draft-ietf-uta-smtp-require-tls-04.txt   draft-ietf-uta-smtp-require-tls-05.txt 
Internet Engineering Task Force J. Fenton Internet Engineering Task Force J. Fenton
Internet-Draft Altmode Networks Internet-Draft Altmode Networks
Intended status: Standards Track September 26, 2018 Intended status: Standards Track November 20, 2018
Expires: March 30, 2019 Expires: May 24, 2019
SMTP Require TLS Option SMTP Require TLS Option
draft-ietf-uta-smtp-require-tls-04 draft-ietf-uta-smtp-require-tls-05
Abstract Abstract
The SMTP STARTTLS option, used in negotiating transport-level The SMTP STARTTLS option, used in negotiating transport-level
encryption of SMTP connections, is not as useful from a security encryption of SMTP connections, is not as useful from a security
standpoint as it might be because of its opportunistic nature; standpoint as it might be because of its opportunistic nature;
message delivery is, by default, prioritized over security. This message delivery is, by default, prioritized over security. This
document describes an SMTP service extension, REQUIRETLS, and message document describes an SMTP service extension, REQUIRETLS, and message
header field, RequireTLS. If the REQUIRETLS option or RequireTLS header field, RequireTLS. If the REQUIRETLS option or RequireTLS
message header field is used when sending a message, it asserts a message header field is used when sending a message, it asserts a
skipping to change at page 1, line 41 skipping to change at page 1, line 41
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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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 30, 2019. This Internet-Draft will expire on May 24, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. The REQUIRETLS Service Extension . . . . . . . . . . . . . . 4 2. The REQUIRETLS Service Extension . . . . . . . . . . . . . . 4
3. The RequireTLS Header Field . . . . . . . . . . . . . . . . . 4 3. The RequireTLS Header Field . . . . . . . . . . . . . . . . . 4
4. REQUIRETLS Semantics . . . . . . . . . . . . . . . . . . . . 5 4. REQUIRETLS Semantics . . . . . . . . . . . . . . . . . . . . 5
4.1. REQUIRETLS Receipt Requirements . . . . . . . . . . . . . 5 4.1. REQUIRETLS Receipt Requirements . . . . . . . . . . . . . 5
4.2. REQUIRETLS Sender Requirements . . . . . . . . . . . . . 5 4.2. REQUIRETLS Sender Requirements . . . . . . . . . . . . . 5
4.2.1. Sending with TLS Required . . . . . . . . . . . . . . 5 4.2.1. Sending with TLS Required . . . . . . . . . . . . . . 5
4.2.2. Sending with TLS Optional . . . . . . . . . . . . . . 6 4.2.2. Sending with TLS Optional . . . . . . . . . . . . . . 6
4.3. REQUIRETLS Submission . . . . . . . . . . . . . . . . . . 7 4.3. REQUIRETLS Submission . . . . . . . . . . . . . . . . . . 7
4.4. Delivery of REQUIRETLS messages . . . . . . . . . . . . . 7 4.4. Delivery of REQUIRETLS messages . . . . . . . . . . . . . 7
5. Non-delivery message handling . . . . . . . . . . . . . . . . 7 5. Non-delivery message handling . . . . . . . . . . . . . . . . 7
6. Mailing list considerations . . . . . . . . . . . . . . . . . 8 6. Mailing list considerations . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8.1. Passive attacks . . . . . . . . . . . . . . . . . . . . . 9 8.1. Passive attacks . . . . . . . . . . . . . . . . . . . . . 10
8.2. Active attacks . . . . . . . . . . . . . . . . . . . . . 10 8.2. Active attacks . . . . . . . . . . . . . . . . . . . . . 10
8.3. Bad Actor MTAs . . . . . . . . . . . . . . . . . . . . . 10 8.3. Bad Actor MTAs . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
10. Revision History . . . . . . . . . . . . . . . . . . . . . . 11 10. Revision History . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Changes since -03 Draft . . . . . . . . . . . . . . . . 11 10.1. Changes since -04 Draft . . . . . . . . . . . . . . . . 11
10.2. Changes since -02 Draft . . . . . . . . . . . . . . . . 11 10.2. Changes since -03 Draft . . . . . . . . . . . . . . . . 11
10.3. Changes since -01 Draft . . . . . . . . . . . . . . . . 11 10.3. Changes since -02 Draft . . . . . . . . . . . . . . . . 11
10.4. Changes since -00 Draft . . . . . . . . . . . . . . . . 11 10.4. Changes since -01 Draft . . . . . . . . . . . . . . . . 12
10.5. Changes since fenton-03 Draft . . . . . . . . . . . . . 12 10.5. Changes since -00 Draft . . . . . . . . . . . . . . . . 12
10.6. Changes Since -02 Draft . . . . . . . . . . . . . . . . 12 10.6. Changes since fenton-03 Draft . . . . . . . . . . . . . 12
10.7. Changes Since -01 Draft . . . . . . . . . . . . . . . . 12 10.7. Changes Since -02 Draft . . . . . . . . . . . . . . . . 12
10.8. Changes Since -00 Draft . . . . . . . . . . . . . . . . 12 10.8. Changes Since -01 Draft . . . . . . . . . . . . . . . . 12
10.9. Changes Since -00 Draft . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.1. Normative References . . . . . . . . . . . . . . . . . . 13 11.1. Normative References . . . . . . . . . . . . . . . . . . 13
11.2. Informative References . . . . . . . . . . . . . . . . . 14 11.2. Informative References . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction 1. Introduction
The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a
means by which an SMTP server and client can establish a Transport means by which an SMTP server and client can establish a Transport
Layer Security (TLS) protected session for the transmission of email Layer Security (TLS) protected session for the transmission of email
messages. By default, TLS is used only upon mutual agreement messages. By default, TLS is used only upon mutual agreement
(successful negotiation) of STARTTLS between the client and server; (successful negotiation) of STARTTLS between the client and server;
if this is not possible, the message is sent without transport if this is not possible, the message is sent without transport
encryption. Furthermore, it is common practice for the client to encryption. Furthermore, it is common practice for the client to
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to a server not also supporting the REQUIRETLS extension. to a server not also supporting the REQUIRETLS extension.
In order to specify REQUIRETLS treatment for a given message, the In order to specify REQUIRETLS treatment for a given message, the
REQUIRETLS option is specified on the MAIL FROM command when that REQUIRETLS option is specified on the MAIL FROM command when that
message is transmitted. This option MUST only be specified in the message is transmitted. This option MUST only be specified in the
context of an SMTP session meeting the security requirements that context of an SMTP session meeting the security requirements that
have been specified: have been specified:
o The session itself MUST employ TLS transmission. o The session itself MUST employ TLS transmission.
o If the SMTP server to which the message is being transmitted is
identified through an MX record lookup, its name MUST be validated
via a DNSSEC signature on the recipient domain's MX record, or the
MX hostname MUST be validated by an MTA-STS policy as described in
Section 4.1 of RFC 8461 [RFC8461]. DNSSEC is defined in RFC 4033
[RFC4033], RFC 4034 [RFC4034], and RFC 4035 [RFC4035].
o The certificate presented by the SMTP server MUST either verify o The certificate presented by the SMTP server MUST either verify
successfully in a trust chain leading to a certificate trusted by successfully in a trust chain leading to a certificate trusted by
the SMTP client or it MUST verify succesfully using DANE as the SMTP client or it MUST verify succesfully using DANE as
specified in RFC 7672 [RFC7672]. For trust chains, the choice of specified in RFC 7672 [RFC7672]. For trust chains, the choice of
trusted (root) certificates is at the discretion of the SMTP trusted (root) certificates is at the discretion of the SMTP
client. client.
o Following the negotiation of STARTTLS, the SMTP server MUST o Following the negotiation of STARTTLS, the SMTP server MUST
advertise in the subsequent EHLO response that it supports advertise in the subsequent EHLO response that it supports
REQUIRETLS. REQUIRETLS.
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4.2. REQUIRETLS Sender Requirements 4.2. REQUIRETLS Sender Requirements
4.2.1. Sending with TLS Required 4.2.1. Sending with TLS Required
When sending a message tagged as requiring TLS, the sending (client) When sending a message tagged as requiring TLS, the sending (client)
MTA MUST: MTA MUST:
1. Look up the SMTP server to which the message is to be sent as 1. Look up the SMTP server to which the message is to be sent as
described in [RFC5321] Section 5.1. described in [RFC5321] Section 5.1.
2. Open an SMTP session with the peer SMTP server using the EHLO 2. If the server lookup is accomplished via the recipient domain's
MX record (the usual case) and is not accompanied by a valid
DNSSEC signature, the client MUST also validate the SMTP server
name using MTA-STS as described in Section 4.1 of RFC 8461
[RFC8461].
3. Open an SMTP session with the peer SMTP server using the EHLO
verb. verb.
3. Establish a TLS-protected SMTP session with its peer SMTP server 4. Establish a TLS-protected SMTP session with its peer SMTP server
and authenticate the server's certificate as specified in and authenticate the server's certificate as specified in
[RFC6125] or [RFC7672] as applicable. [RFC6125] or [RFC7672] as applicable.
4. Ensure that the response to the subsequent EHLO following 5. Ensure that the response to the subsequent EHLO following
establishment of the TLS protection advertises the REQUIRETLS establishment of the TLS protection advertises the REQUIRETLS
capability. capability.
5. The SMTP client SHOULD also require that meaningfully secure 6. The SMTP client SHOULD also require that meaningfully secure
cipher algorithms and key lengths be negotiated with the server. cipher algorithms and key lengths be negotiated with the server.
The choices of key lengths and algorithms change over time, so a The choices of key lengths and algorithms change over time, so a
specific requirement is not presented here. specific requirement is not presented here.
If any of the above steps fail, the client MUST issue a QUIT to the If any of the above steps fail, the client MUST issue a QUIT to the
server and repeat steps 2-4 with each host on the recipient domain's server and repeat steps 2-5 with each host on the recipient domain's
list of MX hosts in an attempt to find a mail path that meets the list of MX hosts in an attempt to find a mail path that meets the
sender's requirements. The client MAY send other, unprotected, sender's requirements. The client MAY send other, unprotected,
messages to that server if it has any prior to issuing the QUIT. If messages to that server if it has any prior to issuing the QUIT. If
there are no more MX hosts, the client MUST NOT transmit the message there are no more MX hosts, the client MUST NOT transmit the message
to the domain. to the domain.
Following such a failure, the SMTP client MUST send a non-delivery Following such a failure, the SMTP client MUST send a non-delivery
notification to the reverse-path of the failed message as described notification to the reverse-path of the failed message as described
in section 3.6 of [RFC5321]. The following status codes [RFC5248] in section 3.6 of [RFC5321]. The following status codes [RFC5248]
SHOULD be used: SHOULD be used:
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insecurely. insecurely.
A second form of attack is a man-in-the-middle attack where the A second form of attack is a man-in-the-middle attack where the
attacker terminates the TLS connection rather than the intended SMTP attacker terminates the TLS connection rather than the intended SMTP
server. This is possible when, as is commonly the case, the SMTP server. This is possible when, as is commonly the case, the SMTP
client either does not verify the server's certificate or establishes client either does not verify the server's certificate or establishes
the connection even when the verification fails. REQUIRETLS requires the connection even when the verification fails. REQUIRETLS requires
successful certificate validation before sending the message. successful certificate validation before sending the message.
Another active attack involves the spoofing of DNS MX records of the Another active attack involves the spoofing of DNS MX records of the
recipient domain. An attacker having this capability could cause the recipient domain. An attacker having this capability could
message to be redirected to a mail server under the attacker's own potentially cause the message to be redirected to a mail server under
control, which would presumably have a valid certificate. REQUIRETLS the attacker's own control, which would presumably have a valid
does not address this attack. certificate. REQUIRETLS requires that the recipient domain's MX
record lookup be validated either using DNSSEC or via a published
MTA-STS policy that specifies the acceptable SMTP server hostname(s)
for the recipient domain.
8.3. Bad Actor MTAs 8.3. Bad Actor MTAs
A bad-actor MTA along the message transmission path could A bad-actor MTA along the message transmission path could
misrepresent its support of REQUIRETLS and/or actively strip misrepresent its support of REQUIRETLS and/or actively strip
REQUIRETLS tags from messages it handles. However, since REQUIRETLS tags from messages it handles. However, since
intermediate MTAs are already trusted with the cleartext of messages intermediate MTAs are already trusted with the cleartext of messages
they handle, and are not part of the threat model for transport-layer they handle, and are not part of the threat model for transport-layer
security, they are also not part of the threat model for REQUIRETLS. security, they are also not part of the threat model for REQUIRETLS.
skipping to change at page 11, line 9 skipping to change at page 11, line 25
The author would like to acknowledge many helpful suggestions on the The author would like to acknowledge many helpful suggestions on the
ietf-smtp and uta mailing lists, in particular those of Viktor ietf-smtp and uta mailing lists, in particular those of Viktor
Dukhovni, Chris Newman, Tony Finch, Jeremy Harris, Arvel Hathcock, Dukhovni, Chris Newman, Tony Finch, Jeremy Harris, Arvel Hathcock,
John Klensin, John Levine, Rolf Sonneveld, and Per Thorsheim. John Klensin, John Levine, Rolf Sonneveld, and Per Thorsheim.
10. Revision History 10. Revision History
To be removed by RFC Editor upon publication as an RFC. To be removed by RFC Editor upon publication as an RFC.
10.1. Changes since -03 Draft 10.1. Changes since -04 Draft
Require validation of SMTP server hostname via DNSSEC or MTA-STS
policy when TLS is required.
10.2. Changes since -03 Draft
Working Group Last Call changes, including: Working Group Last Call changes, including:
o Correct reference for SMTP DANE o Correct reference for SMTP DANE
o Clarify that RequireTLS: NO applies to both MTA-STS and DANE o Clarify that RequireTLS: NO applies to both MTA-STS and DANE
policies policies
o Correct newly-defined status codes o Correct newly-defined status codes
o Update MTA-STS references to RFC o Update MTA-STS references to RFC
10.2. Changes since -02 Draft 10.3. Changes since -02 Draft
o More complete documentation for IANA registration requests. o More complete documentation for IANA registration requests.
o Changed bounce handling to use RET parameters of RFC 3461, along o Changed bounce handling to use RET parameters of RFC 3461, along
with slightly more liberal transmission of bounces even if with slightly more liberal transmission of bounces even if
REQUIRETLS can't be negotiated. REQUIRETLS can't be negotiated.
10.3. Changes since -01 Draft 10.4. Changes since -01 Draft
o Converted DEEP references to RFC 8314. o Converted DEEP references to RFC 8314.
o Removed REQUIRETLS options: CHAIN, DANE, and DNSSEC. o Removed REQUIRETLS options: CHAIN, DANE, and DNSSEC.
o Editorial corrections, notably making the header field name o Editorial corrections, notably making the header field name
consistent (RequireTLS rather than Require-TLS). consistent (RequireTLS rather than Require-TLS).
10.4. Changes since -00 Draft 10.5. Changes since -00 Draft
o Created new header field, Require-TLS, for use by "NO" option. o Created new header field, Require-TLS, for use by "NO" option.
o Removed "NO" option from SMTP service extension. o Removed "NO" option from SMTP service extension.
o Recommend DEEP requirements for delivery of messages requiring o Recommend DEEP requirements for delivery of messages requiring
TLS. TLS.
o Assorted copy edits o Assorted copy edits
10.5. Changes since fenton-03 Draft 10.6. Changes since fenton-03 Draft
o Wording improvements from Rolf Sonneveld review 22 July 2017 o Wording improvements from Rolf Sonneveld review 22 July 2017
o A few copy edits o A few copy edits
o Conversion from individual to UTA WG draft o Conversion from individual to UTA WG draft
10.6. Changes Since -02 Draft 10.7. Changes Since -02 Draft
o Incorporation of "MAY TLS" functionality as REQUIRETLS=NO per o Incorporation of "MAY TLS" functionality as REQUIRETLS=NO per
suggestion on UTA WG mailing list. suggestion on UTA WG mailing list.
o Additional guidance on bounce messages o Additional guidance on bounce messages
10.7. Changes Since -01 Draft 10.8. Changes Since -01 Draft
o Specified retries when multiple MX hosts exist for a given domain. o Specified retries when multiple MX hosts exist for a given domain.
o Clarified generation of non-delivery messages o Clarified generation of non-delivery messages
o Specified requirements for application of REQUIRETLS to mail o Specified requirements for application of REQUIRETLS to mail
forwarders and mailing lists. forwarders and mailing lists.
o Clarified DNSSEC requirements to include MX lookup only. o Clarified DNSSEC requirements to include MX lookup only.
o Corrected terminology regarding message retrieval vs. delivery. o Corrected terminology regarding message retrieval vs. delivery.
o Changed category to standards track. o Changed category to standards track.
10.8. Changes Since -00 Draft 10.9. Changes Since -00 Draft
o Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM o Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM
parameter to better associate REQUIRETLS requirements with parameter to better associate REQUIRETLS requirements with
transmission of individual messages. transmission of individual messages.
o Addition of an option to require DNSSEC lookup of the remote mail o Addition of an option to require DNSSEC lookup of the remote mail
server, since this affects the common name of the certificate that server, since this affects the common name of the certificate that
is presented. is presented.
o Clarified the wording to more clearly state that TLS sessions must o Clarified the wording to more clearly state that TLS sessions must
skipping to change at page 13, line 34 skipping to change at page 14, line 5
[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>.
[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service [RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)", Extension for Delivery Status Notifications (DSNs)",
RFC 3461, DOI 10.17487/RFC3461, January 2003, RFC 3461, DOI 10.17487/RFC3461, January 2003,
<https://www.rfc-editor.org/info/rfc3461>. <https://www.rfc-editor.org/info/rfc3461>.
[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>.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<https://www.rfc-editor.org/info/rfc4034>.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<https://www.rfc-editor.org/info/rfc4035>.
[RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced [RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
Mail System Status Codes", BCP 138, RFC 5248, Mail System Status Codes", BCP 138, RFC 5248,
DOI 10.17487/RFC5248, June 2008, DOI 10.17487/RFC5248, June 2008,
<https://www.rfc-editor.org/info/rfc5248>. <https://www.rfc-editor.org/info/rfc5248>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008, DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>. <https://www.rfc-editor.org/info/rfc5321>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
skipping to change at page 14, line 10 skipping to change at page 14, line 45
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8314] Moore, K. and C. Newman, "Cleartext Considered Obsolete: [RFC8314] Moore, K. and C. Newman, "Cleartext Considered Obsolete:
Use of Transport Layer Security (TLS) for Email Submission Use of Transport Layer Security (TLS) for Email Submission
and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018, and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018,
<https://www.rfc-editor.org/info/rfc8314>. <https://www.rfc-editor.org/info/rfc8314>.
[RFC8461] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
and J. Jones, "SMTP MTA Strict Transport Security (MTA-
STS)", RFC 8461, DOI 10.17487/RFC8461, September 2018,
<https://www.rfc-editor.org/info/rfc8461>.
[SMTPStatusCodes] [SMTPStatusCodes]
Internet Assigned Numbers Authority (IANA), "Simple Mail Internet Assigned Numbers Authority (IANA), "Simple Mail
Transfer Protocol (SMTP) Enhanced Status Codes Registry", Transfer Protocol (SMTP) Enhanced Status Codes Registry",
2008, <http://www.iana.org/assignments/ 2008, <http://www.iana.org/assignments/
smtp-enhanced-status-codes>. smtp-enhanced-status-codes>.
11.2. Informative References 11.2. Informative References
[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,
skipping to change at page 14, line 42 skipping to change at page 15, line 37
Mail Extensions (S/MIME) Version 3.2 Message Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, DOI 10.17487/RFC5751, January Specification", RFC 5751, DOI 10.17487/RFC5751, January
2010, <https://www.rfc-editor.org/info/rfc5751>. 2010, <https://www.rfc-editor.org/info/rfc5751>.
[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>.
[RFC8461] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
and J. Jones, "SMTP MTA Strict Transport Security (MTA-
STS)", RFC 8461, DOI 10.17487/RFC8461, September 2018,
<https://www.rfc-editor.org/info/rfc8461>.
Author's Address Author's Address
Jim Fenton Jim Fenton
Altmode Networks Altmode Networks
Los Altos, California 94024 Los Altos, California 94024
USA USA
Email: fenton@bluepopcorn.net Email: fenton@bluepopcorn.net
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