUST treat
client MUST treat each MX hostname as a separate non-MX destination each MX hostname as a separate non-MX destination for opportunistic
for opportunistic DANE TLS (as described in Section 2.2.2). When, DANE TLS (as described in Section 2.2.2). When, for a given MX
for a given MX hostname, no TLSA records are found, or only hostname, no TLSA records are found, or only "insecure" TLSA records
"insecure" TLSA records are found, DANE TLSA is not applicable with are found, DANE TLSA is not applicable with the SMTP server in
the SMTP server in question and delivery proceeds to that host as question and delivery proceeds to that host as with pre-DANE
with pre-DANE opportunistic TLS. To avoid downgrade attacks, any opportunistic TLS. To avoid downgrade attacks, any errors during
errors during TLSA lookups MUST, as explained in Section 2.1.1, cause TLSA lookups MUST, as explained in Section 2.1.1, cause the SMTP
the SMTP server in question to be treated as unreachable. server in question to be treated as unreachable.
2.2.2. Non-MX destinations 2.2.2. Non-MX destinations
This section describes the algorithm used to locate the TLSA records This section describes the algorithm used to locate the TLSA records
and associated TLSA base domain for an input domain that is not and associated TLSA base domain for an input domain that is not
subject to MX resolution or that lacks MX records. Such domains subject to MX resolution or that lacks MX records. Such domains
include: include:
o Any host configured by the sending MTA administrator as the next- o Any host configured by the sending MTA administrator as the next-
hop relay for some or all domains, that is not subject to MX hop relay for some or all domains, that is not subject to MX
skipping to change at page 31, line 48 skipping to change at page 32, line 10
[RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify [RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify
Conversations about DNS-Based Authentication of Named Conversations about DNS-Based Authentication of Named
Entities (DANE)", RFC 7218, April 2014. Entities (DANE)", RFC 7218, April 2014.
13.2. Informative References 13.2. Informative References
[I-D.ietf-dane-srv] [I-D.ietf-dane-srv]
Finch, T., Miller, M., and P. Saint-Andre, "Using DNS- Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
Based Authentication of Named Entities (DANE) TLSA Records Based Authentication of Named Entities (DANE) TLSA Records
with SRV Records", draft-ietf-dane-srv-13 (work in with SRV Records", draft-ietf-dane-srv-14 (work in
progress), April 2015. progress), April 2015.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987. STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987. specification", STD 13, RFC 1035, November 1987.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997. Specification", RFC 2181, July 1997.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC
4949, August 2007.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July
2009. 2009.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, November 2011. STD 72, RFC 6409, November 2011.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection [RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, December 2014. Most of the Time", RFC 7435, December 2014.
Authors' Addresses Authors' Addresses
 End of changes. 14 change blocks. 
23 lines changed or deleted 35 lines changed or added

This html diff was produced by rfcdiff 1.48. The latest version is available from http://tools.ietf.org/tools/rfcdiff/