Internet Engineering Task Force S. Nurpmeso, Ed.
Internet-Draft 13 February 2025
Updates: 5321, 3207 (if approved)
Intended status: Informational
Expires: 17 August 2025
Secure SMTP/TLS SRV Announcement
draft-nurpmeso-smtp-tls-srv-05
Abstract
This specification defines a DNS (RFC 1035) SRV (RFC 2782) record
that announces TLS (RFC 9325) secured SMTP (RFC 5321, RFC 3207),
optionally including Implicit TLS.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 17 August 2025.
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document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions and Terminology . . . . . . . . . . . . . . . 2
2. SMTP/TLS SRV Service Name . . . . . . . . . . . . . . . . . . 3
3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. STARTTLS . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Implicit TLS . . . . . . . . . . . . . . . . . . . . . . 4
3.3. Prioritized Server Selection . . . . . . . . . . . . . . 4
4. Guidance for MTAs . . . . . . . . . . . . . . . . . . . . . . 4
5. Guidance for Service Providers . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Rationale . . . . . . . . . . . . . . . . . . . . . 9
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
[RFC2782] defines a widely adopted DNS-based service discovery
protocol. [RFC6186] is a specification of SRV[RFC2782] records for
the email protocols IMAP[RFC9051], POP3[RFC1939], and
SUBMISSION[RFC6409]. This includes DNS service names for Implicit
TLS protocol variants. SMTP[RFC5321] connections to MTAs ([RFC5598])
are yet excluded from their share on the widely supported SRV record,
at least from the IETF point of view. (According usage however
exists for many years, see for example the German De-Mail definition
law; and in recent years notable FOSS MTA implementations gained
support for SRV lookups.) Moreover, no Implicit TLS SMTP protocol
variant has ever been specified by the IETF, even though the
achievable non-batchable packet roundtrip savings for the SMTP
protocol are immense, and despite the fact that many (most FOSS) SMTP
implementations already have, or can easily implement, built-in
support for this mode. This specification adds a SMTP/TLS service
name for SRV[RFC2782] records.
1.1. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"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.
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The term "Implicit TLS" refers to the automatic negotiation of TLS
whenever a TCP connection is made on a particular TCP port that is
used exclusively by that server for TLS connections. The term
"Implicit TLS" is intended to contrast with the use of the STARTTLS
command in SMTP that is used by the client and the server to
explicitly negotiate TLS on an established cleartext TCP connection.
The term "FOSS" refers to Free and Open Source Software.
2. SMTP/TLS SRV Service Name
The service name for TLS[RFC9325] enabled Secure[RFC3207]
SMTP[RFC5321] is smtp-tls, the resulting DNS label _smtp-tls.
STARTTLS
Whenever a domain publishes an according DNS SRV[RFC2782] resource
record it asserts availability of Secure SMTP, that is, of the
STARTTLS[RFC3207] SMTP service extension on the normal
SMTP[RFC5321] port, specified by IANA as port 25. The port number
MUST be given as 25.
Implicit TLS
If the port number of the published SRV resource record does not
equal the IANA port 25,0 the domain announces support for Implicit
TLS on the given port in addition to STARTTLS[RFC3207] support on
the IANA port. Servers SHOULD NOT announce STARTTLS in the EHLO
command response of an Implicit TLS connection. If a client
issues a STARTTLS[RFC3207] extension command during an Implicit
TLS connection, the SMTP reply code 554 SHOULD be returned, if
enhanced status codes[RFC3463] are used, 5.5.1 SHOULD be used.
DNS SRV RRs MUST take priority over MX[RFC5321] ones: no MX lookup
SHOULD be performed, and no otherwise available MX RR MUST be used in
the presence of a SRV RR. After a successful _smtp-tls DNS SRV
lookup cleartext communication SHOULD NOT be used. Servers and
clients which make use of the _smtp-tls DNS SRV SHOULD follow the
guidelines of TLS[RFC9325].
3. Examples
3.1. STARTTLS
This example announces support for the STARTTLS SMTP service
extension.
_smtp-tls._tcp SRV 0 0 25 mail.example.com.
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3.2. Implicit TLS
This example specifies port number 842 to indicate an Implicit TLS
port in addition to the STARTTLS SMTP service extension.
_smtp-tls._tcp SRV 0 0 842 mail.example.com.
3.3. Prioritized Server Selection
A multi-server scenario where the main server supports Implicit TLS,
and the backup server only supports STARTTLS.
_smtp-tls._tcp SRV 0 0 842 mail.example.com.
_smtp-tls._tcp SRV 1 0 25 backup.example.com.
4. Guidance for MTAs
If, after a successful _smtp-tls DNS SRV lookup that announces
Implicit TLS support, connecting to the announced port fails, a
connection to the IANA SMTP port 25 SHOULD be established which MUST
use the STARTTLS SMTP extension. If none of the shortcomings
described next apply, the fallback connection MAY be omitted.
| _Informative remark:_ This is meant to overcome two shortcomings:
| first the given port may be blocked along the network path; it may
| take time until an IANA registration takes places, and/or the
| network adapts; whereas expected to be a rare event for the System
| Ports range ([RFC6335]), defining a recover strategy seems useful.
| Second DNS SRV lookups could return results unprotected by
| DNSSEC[RFC4033][RFC4034][RFC4035], or without perceived knowledge
| of whether DNSSEC was actually used, for example, when the DNS is
| accessed via some kind of furtherly unspecified intermediate proxy
| that needs to be trusted: in either case the possibility of DNS
| forge attacks exist; if the STARTTLS secured connection to the
| IANA SMTP port fails, the DNS result SHOULD be treated with
| maximum suspicion, if at all applicable, for example by treating
| it as a negative result ([RFC2308]). The mail log record may give
| useful insight. If the STARTTLS secured connection succeeds, it
| may make sense to add a caching mechanism in order to avoid
| retrying Implicit TLS in situations where connections repeatedly
| cannot be established.
The section of [RFC6186] named "Guidance for MUAs" (section 4) in
parts also applies to this specification.
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5. Guidance for Service Providers
The equally named section of [RFC6186] (section 5) also applies to
this specification.
6. IANA Considerations
The author does not expect IANA to add a dedicated port for "Implicit
TLS SMTP", given that it has registered according ones for other
email protocols, for example POP3S, no sooner but 1999, a quarter of
a century ago. The author nonetheless asks for assigning a dedicated
port for Implicit TLS SMTP, and suggests port 842 in the System Ports
range. The author wants to point out that the contra arguments given
in section 7 of [RFC2595] that created according POP3S and IMAPS
assignments in 1999 are contradicted by operational reality in the
internet, and here that includes the IETF by means of [RFC8314]. A
dedicated port enables administrators to apply strict policies, for
example in firewalls.
7. Security Considerations
First of all, the equally named section of [RFC6186] (section 6) also
applies to this specification.
Due to fact that one and a half decade passed since RFC 6186 it
follows a reiteration of the reasoning. This specification avoids
downgrade attacks on the opportunistic approach of STARTTLS,
accomplished via the mechanism used for many other IETF standardized
protocols, most notably [RFC2782] (IMAP, POP3, SUBMISSION). With its
Implicit TLS capability it grants the SMTP protocol the same level of
confidentiality through TLS[RFC9325] as is already standardized for
the other email protocols; this is considered a value by itself, even
for the possibly lesser sensitive MTA-to-MTA communication. Implicit
TLS reduces the number of packet roundtrips, that at a protocol stage
where the widely used command pipelining[RFC2920] performance
improvement extension cannot be used; it must be said that today
these roundtrips are anachronistic (also environmentally): for
example the about 4.2 million known DANE for SMTP[RFC7672] enabled
domains at the time of this writing alone, which must use TLS by
standard definition, likely generate (several) billion(s) of useless
sequential and blocking roundtrip packets each and every day. The
security of DNS[RFC1035] is out of scope for this specification, but
DNSSEC[RFC4033][RFC4034][RFC4035] and secure DNS
transport[RFC7858][RFC8094][RFC8310][RFC8484][RFC9250] etc exists.
Selection of the appropriate transport layer security protocol is out
of scope for this specification, please see for example TLS[RFC9325].
8. References
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8.1. Normative References
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/info/rfc2782>.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <https://www.rfc-editor.org/info/rfc3207>.
[RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes",
RFC 3463, DOI 10.17487/RFC3463, January 2003,
<https://www.rfc-editor.org/info/rfc3463>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>.
8.2. Informative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
<https://www.rfc-editor.org/info/rfc1939>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
<https://www.rfc-editor.org/info/rfc2308>.
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, DOI 10.17487/RFC2595, June 1999,
<https://www.rfc-editor.org/info/rfc2595>.
[RFC2920] Freed, N., "SMTP Service Extension for Command
Pipelining", STD 60, RFC 2920, DOI 10.17487/RFC2920,
September 2000, <https://www.rfc-editor.org/info/rfc2920>.
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[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>.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
DOI 10.17487/RFC5598, July 2009,
<https://www.rfc-editor.org/info/rfc5598>.
[RFC6186] Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186,
DOI 10.17487/RFC6186, March 2011,
<https://www.rfc-editor.org/info/rfc6186>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
<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>.
[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <https://www.rfc-editor.org/info/rfc7250>.
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[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via
Opportunistic DNS-Based Authentication of Named Entities
(DANE) Transport Layer Security (TLS)", RFC 7672,
DOI 10.17487/RFC7672, October 2015,
<https://www.rfc-editor.org/info/rfc7672>.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.
[RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
Transport Layer Security (DTLS)", RFC 8094,
DOI 10.17487/RFC8094, February 2017,
<https://www.rfc-editor.org/info/rfc8094>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
for DNS over TLS and DNS over DTLS", RFC 8310,
DOI 10.17487/RFC8310, March 2018,
<https://www.rfc-editor.org/info/rfc8310>.
[RFC8314] Moore, K. and C. Newman, "Cleartext Considered Obsolete:
Use of Transport Layer Security (TLS) for Email Submission
and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018,
<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>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.
[RFC9051] Melnikov, A., Ed. and B. Leiba, Ed., "Internet Message
Access Protocol (IMAP) - Version 4rev2", RFC 9051,
DOI 10.17487/RFC9051, August 2021,
<https://www.rfc-editor.org/info/rfc9051>.
[RFC9250] Huitema, C., Dickinson, S., and A. Mankin, "DNS over
Dedicated QUIC Connections", RFC 9250,
DOI 10.17487/RFC9250, May 2022,
<https://www.rfc-editor.org/info/rfc9250>.
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[RFC9325] Sheffer, Y., Saint-Andre, P., and T. Fossati,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
2022, <https://www.rfc-editor.org/info/rfc9325>.
[RFC9460] Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
and Parameter Specification via the DNS (SVCB and HTTPS
Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
November 2023, <https://www.rfc-editor.org/info/rfc9460>.
Appendix A. Rationale
Plenty of methods have been standardized by the IETF to perform
service TLS capability discovery for SMTP. They have in common that
they represent SMTP specific solutions to a problem that other
protocols address by means this document thus reiterates for SMTP.
They represent further development effort and error surfaces. They
also impose increased permanent administration effort, and, due to
this, are inaccessible to a large number, if not the majority, of
private email server operators: either through additional costs, the
impossibility to set up a dedicated name server, if so required, and,
also as a superset of the former, restrictive web interfaces that
support only a minimal set of DNS[RFC1035] resource record types.
Regardless the fact that several of these extensions switch the SMTP
protocol to a MUST usage of TLS, they still require the additional
STARTTLS plus EHLO roundtrips that for example [RFC8314] disregards;
and even though that RFC explicitly refers to DANE for SMTP[RFC7672],
giving no reason for why SMTP "requires a different approach", except
for the TLS certificate discovery that could and should be hoped for
as a general solution, including for clients for any sort of network
protocol, DANE falls in this category, too: what this specification
tries to achieve is therefore not counteracting [RFC7672], but
accompanying it; the author wants to state again that according
mechanisms already exist in active use, and that many SMTP client/
server software implementations already support SRV RR lookups:
activating this existing code base by standard means is only a
consequence.
The other extension that [RFC8314] mentions, MTA-STS[RFC8461],
requires SMTP operators to install and maintain an additional HTTP
protocol server, accessible on the usual HTTP/S ports and the same
domain as the email service. Dependent on the software this may
require non-trivial and error prone path access policy server rules.
It is an approach that is complicated to implement, and that brings
the complexity of another network protocol in the SMTP path; given
that several different HTTP protocol exist, which in turn base upon
several different transport protocols, the complexity and protocol is
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alien to SMTP. The SVCB[RFC9460] DNS approach would not bring
additional value to the problem in question, but on the other hand
requires additional implementation efforts as email protocols do not
make any use of it as of today.
Coming back to DANE for SMTP[RFC7672] it must be said that
unfortunately DNSSEC[RFC4033][RFC4034][RFC4035] is still consciously
unsupported by major players as of today. Furthermore provider web
interfaces do often not allow users creation of the necessary DANE
TLSA resource records: not always can the answer be "run your own
name server", also letting aside the administrative effort of doing
so. However, discovering an Implicit TLS enabled email service via
the _smtp-tls._tcp SRV record, thereafter checking for applicability
of DNS-Based Authentication of Named Entities (DANE)[RFC6698],
possibly even Using Raw Public Keys in Transport Layer Security
(TLS)[RFC7250] would be a bright future that addresses possibly all
of today's issues.
Appendix B. Acknowledgements
Thanks to Jan Ingvoldstad for remarks on the missing rationale.
Thanks to Jeremy Harris for spending time and revealing the many
problems of early draft variants, as well as comments on how to do it
better; very special thanks to him for a kickstart implementation of
this very draft in the widely used FOSS MTA Exim. Jeremy Harris,
Viktor Dukhovni and Wietse Venema commented on the initial odd usage
of port 0 for the STARTTLS discovery case. Thanks to Alex Brotman
for hinting on the fallback strategy. Many thanks to Jonas Stalder
for dozens of corrections and suggestions, and also for finding IETF
tooling bugs.
Author's Address
Steffen Nurpmeso (editor)
Email: steffen@sdaoden.eu
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