draft-ietf-uta-email-deep-09.txt   draft-ietf-uta-email-deep-10.txt 
Network Working Group K. Moore Network Working Group K. Moore
Internet-Draft Windrock, Inc. Internet-Draft Windrock, Inc.
Updates: 1939, 2595, 3464, 3501, 5068, C. Newman Updates: 1939, 2595, 3464, 3501, 5068, C. Newman
6186, 6409 (if approved) Oracle 6186, 6409 (if approved) Oracle
Intended status: Standards Track September 12, 2017 Intended status: Standards Track October 26, 2017
Expires: March 16, 2018 Expires: April 29, 2018
Cleartext Considered Obsolete: Use of TLS for Email Submission and Cleartext Considered Obsolete: Use of TLS for Email Submission and
Access Access
draft-ietf-uta-email-deep-09 draft-ietf-uta-email-deep-10
Abstract Abstract
This specification outlines current recommendations for use of This specification outlines current recommendations for the use of
Transport Layer Security (TLS) to provide confidentiality of email Transport Layer Security (TLS) to provide confidentiality of email
traffic between a mail user agent (MUA) and a mail submission or mail traffic between a mail user agent (MUA) and a mail submission or mail
access server. access server.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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 https://datatracker.ietf.org/drafts/current/. Drafts is at https://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 March 16, 2018. This Internet-Draft will expire on April 29, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 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
(https://trustee.ietf.org/license-info) in effect on the date of (https://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
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology Used in This Document . . . . . . 3 2. Conventions and Terminology Used in This Document . . . . . . 3
3. Implicit TLS . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Implicit TLS . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Implicit TLS for POP . . . . . . . . . . . . . . . . . . 5 3.1. Implicit TLS for POP . . . . . . . . . . . . . . . . . . 5
3.2. Implicit TLS for IMAP . . . . . . . . . . . . . . . . . . 5 3.2. Implicit TLS for IMAP . . . . . . . . . . . . . . . . . . 5
3.3. Implicit TLS for SMTP Submission . . . . . . . . . . . . 5 3.3. Implicit TLS for SMTP Submission . . . . . . . . . . . . 5
3.4. Implicit TLS Connection Closure for POP, IMAP and SMTP 3.4. Implicit TLS Connection Closure for POP, IMAP and SMTP
Submission . . . . . . . . . . . . . . . . . . . . . . . 6 Submission . . . . . . . . . . . . . . . . . . . . . . . 6
4. Recommendations for Use of TLS by Mail Access 4. Use of TLS by Mail Access Services and
Services and Message Submission Services . . . . . . . . . . 6 Message Submission Services . . . . . . . . . . . . . . . . . 6
4.1. Deprecation of Services Using Cleartext and TLS Versions 4.1. Deprecation of Services Using Cleartext and TLS Versions
< 1.1 . . . . . . . . . . . . . . . . . . . . . . . . . . 8 < 1.1 . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Mail Server Use of Client Certificate Authentication . . 8 4.2. Mail Server Use of Client Certificate Authentication . . 9
4.3. Recording TLS Cipher Suite in Received Header . . . . . . 9 4.3. Recording TLS Cipher Suite in Received Header . . . . . . 9
4.4. TLS Server Certificate Requirements . . . . . . . . . . . 9 4.4. TLS Server Certificate Requirements . . . . . . . . . . . 10
4.5. Recommended DNS records for mail protocol servers . . . . 10 4.5. Recommended DNS records for mail protocol servers . . . . 10
4.5.1. MX records . . . . . . . . . . . . . . . . . . . . . 10 4.5.1. MX records . . . . . . . . . . . . . . . . . . . . . 10
4.5.2. SRV records . . . . . . . . . . . . . . . . . . . . . 10 4.5.2. SRV records . . . . . . . . . . . . . . . . . . . . . 10
4.5.3. DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 10 4.5.3. DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 10
4.5.4. TLSA records . . . . . . . . . . . . . . . . . . . . 10 4.5.4. TLSA records . . . . . . . . . . . . . . . . . . . . 10
4.6. Changes to Internet Facing Servers . . . . . . . . . . . 10 4.6. Changes to Internet Facing Servers . . . . . . . . . . . 11
5. Recommendations for use of TLS by Mail User Agents . . . . . 10 5. Use of TLS by Mail User Agents . . . . . . . . . . . . . . . 11
5.1. Use of SRV records in Establishing Configuration . . . . 12 5.1. Use of SRV records in Establishing Configuration . . . . 12
5.2. Minimum Confidentiality Level . . . . . . . . . . . . . . 13 5.2. Minimum Confidentiality Level . . . . . . . . . . . . . . 13
5.3. Certificiate Validation . . . . . . . . . . . . . . . . . 14 5.3. Certificiate Validation . . . . . . . . . . . . . . . . . 14
5.4. Certificate Pinning . . . . . . . . . . . . . . . . . . . 14 5.4. Certificate Pinning . . . . . . . . . . . . . . . . . . . 14
5.5. Client Certificate Authentication . . . . . . . . . . . . 15 5.5. Client Certificate Authentication . . . . . . . . . . . . 15
6. Considerations related to Anti-Virus/Anti-Spam Software and 6. Considerations related to Anti-Virus/Anti-Spam Software and
Services . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Services . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
7.1. POP3S Port Registration Update . . . . . . . . . . . . . 16 7.1. POP3S Port Registration Update . . . . . . . . . . . . . 16
7.2. IMAPS Port Registration Update . . . . . . . . . . . . . 16 7.2. IMAPS Port Registration Update . . . . . . . . . . . . . 17
7.3. Submissions Port Registration . . . . . . . . . . . . . . 16 7.3. Submissions Port Registration . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17 8. Security Considerations . . . . . . . . . . . . . . . . . . . 18
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . 17 9.1. Normative References . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . 19 9.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Design Considerations . . . . . . . . . . . . . . . 20 Appendix A. Design Considerations . . . . . . . . . . . . . . . 22
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 21 Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 23
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 27 Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction 1. Introduction
Software that provides email service via Internet Message Access Software that provides email service via Internet Message Access
Protocol (IMAP) [RFC3501], Post Office Protocol (POP) [RFC1939] and/ Protocol (IMAP) [RFC3501], Post Office Protocol (POP) [RFC1939] and/
or Simple Mail Transfer Protocol (SMTP) Submission [RFC6409] usually or Simple Mail Transfer Protocol (SMTP) Submission [RFC6409] usually
has Transport Layer Security (TLS) [RFC5246] support but often does has Transport Layer Security (TLS) [RFC5246] support but often does
not use it in a way that maximizes end-user confidentiality. This not use it in a way that maximizes end-user confidentiality. This
specification describes current recommendations for use of TLS in specification describes current recommendations for the use of TLS in
interactions between Mail User Agents and Mail Access Services, and interactions between Mail User Agents and Mail Access Services, and
between Mail User Agents and Mail Submission Services. between Mail User Agents and Mail Submission Services.
In brief, this memo now recommends that: In brief, this memo now recommends that:
o TLS version 1.2 or greater be used for all traffic between mail o TLS version 1.2 or greater be used for all traffic between mail
user agents (MUAs) and mail submission servers, and also between user agents (MUAs) and mail submission servers, and also between
MUAs and mail access servers. MUAs and mail access servers.
o MUAs and mail service providers discourage use of cleartext o MUAs and mail service providers discourage use of cleartext
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approach to address that topic is described in [RFC7672]; another is approach to address that topic is described in [RFC7672]; another is
in [I-D.ietf-uta-mta-sts]. in [I-D.ietf-uta-mta-sts].
The recommendations in this memo do not replace the functionality of, The recommendations in this memo do not replace the functionality of,
and are not intended as a substitute for, end-to-end encryption of and are not intended as a substitute for, end-to-end encryption of
electronic mail. electronic mail.
2. Conventions and Terminology Used in This Document 2. Conventions and Terminology Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
document are to be interpreted as described in [RFC2119]. "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The term "Implicit TLS" refers to the automatic negotiation of TLS The term "Implicit TLS" refers to the automatic negotiation of TLS
whenever a TCP connection is made on a particular TCP port that is whenever a TCP connection is made on a particular TCP port that is
used exclusively by that server for TLS connections. The term used exclusively by that server for TLS connections. The term
"Implicit TLS" is intended to contrast with use of STARTTLS and "Implicit TLS" is intended to contrast with use of STARTTLS and
similar commands in POP, IMAP, SMTP message submission, and other similar commands in POP, IMAP, SMTP message submission, and other
protocols, that are used by client and server to explicitly negotiate protocols, that are used by client and server to explicitly negotiate
TLS on an established cleartext TCP connection. TLS on an established cleartext TCP connection.
The term "Mail Access Services" includes POP, IMAP and any other The term "Mail Access Services" includes POP, IMAP and any other
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3.1. Implicit TLS for POP 3.1. Implicit TLS for POP
When a TCP connection is established for the "pop3s" service (default When a TCP connection is established for the "pop3s" service (default
port 995), a TLS handshake begins immediately. Clients MUST port 995), a TLS handshake begins immediately. Clients MUST
implement the certificate validation mechanism described in implement the certificate validation mechanism described in
[RFC7817]. Once the TLS session is established, POP3 [RFC1939] [RFC7817]. Once the TLS session is established, POP3 [RFC1939]
protocol messages are exchanged as TLS application data for the protocol messages are exchanged as TLS application data for the
remainder of the TCP connection. After the server sends a +OK remainder of the TCP connection. After the server sends a +OK
greeting, the server and client MUST enter AUTHORIZATION state, even greeting, the server and client MUST enter AUTHORIZATION state, even
if client credentials were supplied during the TLS handshake. if a client certificate was supplied during the TLS handshake.
See Section 5.5 and Section 4.2 for additional information on client See Section 5.5 and Section 4.2 for additional information on client
certificate authentication. See Section 7.1 for port registration certificate authentication. See Section 7.1 for port registration
information. information.
3.2. Implicit TLS for IMAP 3.2. Implicit TLS for IMAP
When a TCP connection is established for the "imaps" service (default When a TCP connection is established for the "imaps" service (default
port 993), a TLS handshake begins immediately. Clients MUST port 993), a TLS handshake begins immediately. Clients MUST
implement the certificate validation mechanism described in [RFC3501] implement the certificate validation mechanism described in
and SHOULD implement the certificate validation mechanism described [RFC7817]. Once the TLS session is established, IMAP [RFC3501]
in [RFC7817]. Once the TLS session is established, IMAP [RFC3501]
protocol messages are exchanged as TLS application data for the protocol messages are exchanged as TLS application data for the
remainder of the TCP connection. If client credentials were provided remainder of the TCP connection. If a client certificate was
during the TLS handshake that the server finds acceptable, the server provided during the TLS handshake that the server finds acceptable,
MAY issue a PREAUTH greeting in which case both the server and client the server MAY issue a PREAUTH greeting in which case both the server
enter AUTHENTICATED state. If the server issues an OK greeting then and client enter AUTHENTICATED state. If the server issues an OK
both server and client enter NOT AUTHENTICATED state. greeting then both server and client enter NOT AUTHENTICATED state.
See Section 5.5 and Section 4.2 for additional information on client See Section 5.5 and Section 4.2 for additional information on client
certificate authentication. See Section 7.1 and Section 7.2 for port certificate authentication. See Section 7.1 and Section 7.2 for port
registration information. registration information.
3.3. Implicit TLS for SMTP Submission 3.3. Implicit TLS for SMTP Submission
When a TCP connection is established for the "submissions" service When a TCP connection is established for the "submissions" service
(default port 465), a TLS handshake begins immediately. Clients MUST (default port 465), a TLS handshake begins immediately. Clients MUST
implement the certificate validation mechanism described in implement the certificate validation mechanism described in
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See Section 5.5 and Section 4.2 for additional information on client See Section 5.5 and Section 4.2 for additional information on client
certificate authentication. See Section 7.3 for port registration certificate authentication. See Section 7.3 for port registration
information. information.
3.4. Implicit TLS Connection Closure for POP, IMAP and SMTP Submission 3.4. Implicit TLS Connection Closure for POP, IMAP and SMTP Submission
When a client or server wishes to close the connection, it SHOULD When a client or server wishes to close the connection, it SHOULD
initiate the exchange of TLS close alerts before TCP connection initiate the exchange of TLS close alerts before TCP connection
termination. The client MAY, after sending a TLS close alert, termination. The client MAY, after sending a TLS close alert,
gracefully close the TCP connection without waiting for a TLS gracefully close the TCP connection (e.g. call the close() function
response from the server. on the TCP socket or otherwise issue a TCP CLOSE ([RFC0793] section
3.5) without waiting for a TLS response from the server.
4. Recommendations for Use of TLS by Mail Access Services and Message 4. Use of TLS by Mail Access Services and Message Submission Services
Submission Services
The following practices are recommended for Mail Access Services and The following requirements and recommendations apply to Mail Access
Mail Submission Services: Services and Mail Submission Services:
o Mail Service Providers (MSPs) which support POP, IMAP, and/or o Mail Service Providers (MSPs) that support POP, IMAP, and/or
Message Submission, MUST support TLS access for those services. Message Submission, MUST support TLS access for those services.
o Other services than POP, IMAP and/or Message Submission provided o Other services than POP, IMAP and/or Message Submission provided
by MSPs SHOULD support TLS access, and MUST support TLS access for by MSPs SHOULD support TLS access, and MUST support TLS access for
those services which support authentication via username and those services which support authentication via username and
password. password.
o MSPs which support POP, IMAP, and/or Message Submission, SHOULD o MSPs that support POP, IMAP, and/or Message Submission, SHOULD
provide and support instances of those services which use Implicit provide and support instances of those services which use Implicit
TLS. (See Section 3.) TLS. (See Section 3.)
o For compatibility with existing MUAs and existing MUA o For compatibility with existing MUAs and existing MUA
configurations, MSPs SHOULD also, in the near term, provide configurations, MSPs SHOULD also, in the near term, provide
instances of these services which support STARTTLS. This will instances of these services which support STARTTLS. This will
permit legacy MUAs to discover new availability of TLS capability permit legacy MUAs to discover new availability of TLS capability
on servers, and may increase use of TLS by such MUAs. However, on servers, and may increase use of TLS by such MUAs. However,
servers SHOULD NOT advertise STARTTLS if use of the STARTTLS servers SHOULD NOT advertise STARTTLS if use of the STARTTLS
command by a client is likely to fail (for example, if the server command by a client is likely to fail (for example, if the server
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After a user's ability to authenticate to a service using cleartext After a user's ability to authenticate to a service using cleartext
is revoked, the server denying such access MUST NOT provide any is revoked, the server denying such access MUST NOT provide any
indication over a cleartext channel of whether the user's indication over a cleartext channel of whether the user's
authentication credentials were valid. An attempt to authenticate as authentication credentials were valid. An attempt to authenticate as
such a user using either invalid credentials or valid credentials such a user using either invalid credentials or valid credentials
MUST both result in the same indication of access being denied. MUST both result in the same indication of access being denied.
Also, users previously authenticating with passwords sent as Also, users previously authenticating with passwords sent as
cleartext SHOULD be required to change those passwords when migrating cleartext SHOULD be required to change those passwords when migrating
to TLS, since the old passwords were likely to have been compromised. to TLS, if the old passwords were likely to have been compromised.
(For any large community of users using public Internet to access
mail without encryption, compromise of at least some of those
passwords should be assumed.)
Transition of users from SSL or TLS 1.0 to later versions of TLS MAY Transition of users from SSL or TLS 1.0 to later versions of TLS MAY
be accomplished by a means similar to that described above. There be accomplished by a means similar to that described above. There
are multiple ways to accomplish this. One way is for the server to are multiple ways to accomplish this. One way is for the server to
refuse a ClientHello message from any client sending a protocol refuse a ClientHello message from any client sending a
version number corresponding to any version of SSL or TLS 1.0. ClientHello.version field corresponding to any version of SSL or TLS
Another way is for the server to accept ClientHello messages from 1.0. Another way is for the server to accept ClientHello messages
some client versions that it does not wish to support, but later from some client versions that it does not wish to support, but later
refuse to allow the user to authenticate. The latter method may refuse to allow the user to authenticate. The latter method may
provide a better indication to the user of the reason for the failure provide a better indication to the user of the reason for the failure
but (depending on the protocol and method of authentication used) may but (depending on the protocol and method of authentication used) may
also risk exposure of the user's password over an channel which is also risk exposure of the user's password over an channel which is
known to not provide adequate confidentiality. known to not provide adequate confidentiality.
It is RECOMMENDED that new users be required to use TLS version 1.1 It is RECOMMENDED that new users be required to use TLS version 1.1
or greater from the start. However an MSP may find it necessary to or greater from the start. However an MSP may find it necessary to
make exceptions to accommodate some legacy systems which support only make exceptions to accommodate some legacy systems which support only
earlier versions of TLS, or only cleartext. earlier versions of TLS, or only cleartext.
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4.3. Recording TLS Cipher Suite in Received Header 4.3. Recording TLS Cipher Suite in Received Header
The ESMTPS transmission type [RFC3848] provides trace information The ESMTPS transmission type [RFC3848] provides trace information
that can indicate TLS was used when transferring mail. However, TLS that can indicate TLS was used when transferring mail. However, TLS
usage by itself is not a guarantee of confidentiality or security. usage by itself is not a guarantee of confidentiality or security.
The TLS cipher suite provides additional information about the level The TLS cipher suite provides additional information about the level
of security made available for a connection. This defines a new SMTP of security made available for a connection. This defines a new SMTP
"tls" Received header additional-registered-clause that is used to "tls" Received header additional-registered-clause that is used to
record the TLS cipher suite that was negotiated for the connection. record the TLS cipher suite that was negotiated for the connection.
The value included in this additional clause SHOULD be the registered This clause SHOULD be included whenever a Submission server generates
cipher suite name (e.g., TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) a Received header field for a message received via TLS. The value
included in the TLS cipher suite registry. In the event the included in this additional clause SHOULD be the registered cipher
implementation does not know the name of the cipher suite (a suite name (e.g., TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) included in
situation that should be remedied promptly), a four-digit hexadecimal the TLS cipher suite registry. In the event the implementation does
cipher suite identifier MAY be used. The ABNF for the field follows: not know the name of the cipher suite (a situation that should be
remedied promptly), a four-digit hexadecimal cipher suite identifier
MAY be used. In addition, the Diffie-Hellman group name associated
with the ciphersuite MAY be included (when applicable and known)
following the ciphersuite name. The ABNF for the field follows:
tls-cipher-clause = CFWS "tls" FWS tls-cipher tls-cipher-clause = CFWS "tls" FWS tls-cipher [ FWS "group" FWS dh-group ]
tls-cipher = tls-cipher-suite-name / tls-cipher-suite-hex tls-cipher = tls-cipher-name / tls-cipher-hex
tls-cipher-name = ALPHA *(ALPHA / DIGIT / "_") tls-cipher-name = ALPHA *(ALPHA / DIGIT / "_")
; as registered in IANA cipher suite registry ; as registered in IANA cipher suite registry
tls-cipher-hex = "0x" 4HEXDIG tls-cipher-hex = "0x" 4HEXDIG
dh-group = ALPHA *(ALPHA / DIGIT / "_" / "-")
; as registered in IANA TLS Supported Groups Registry
4.4. TLS Server Certificate Requirements 4.4. TLS Server Certificate Requirements
MSPs MUST maintain valid server certificates for all servers. See MSPs MUST maintain valid server certificates for all servers. See
[RFC7817] for the recommendations and requirements necessary to [RFC7817] for the recommendations and requirements necessary to
achieve this. achieve this.
If a protocol server provides service for more than one mail domain, If a protocol server provides service for more than one mail domain,
it MAY use a separate IP address for each domain and/or a server it MAY use a separate IP address for each domain and/or a server
certificate that advertises multiple domains. This will generally be certificate that advertises multiple domains. This will generally be
necessary unless and until it is acceptable to impose the constraint necessary unless and until it is acceptable to impose the constraint
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4.5. Recommended DNS records for mail protocol servers 4.5. Recommended DNS records for mail protocol servers
This section discusses not only the DNS records that are recommended, This section discusses not only the DNS records that are recommended,
but also implications of DNS records for server configuration and TLS but also implications of DNS records for server configuration and TLS
server certificates. server certificates.
4.5.1. MX records 4.5.1. MX records
It is recommended that MSPs advertise MX records for handling of It is recommended that MSPs advertise MX records for handling of
inbound mail (instead of relying entirely on A or AAAA records), and inbound mail (instead of relying entirely on A or AAAA records), and
that those MX records be signed using DNSSEC. This is mentioned here that those MX records be signed using DNSSEC [RFC4033]. This is
only for completeness, as handling of inbound mail is out of scope mentioned here only for completeness, as handling of inbound mail is
for this document. out of scope for this document.
4.5.2. SRV records 4.5.2. SRV records
MSPs SHOULD advertise SRV records to aid MUAs in determination of MSPs SHOULD advertise SRV records to aid MUAs in determination of
proper configuration of servers, per the instructions in [RFC6186]. proper configuration of servers, per the instructions in [RFC6186].
MSPs SHOULD advertise servers that support Implicit TLS in preference MSPs SHOULD advertise servers that support Implicit TLS in preference
to those which support cleartext and/or STARTTLS operation. to those which support cleartext and/or STARTTLS operation.
4.5.3. DNSSEC 4.5.3. DNSSEC
All DNS records advertised by an MSP as a means of aiding clients in All DNS records advertised by an MSP as a means of aiding clients in
communicating with the MSP's servers, SHOULD be signed using DNSSEC. communicating with the MSP's servers, SHOULD be signed using DNSSEC
if and when the parent DNS zone supports doing so.
4.5.4. TLSA records 4.5.4. TLSA records
MSPs SHOULD advertise TLSA records to provide an additional trust MSPs SHOULD advertise TLSA records to provide an additional trust
anchor for public keys used in TLS server certificates. However, anchor for public keys used in TLS server certificates. However,
TLSA records MUST NOT be advertised unless they are signed using TLSA records MUST NOT be advertised unless they are signed using
DNSSEC. DNSSEC.
4.6. Changes to Internet Facing Servers 4.6. Changes to Internet Facing Servers
When an MSP changes the Internet Facing Servers providing mail access When an MSP changes the Internet Facing Servers providing mail access
and mail submission services, including SMTP-based spam/virus and mail submission services, including SMTP-based spam/virus
filters, it is generally necessary to support the same and/or a newer filters, it is generally necessary to support the same and/or a newer
version of TLS and the same security directives that were previously version of TLS and the same security directives that were previously
advertised. advertised.
5. Recommendations for use of TLS by Mail User Agents 5. Use of TLS by Mail User Agents
It is recommended that Mail User Agents implement the following The following requirements and recommendations apply to Mail User
practices: Agents:
o MUAs SHOULD be capable of using DNS SRV records to discover Mail o MUAs SHOULD be capable of using DNS SRV records to discover Mail
Access Services and Mail Submission Services that are advertised Access Services and Mail Submission Services that are advertised
by a MSP for an account being configured. Other means of by a MSP for an account being configured. Other means of
discovering server configuration information (e.g. a database discovering server configuration information (e.g. a database
maintained by the MUA vendor) MAY also be supported. (See maintained by the MUA vendor) MAY also be supported. (See
Section 5.1 for more information.) Section 5.1 for more information.)
o MUAs SHOULD be configurable to require a minimum level of o MUAs SHOULD be configurable to require a minimum level of
confidentiality for any particular Mail Account, and refuse to confidentiality for any particular Mail Account, and refuse to
exchange information via any service associated with that Mail exchange information via any service associated with that Mail
Account if the session does not provide that minimum level of Account if the session does not provide that minimum level of
confidentiality. (See Section 5.2.) confidentiality. (See Section 5.2.)
o MUAs MUST NOT consider a session as meeting a minimum level of o MUAs MUST NOT treat a session as meeting a minimum level of
confidentiality if the server's TLS certificate cannot be confidentiality if the server's TLS certificate cannot be
validated. (See Section 5.3.) validated. (See Section 5.3.)
o MUAs MAY impose other minimum confidentiality requirements in the o MUAs MAY impose other minimum confidentiality requirements in the
future, e.g. in order to discourage use of TLS versions or future, e.g. in order to discourage use of TLS versions or
cryptographic algorithms in which weaknesses have been discovered. cryptographic algorithms in which weaknesses have been discovered.
o MUAs SHOULD provide a prominent visual indication of the level of o MUAs SHOULD provide a prominent indication of the level of
confidentiality associated with an account configuration (for confidentiality associated with an account configuration that is
example, indications such as "lock" icons or changed background appropriate for the user interface (for example, a "lock" icon or
colors similar to those used by some browsers), at appropriate changed background color for a visual interface, or some sort of
times and locations in order to inform the user of the audible indication for an audio user interface), at appropriate
times and/or locations in order to inform the user of the
confidentiality of the communications associated with that confidentiality of the communications associated with that
account. For example, this might be done whenever (a) prompting account. For example, this might be done whenever (a) prompting
the user for authentication credentials, (b) the user is composing the user for authentication credentials, (b) the user is composing
mail that will be sent to a particular submission server, (c) a mail that will be sent to a particular submission server, (c) a
list of accounts is displayed (particularly if the user can select list of accounts is displayed (particularly if the user can select
from that list to read mail), or (d) the user is requesting to from that list to read mail), or (d) the user is requesting to
view or update any configuration data that will be stored on a view or update any configuration data that will be stored on a
remote server. remote server. If, however, an MUA provides such an indication,
it MUST NOT indicate confidentiality for any connection that does
not at least use TLS 1.1 with certificate verification and also
meet the minimum confidentiality requirements associated with that
account.
o MUAs MUST implement TLS 1.2 [RFC5246] or later. Earlier TLS and o MUAs MUST implement TLS 1.2 [RFC5246] or later. Earlier TLS and
SSL versions MAY also be supported so long as the MUA requires at SSL versions MAY also be supported so long as the MUA requires at
least TLS 1.1 [RFC4346] when accessing accounts that are least TLS 1.1 [RFC4346] when accessing accounts that are
configured to impose minimum confidentiality requirements. Per configured to impose minimum confidentiality requirements.
[RFC7525], TLS 1.1 (or earlier) SHOULD NOT be used unless no
higher version is available during TLS protocol negotiation.
o All MUAs SHOULD implement the recommended TLS cipher suites o All MUAs SHOULD implement the recommended TLS cipher suites
described in [RFC7525] or a future BCP or standards track revision described in [RFC7525] or a future BCP or standards track revision
of that document. of that document.
o MUAs that are configured to not require minimum confidentiality o MUAs that are configured to not require minimum confidentiality
for one or more accounts SHOULD detect when TLS becomes available for one or more accounts SHOULD detect when TLS becomes available
on those accounts (using [RFC6186] or other means), and offer to on those accounts (using [RFC6186] or other means), and offer to
upgrade the account to require TLS. upgrade the account to require TLS.
skipping to change at page 12, line 47 skipping to change at page 13, line 15
Similarly, a MUA MUST NOT attempt to "test" a particular mail account Similarly, a MUA MUST NOT attempt to "test" a particular mail account
configuration by submitting the user's authentication credentials to configuration by submitting the user's authentication credentials to
a server, unless a TLS session meeting minimum confidentiality levels a server, unless a TLS session meeting minimum confidentiality levels
has been established with that server. If minimum confidentiality has been established with that server. If minimum confidentiality
requirements have not been satisfied, the MUA must explicitly warn requirements have not been satisfied, the MUA must explicitly warn
the user that his password may be exposed to attackers before testing the user that his password may be exposed to attackers before testing
the new configuration. the new configuration.
When establishing a new configuration for connecting to an IMAP, POP, When establishing a new configuration for connecting to an IMAP, POP,
or SMTP submission server, based on SRV records, an MUA SHOULD either or SMTP submission server, based on SRV records, an MUA SHOULD either
verify that the SRV records are verifiably signed using DNSSEC, or verify that the SRV records are verifiability signed using DNSSEC, or
that the target FQDN of the SRV record matches the original server that the target FQDN of the SRV record matches the original server
FQDN for which the SRV queries were made. If the target FQDN is not FQDN for which the SRV queries were made. If the target FQDN is not
in the queried domain, the MUA SHOULD verify with the user that the in the queried domain, the MUA SHOULD verify with the user that the
SRV target FQDN is suitable for use, before executing any connections SRV target FQDN is suitable for use, before executing any connections
to the host. (See [RFC6186] section 6). to the host. (See [RFC6186] section 6).
An MUA MUST NOT consult SRV records to determine which servers to use An MUA MUST NOT consult SRV records to determine which servers to use
on every connection attempt, unless those SRV records are signed by on every connection attempt, unless those SRV records are signed by
DNSSEC and have a valid signature. However, an MUA MAY consult SRV DNSSEC and have a valid signature. However, an MUA MAY consult SRV
records from time to time to determine if an MSP's server records from time to time to determine if an MSP's server
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support it. support it.
5.2. Minimum Confidentiality Level 5.2. Minimum Confidentiality Level
MUAs SHOULD, by default, require a minimum level of confidentiality MUAs SHOULD, by default, require a minimum level of confidentiality
for services accessed by each account. For MUAs supporting the for services accessed by each account. For MUAs supporting the
ability to access multiple mail accounts, this requirement SHOULD be ability to access multiple mail accounts, this requirement SHOULD be
configurable on a per-account basis. configurable on a per-account basis.
The default minimum expected level of confidentiality for all new The default minimum expected level of confidentiality for all new
accounts SHOULD be at least use of TLS version 1.1 or greater, and accounts MUST require successful validation of the server's
successful validation of the server's certificate. (Future revisions certificate and SHOULD require negotiation of TLS version 1.1 or
to this specification may raise these requirements or impose greater. (Future revisions to this specification may raise these
additional requirements to address newly-discovered weaknesses in requirements or impose additional requirements to address newly-
protocols or cryptographic algorithms.) discovered weaknesses in protocols or cryptographic algorithms.)
MUAs MAY permit the user to disable this minimum confidentiality MUAs MAY permit the user to disable this minimum confidentiality
requirement during initial account configuration, or subsequently requirement during initial account configuration, or subsequently
editing an account configuration, but MUST warn users that such a editing an account configuration, but MUST warn users that such a
configuration will not assure privacy for either passwords or configuration will not assure privacy for either passwords or
messages. messages.
An MUA which is configured to require a minimum level of An MUA which is configured to require a minimum level of
confidentiality for a mail account MUST NOT attempt to perform any confidentiality for a mail account MUST NOT attempt to perform any
operation other than capability discovery, or STARTTLS for servers operation other than capability discovery, or STARTTLS for servers
not using Implicit TLS, unless the minimum level of confidentiality not using Implicit TLS, unless the minimum level of confidentiality
is provided by that connection. is provided by that connection.
MUAs SHOULD NOT allow users to easily access or send mail via an MUAs SHOULD NOT allow users to easily access or send mail via an
connection, or authenticate to any service using a password, if that connection, or authenticate to any service using a password, if that
account is configured to impose minimum confidentiality requirements account is configured to impose minimum confidentiality requirements
and that connection does not meet all of those requirements. An and that connection does not meet all of those requirements. An
example of "easily access" would be to display a dialog informing the example of "easily access" would be to display a dialog informing the
user that the security requirements of the account were not met by user that the security requirements of the account were not met by
the connecting, but allowing the user to "click through" to send mail the connection, but allowing the user to "click through" to send mail
or access the service anyway. Experience indicates that users or access the service anyway. Experience indicates that users
presented with such an option often "click through" without presented with such an option often "click through" without
understanding the risks that they're accepting by doing so. understanding the risks that they're accepting by doing so.
Furthermore, users who frequently find the need to "click through" to Furthermore, users who frequently find the need to "click through" to
use an insecure connection may become conditioned to do so as a use an insecure connection may become conditioned to do so as a
matter of habit, before considering whether the risks are reasonable matter of habit, before considering whether the risks are reasonable
in each specific instance. in each specific instance.
An MUA which is not configured to require a minimum level of An MUA which is not configured to require a minimum level of
confidentiality for a mail account SHOULD still attempt to connect to confidentiality for a mail account SHOULD still attempt to connect to
the services associated with that account using the most secure means the services associated with that account using the most secure means
available, e.g. by using Implicit TLS or STARTTLS. available, e.g. by using Implicit TLS or STARTTLS.
skipping to change at page 14, line 41 skipping to change at page 15, line 7
account services such as mail access and mail submission (the account services such as mail access and mail submission (the
previous section describes one way to do this). The certificates for previous section describes one way to do this). The certificates for
these servers are verified using the rules described in [RFC7817] and these servers are verified using the rules described in [RFC7817] and
PKIX [RFC5280]. In the event the certificate does not validate due PKIX [RFC5280]. In the event the certificate does not validate due
to an expired certificate, lack of appropriate chain of trust, or to an expired certificate, lack of appropriate chain of trust, or
lack of identifier match, the MUA MAY offer to create a persistent lack of identifier match, the MUA MAY offer to create a persistent
binding between that certificate and the saved host name for the binding between that certificate and the saved host name for the
server, for use when accessing that account's servers. This is server, for use when accessing that account's servers. This is
called certificate pinning. called certificate pinning.
(Note: This use of the term "certificate pinning" means something
subtly different than "HTTP Public Key Pinning" [RFC7469]. The dual
use of the same term is confusing, but unfortunately both uses are
well-established.)
Certificate pinning is only appropriate during mail account setup and Certificate pinning is only appropriate during mail account setup and
MUST NOT be offered as an option in response to a failed certificate MUST NOT be offered as an option in response to a failed certificate
validation for an existing mail account. An MUA that allows validation for an existing mail account. An MUA that allows
certificate pinning MUST NOT allow a certificate pinned for one certificate pinning MUST NOT allow a certificate pinned for one
account to validate connections for other accounts. account to validate connections for other accounts. An MUA that
allows certificate pinning MUST also allow a user to undo the
pinning, i.e. to revoke trust in a certificate that has previously
been pinned.
A pinned certificate is subject to a man-in-the-middle attack at A pinned certificate is subject to a man-in-the-middle attack at
account setup time, and typically lacks a mechanism to automatically account setup time, and typically lacks a mechanism to automatically
revoke or securely refresh the certificate. Note also that a man-in- revoke or securely refresh the certificate. Note also that a man-in-
the-middle attack at account setup time will expose the user's the-middle attack at account setup time will expose the user's
password to the attacker (if a password is used). Therefore use of a password to the attacker (if a password is used). Therefore use of a
pinned certificate does not meet the requirement for a minimum pinned certificate does not meet the requirement for a minimum
confidentiality level, and an MUA MUST NOT indicate to the user that confidentiality level, and an MUA MUST NOT indicate to the user that
the such confidentiality is provided. Additional advice on the such confidentiality is provided. Additional advice on
certificate pinning is present in [RFC6125]. certificate pinning is present in [RFC6125].
5.5. Client Certificate Authentication 5.5. Client Certificate Authentication
MUAs MAY implement client certificate authentication on the Implicit MUAs MAY implement client certificate authentication on the Implicit
TLS port. An MUA MUST NOT provide a client certificate during the TLS port. An MUA MUST NOT provide a client certificate during the
TLS handshake unless the server requests one and the client has TLS handshake unless the server requests one and the MUA has been
determined the certificate can be safely used with that specific authorized to use that client certificate with that account. Having
server, OR the client has been explicitly configured by the user to the end-user explicitly configure a client certificate for use with a
use that particular certificate with that server. How to make this given account is sufficient to meet this requirement. However,
determination is presently implementation specific. installing a client certificate for use with one account MUST NOT
automatically authorize use of that certificate with other accounts.
This is not intended to prohibit site-specific authorization
mechanisms, such as a site-administrator-controlled mechanism to
authorize use of a client certificate with a given account, or a
domain-name matching mechanism.
Note: The requirement that the server request a certificate is just a
restatement of the TLS protocol rules, e.g. [RFC5246] section 7.4.6.
The requirement that the client not send a certificate not known to
be acceptable to the server is pragmatic in multiple ways: the
current TLS protocol provides no way for the client to know which of
potentially multiple certificates it should use; also, when the
client sends a certificate it is potentially disclosing its identity
(or its user's identity) to both the server and to any party with
access to the transmission medium, perhaps unnecessarily and for no
useful purpose.
A client supporting client certificate authentication with Implicit A client supporting client certificate authentication with Implicit
TLS MUST implement the SASL EXTERNAL [RFC4422] mechanism using the TLS MUST implement the SASL EXTERNAL [RFC4422] mechanism using the
appropriate authentication command (AUTH for POP3 [RFC5034], AUTH for appropriate authentication command (AUTH for POP3 [RFC5034], AUTH for
SMTP Submission [RFC4954], AUTHENTICATE for IMAP [RFC3501]). SMTP Submission [RFC4954], AUTHENTICATE for IMAP [RFC3501]).
6. Considerations related to Anti-Virus/Anti-Spam Software and Services 6. Considerations related to Anti-Virus/Anti-Spam Software and Services
There are multiple ways to connect an Anti-Virus and/or Anti-Spam There are multiple ways to connect an Anti-Virus and/or Anti-Spam
(AVAS) service to a mail server. Some mechanisms, such as the de- (AVAS) service to a mail server. Some mechanisms, such as the de-
skipping to change at page 16, line 4 skipping to change at page 16, line 38
Note that end-to-end email encryption prevents AVAS software and Note that end-to-end email encryption prevents AVAS software and
services from using email content as part of a spam or virus services from using email content as part of a spam or virus
assessment. Furthermore, while a minimum confidentiality level can assessment. Furthermore, while a minimum confidentiality level can
prevent a man-in-the-middle from introducing spam or virus content prevent a man-in-the-middle from introducing spam or virus content
between the MUA and Submission server, it does not prevent other between the MUA and Submission server, it does not prevent other
forms of client or account compromise. Use of AVAS services for forms of client or account compromise. Use of AVAS services for
submitted email therefore remains necessary. submitted email therefore remains necessary.
7. IANA Considerations 7. IANA Considerations
7.1. POP3S Port Registration Update 7.1. POP3S Port Registration Update
IANA is asked to update the registration of the TCP well-known port IANA is asked to update the registration of the TCP well-known port
995 using the following template ([RFC6335]): 995, and also UDP well-known port 995, using the following templates
([RFC6335]):
Service Name: pop3s Service Name: pop3s
Transport Protocol: TCP Transport Protocol: TCP
Assignee: IETF <iesg@ietf.org> Assignee: IETF <iesg@ietf.org>
Contact: IESG <iesg@ietf.org> Contact: IESG <iesg@ietf.org>
Description: POP3 over TLS protocol Description: POP3 over TLS protocol
Reference: RFC XXXX (this document once published) Reference: RFC XXXX (this document once published)
Port Number: 995 Port Number: 995
Service Name: pop3s
Transport Protocol: UDP
Assignee: IETF <iesg@ietf.org>
Contact: IESG <iesg@ietf.org>
Description: POP3 over TLS protocol
Reference: RFC XXXX (this document once published)
Port Number: 995
7.2. IMAPS Port Registration Update 7.2. IMAPS Port Registration Update
IANA is asked to update the registration of the TCP well-known port IANA is asked to update the registration of the TCP well-known port
993 using the following template ([RFC6335]): 993, and also UDP well-known port 993, using the following templates
([RFC6335]):
Service Name: imaps Service Name: imaps
Transport Protocol: TCP Transport Protocol: TCP
Assignee: IETF <iesg@ietf.org> Assignee: IETF <iesg@ietf.org>
Contact: IESG <iesg@ietf.org> Contact: IESG <iesg@ietf.org>
Description: IMAP over TLS protocol Description: IMAP over TLS protocol
Reference: RFC XXXX (this document once published) Reference: RFC XXXX (this document once published)
Port Number: 993 Port Number: 993
Service Name: imaps
Transport Protocol: UDP
Assignee: IETF <iesg@ietf.org>
Contact: IESG <iesg@ietf.org>
Description: IMAP over TLS protocol
Reference: RFC XXXX (this document once published)
Port Number: 993
Note: the updates to the UDP port assignments preserve the pre-
existing alignment in which both TCP and UDP ports were allocated to
the same protocols (in this case POP3+TLS and IMAP4+TLS) even though
there is no specification for using either protocol over UDP. This
seems undesirable as it wastes UDP port assignments. However, it is
not within the scope of this document to revisit old conventions for
port assignments.
7.3. Submissions Port Registration 7.3. Submissions Port Registration
IANA is asked to assign an alternate usage of port 465 in addition to IANA is asked to assign an alternate usage of TCP port 465 in
the current assignment using the following template ([RFC6335]): addition to the current assignment using the following template
([RFC6335]):
Service Name: submissions Service Name: submissions
Transport Protocol: TCP Transport Protocol: TCP
Assignee: IETF <iesg@ietf.org> Assignee: IETF <iesg@ietf.org>
Contact: IESG <iesg@ietf.org> Contact: IESG <iesg@ietf.org>
Description: Message Submission over TLS protocol Description: Message Submission over TLS protocol
Reference: RFC XXXX (this document once published) Reference: RFC XXXX (this document once published)
Port Number: 465 Port Number: 465
This is a one-time procedural exception to the rules in RFC 6335. This is a one-time procedural exception to the rules in RFC 6335.
This requires explicit IESG approval and does not set a precedent. This requires explicit IESG approval and does not set a precedent.
Note: Since the purpose of this alternate usage assignment is to
align with widespread existing practice, and there is no known usage
of UDP port 465 for message submission over TLS, IANA is not being
asked to assign an alternate usage of UDP port 465.
Historically, port 465 was briefly registered as the "smtps" port. Historically, port 465 was briefly registered as the "smtps" port.
This registration made no sense as the SMTP transport MX This registration made no sense as the SMTP transport MX
infrastructure has no way to specify a port, so port 25 is always infrastructure has no way to specify a port, so port 25 is always
used. As a result, the registration was revoked and was subsequently used. As a result, the registration was revoked and was subsequently
reassigned to a different service. In hindsight, the "smtps" reassigned to a different service. In hindsight, the "smtps"
registration should have been renamed or reserved rather than registration should have been renamed or reserved rather than
revoked. Unfortunately, some widely deployed mail software revoked. Unfortunately, some widely deployed mail software
interpreted "smtps" as "submissions" [RFC6409] and used that port for interpreted "smtps" as "submissions" [RFC6409] and used that port for
email submission by default when an end-user requests security during email submission by default when an end-user requests security during
skipping to change at page 17, line 26 skipping to change at page 18, line 48
replaced deployed use of Implicit TLS submission on port 465. replaced deployed use of Implicit TLS submission on port 465.
8. Security Considerations 8. Security Considerations
This entire document is about security considerations. In general, This entire document is about security considerations. In general,
this is targeted to improve mail confidentiality and to mitigate this is targeted to improve mail confidentiality and to mitigate
threats external to the email system such as network-level snooping threats external to the email system such as network-level snooping
or interception; this is not intended to mitigate active attackers or interception; this is not intended to mitigate active attackers
who have compromised service provider systems. who have compromised service provider systems.
Implementers should be aware that use of client certificates with TLS
1.2 reveals the user's identity to any party with ability to read
packets from the transmission medium, and therefore may compromise
the user's privacy. There seems to be no easy fix with TLS 1.2 or
earlier versions other than to avoid presenting client certificates
except when there is explicit authorization to do so. TLS 1.3
[I-D.ietf-tls-tls13] appears to reduce the privacy risk somewhat.
9. References 9. References
9.1. Normative References 9.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
<https://www.rfc-editor.org/info/rfc1939>. <https://www.rfc-editor.org/info/rfc1939>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <https://www.rfc-editor.org/info/rfc3207>. February 2002, <https://www.rfc-editor.org/info/rfc3207>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<https://www.rfc-editor.org/info/rfc3501>. <https://www.rfc-editor.org/info/rfc3501>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>.
[RFC5034] Siemborski, R. and A. Menon-Sen, "The Post Office Protocol [RFC5034] Siemborski, R. and A. Menon-Sen, "The Post Office Protocol
(POP3) Simple Authentication and Security Layer (SASL) (POP3) Simple Authentication and Security Layer (SASL)
Authentication Mechanism", RFC 5034, DOI 10.17487/RFC5034, Authentication Mechanism", RFC 5034, DOI 10.17487/RFC5034,
July 2007, <https://www.rfc-editor.org/info/rfc5034>. July 2007, <https://www.rfc-editor.org/info/rfc5034>.
[RFC5068] Hutzler, C., Crocker, D., Resnick, P., Allman, E., and T. [RFC5068] Hutzler, C., Crocker, D., Resnick, P., Allman, E., and T.
Finch, "Email Submission Operations: Access and Finch, "Email Submission Operations: Access and
Accountability Requirements", BCP 134, RFC 5068, Accountability Requirements", BCP 134, RFC 5068,
DOI 10.17487/RFC5068, November 2007, DOI 10.17487/RFC5068, November 2007,
<https://www.rfc-editor.org/info/rfc5068>. <https://www.rfc-editor.org/info/rfc5068>.
skipping to change at page 18, line 40 skipping to change at page 20, line 29
[RFC6186] Daboo, C., "Use of SRV Records for Locating Email [RFC6186] Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186, Submission/Access Services", RFC 6186,
DOI 10.17487/RFC6186, March 2011, DOI 10.17487/RFC6186, March 2011,
<https://www.rfc-editor.org/info/rfc6186>. <https://www.rfc-editor.org/info/rfc6186>.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011, STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
<https://www.rfc-editor.org/info/rfc6409>. <https://www.rfc-editor.org/info/rfc6409>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <https://www.rfc-editor.org/info/rfc6698>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer "Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>. 2015, <https://www.rfc-editor.org/info/rfc7525>.
[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via
Opportunistic DNS-Based Authentication of Named Entities Opportunistic DNS-Based Authentication of Named Entities
(DANE) Transport Layer Security (TLS)", RFC 7672, (DANE) Transport Layer Security (TLS)", RFC 7672,
DOI 10.17487/RFC7672, October 2015, DOI 10.17487/RFC7672, October 2015,
<https://www.rfc-editor.org/info/rfc7672>. <https://www.rfc-editor.org/info/rfc7672>.
[RFC7817] Melnikov, A., "Updated Transport Layer Security (TLS) [RFC7817] Melnikov, A., "Updated Transport Layer Security (TLS)
Server Identity Check Procedure for Email-Related Server Identity Check Procedure for Email-Related
Protocols", RFC 7817, DOI 10.17487/RFC7817, March 2016, Protocols", RFC 7817, DOI 10.17487/RFC7817, March 2016,
<https://www.rfc-editor.org/info/rfc7817>. <https://www.rfc-editor.org/info/rfc7817>.
9.2. Informative References 9.2. Informative References
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-21 (work in progress),
July 2017.
[I-D.ietf-uta-mta-sts] [I-D.ietf-uta-mta-sts]
Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A., Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
and J. Jones, "SMTP MTA Strict Transport Security (MTA- and J. Jones, "SMTP MTA Strict Transport Security (MTA-
STS)", draft-ietf-uta-mta-sts-09 (work in progress), STS)", draft-ietf-uta-mta-sts-09 (work in progress),
September 2017. September 2017.
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", [RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, DOI 10.17487/RFC2595, June 1999, RFC 2595, DOI 10.17487/RFC2595, June 1999,
<https://www.rfc-editor.org/info/rfc2595>. <https://www.rfc-editor.org/info/rfc2595>.
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Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>. 2011, <https://www.rfc-editor.org/info/rfc6125>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA) Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165, Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011, RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>. <https://www.rfc-editor.org/info/rfc6335>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication [RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
of Named Entities (DANE) Transport Layer Security (TLS) Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 2015, <https://www.rfc-editor.org/info/rfc7469>.
2012, <https://www.rfc-editor.org/info/rfc6698>.
Appendix A. Design Considerations Appendix A. Design Considerations
This section is not normative. This section is not normative.
The first version of this was written independently from draft-moore- The first version of this was written independently from draft-moore-
email-tls-00.txt; subsequent versions merge ideas from both drafts. email-tls-00.txt; subsequent versions merge ideas from both drafts.
One author of this document was also the author of RFC 2595 that One author of this document was also the author of RFC 2595 that
became the standard for TLS usage with POP and IMAP, and the other became the standard for TLS usage with POP and IMAP, and the other
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wrong with STARTTLS, the fact it resulted in a common implementation wrong with STARTTLS, the fact it resulted in a common implementation
error (made independently by multiple implementers) suggests it is a error (made independently by multiple implementers) suggests it is a
less secure architecture than Implicit TLS. less secure architecture than Implicit TLS.
Section 7 of RFC 2595 critiques the separate-port approach to TLS. Section 7 of RFC 2595 critiques the separate-port approach to TLS.
The first bullet was a correct critique. There are proposals in the The first bullet was a correct critique. There are proposals in the
http community to address that, and use of SRV records as described http community to address that, and use of SRV records as described
in RFC 6186 resolves that critique for email. The second bullet is in RFC 6186 resolves that critique for email. The second bullet is
correct as well, but not very important because useful deployment of correct as well, but not very important because useful deployment of
security layers other than TLS in email is small enough to be security layers other than TLS in email is small enough to be
effectively irrelevant. The third bullet is incorrect because it effectively irrelevant. (Also it's less correct than it used to be
misses the desirable option of "use and latch-on TLS if available". because "export" ciphersuites are no longer supported in modern
The fourth bullet may be correct, but is not a problem yet with versions of TLS.) The third bullet is incorrect because it misses
current port consumption rates. The fundamental error was the desirable option of "use and latch-on TLS if available". The
prioritizing a perceived better design based on a mostly valid fourth bullet may be correct, but is not a problem yet with current
critique over real-world deployability. But getting security and port consumption rates. The fundamental error was prioritizing a
confidentiality facilities actually deployed is so important it perceived better design based on a mostly valid critique over real-
should trump design purity considerations. world deployability. But getting security and confidentiality
facilities actually deployed is so important it should trump design
purity considerations.
Port 465 is presently used for two purposes: for submissions by a Port 465 is presently used for two purposes: for submissions by a
large number of clients and service providers and for the "urd" large number of clients and service providers and for the "urd"
protocol by one vendor. Actually documenting this current state is protocol by one vendor. Actually documenting this current state is
controversial as discussed in the IANA considerations section. controversial as discussed in the IANA considerations section.
However, there is no good alternative. Registering a new port for However, there is no good alternative. Registering a new port for
submissions when port 465 is widely used for that purpose already submissions when port 465 is widely used for that purpose already
will just create interoperability problems. Registering a port will just create interoperability problems. Registering a port
that's only used if advertised by an SRV record (RFC 6186) would not that's only used if advertised by an SRV record (RFC 6186) would not
create interoperability problems but would require all client and create interoperability problems but would require all client and
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