Internet Engineering Task Force (IETF) D. Margolis
Request for Comments: 8460 Google, Inc.
Category: Standards Track A. Brotman
ISSN: 2070-1721 Comcast, Inc.
B. Ramakrishnan
Oath, Inc.
J. Jones
Microsoft, Inc.
M. Risher
Google, Inc.
September 2018
SMTP TLS Reporting
Abstract
A number of protocols exist for establishing encrypted channels
between SMTP Mail Transfer Agents (MTAs), including STARTTLS, DNS-
Based Authentication of Named Entities (DANE) TLSA, and MTA Strict
Transport Security (MTA-STS). These protocols can fail due to
misconfiguration or active attack, leading to undelivered messages or
delivery over unencrypted or unauthenticated channels. This document
describes a reporting mechanism and format by which sending systems
can share statistics and specific information about potential
failures with recipient domains. Recipient domains can then use this
information to both detect potential attacks and diagnose
unintentional misconfigurations.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8460.
Margolis, et al. Standards Track [Page 1]RFC 8460 SMTP TLS Reporting September 2018
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Margolis, et al. Standards Track [Page 2]
RFC 8460 SMTP TLS Reporting September 2018
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Related Technologies . . . . . . . . . . . . . . . . . . . . 5
3. Reporting Policy . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Example Reporting Policy . . . . . . . . . . . . . . . . 8
3.1.1. Report Using MAILTO . . . . . . . . . . . . . . . . . 8
3.1.2. Report Using HTTPS . . . . . . . . . . . . . . . . . 8
4. Reporting Schema . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Report Time Frame . . . . . . . . . . . . . . . . . . . . 9
4.2. Delivery Summary . . . . . . . . . . . . . . . . . . . . 10
4.2.1. Success Count . . . . . . . . . . . . . . . . . . . . 10
4.2.2. Failure Count . . . . . . . . . . . . . . . . . . . . 10
4.3. Result Types . . . . . . . . . . . . . . . . . . . . . . 10
4.3.1. Negotiation Failures . . . . . . . . . . . . . . . . 10
4.3.2. Policy Failures . . . . . . . . . . . . . . . . . . . 11
4.3.3. General Failures . . . . . . . . . . . . . . . . . . 11
4.3.4. Transient Failures . . . . . . . . . . . . . . . . . 12
4.4. JSON Report Schema . . . . . . . . . . . . . . . . . . . 12
4.5. Policy Samples . . . . . . . . . . . . . . . . . . . . . 15
5. Report Delivery . . . . . . . . . . . . . . . . . . . . . . . 15
5.1. Report Filename . . . . . . . . . . . . . . . . . . . . . 16
5.2. Compression . . . . . . . . . . . . . . . . . . . . . . . 17
5.3. Email Transport . . . . . . . . . . . . . . . . . . . . . 17
5.3.1. Example Report . . . . . . . . . . . . . . . . . . . 19
5.4. HTTPS Transport . . . . . . . . . . . . . . . . . . . . . 19
5.5. Delivery Retry . . . . . . . . . . . . . . . . . . . . . 20
5.6. Metadata Variances . . . . . . . . . . . . . . . . . . . 20
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
6.1. Message Headers . . . . . . . . . . . . . . . . . . . . . 20
6.2. Report Type . . . . . . . . . . . . . . . . . . . . . . . 21
6.3. +gzip Media Type Suffix . . . . . . . . . . . . . . . . . 22
6.4. application/tlsrpt+json Media Type . . . . . . . . . . . 23
6.5. application/tlsrpt+gzip Media Type . . . . . . . . . . . 24
6.6. STARTTLS Validation Result Types . . . . . . . . . . . . 25
7. Security Considerations . . . . . . . . . . . . . . . . . . . 26
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 27
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
9.1. Normative References . . . . . . . . . . . . . . . . . . 28
9.2. Informative References . . . . . . . . . . . . . . . . . 30
Appendix A. Example Reporting Policy . . . . . . . . . . . . . . 32
A.1. Report Using MAILTO . . . . . . . . . . . . . . . . . . . 32
A.2. Report Using HTTPS . . . . . . . . . . . . . . . . . . . 32
Appendix B. Example JSON Report . . . . . . . . . . . . . . . . 32
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
Margolis, et al. Standards Track [Page 3]RFC 8460 SMTP TLS Reporting September 2018
1. Introduction
The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and
hosts to establish secure SMTP sessions over TLS. The protocol
design uses an approach that has come to be known as "Opportunistic
Security" (OS) [RFC7435]. This method maintains interoperability
with clients that do not support STARTTLS, but it means that any
attacker could potentially eavesdrop on a session. An attacker could
perform a downgrade or interception attack by deleting parts of the
SMTP session (such as the "250 STARTTLS" response) or redirect the
entire SMTP session (perhaps by overwriting the resolved MX record of
the delivery domain).
Because such "downgrade attacks" are not necessarily apparent to the
receiving MTA, this document defines a mechanism for sending domains
to report on failures at multiple stages of the MTA-to-MTA
conversation.
Recipient domains may also use the mechanisms defined by MTA-STS
[RFC8461] or DANE [RFC6698] to publish additional encryption and
authentication requirements; this document defines a mechanism for
sending domains that are compatible with MTA-STS or DANE to share
success and failure statistics with recipient domains.
Specifically, this document defines a reporting schema that covers
failures in routing, DNS resolution, and STARTTLS negotiation; policy
validation errors for both DANE [RFC6698] and MTA-STS [RFC8461]; and
a standard TXT record that recipient domains can use to indicate
where reports in this format should be sent. The report can also
serve as a heartbeat to indicate that systems are successfully
negotiating TLS during sessions as expected.
This document is intended as a companion to the specification for
SMTP MTA-STS [RFC8461] and adds reporting abilities for those
implementing DANE [RFC7672].
1.1. 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.
Margolis, et al. Standards Track [Page 4]
RFC 8460 SMTP TLS Reporting September 2018
We also define the following terms for further use in this document:
o MTA-STS Policy: A mechanism by which administrators can specify
the expected TLS availability, presented identity, and desired
actions for a given email recipient domain. MTA-STS is defined in
[RFC8461].
o DANE Policy: A mechanism by which administrators can use DNSSEC to
commit an MTA to support STARTTLS and to publish criteria to be
used to validate its presented certificates. DANE for SMTP is
defined in [RFC7672], with the base specification defined in
[RFC6698] (and updated by [RFC7671]).
o TLSRPT (TLS Reporting) Policy: A policy specifying the endpoint to
which Sending MTAs should deliver reports.
o Policy Domain: The domain against which a TLSRPT, an MTA-STS, or a
DANE policy is defined. For TLSRPT and MTA-STS, this is typically
the same as the envelope recipient domain [RFC5321], but when mail
is routed to a "smarthost" gateway by local policy, the
"smarthost" domain name is used instead. For DANE, the Policy
Domain is the "TLSA base domain" of the receiving SMTP server as
described in Section 2.2.3 of RFC 7672 and Section 3 of RFC 6698.
o Sending MTA: The MTA initiating the relay of an email message.
o Aggregate Report URI (rua): A comma-separated list of locations
where the report is to be submitted.
o ABNF: Augmented Backus-Naur Form, a syntax for formally specifying
syntax, defined in [RFC5234] and [RFC7405].
2. Related Technologies
o This document is intended as a companion to the specification for
SMTP MTA-STS [RFC8461].
o SMTP TLSRPT defines a mechanism for sending domains that are
compatible with MTA-STS or DANE to share success and failure
statistics with recipient domains. DANE is defined in [RFC6698],
and MTA-STS is defined in [RFC8461].
Margolis, et al. Standards Track [Page 5]RFC 8460 SMTP TLS Reporting September 2018
3. Reporting Policy
A domain publishes a record to its DNS indicating that it wishes to
receive reports. These SMTP TLSRPT policies are distributed via DNS
from the Policy Domain's zone as TXT records (similar to Domain-based
Message Authentication, Reporting, and Conformance (DMARC) policies)
under the name "_smtp._tls". For example, for the Policy Domain
"example.com", the recipient's TLSRPT policy can be retrieved from
"_smtp._tls.example.com".
Policies consist of the following directives:
o "v": This document defines version 1 of TLSRPT, for which this
value MUST be equal to "TLSRPTv1". Other versions may be defined
in later documents.
o "rua": A URI specifying the endpoint to which aggregate
information about policy validation results should be sent (see
Section 4, "Reporting Schema", for more information). Two URI
schemes are supported: "mailto" and "https". As with DMARC
[RFC7489], the Policy Domain can specify a comma-separated list of
URIs.
o In the case of "https", reports should be submitted via POST
[RFC7231] to the specified URI. Report submitters MAY ignore
certificate validation errors when submitting reports via HTTPS
POST.
o In the case of "mailto", reports should be submitted to the
specified email address [RFC6068]. When sending failure reports
via SMTP, Sending MTAs MUST deliver reports despite any TLS-
related failures and SHOULD NOT include this SMTP session in the
next report. This may mean that the reports are delivered
unencrypted. Reports sent via SMTP MUST contain a valid
DomainKeys Identified Mail (DKIM) [RFC6376] signature by the
reporting domain. Reports lacking such a signature MUST be
ignored by the recipient. DKIM signatures MUST NOT use the "l="
attribute to limit the body length used in the signature. This
ensures attackers cannot append extraneous or misleading data to a
report without breaking the signature. The DKIM TXT record SHOULD
contain the appropriate service type declaration, "s=tlsrpt". If
not present, the receiving system MAY ignore reports lacking that
service type.
Sample DKIM record:
dkim_selector._domainkey.example.com TXT
"v=DKIM1;k=rsa;s=tlsrpt;p=Mlf4qwSZfase4fa=="
Margolis, et al. Standards Track [Page 6]RFC 8460 SMTP TLS Reporting September 2018
The formal definition of the "_smtp._tls" TXT record, defined using
[RFC5234] and [RFC7405], is as follows:
tlsrpt-record = tlsrpt-version 1*(field-delim tlsrpt-field)
[field-delim]
field-delim = *WSP ";" *WSP
tlsrpt-field = tlsrpt-rua / ; Note that the
tlsrpt-extension ; tlsrpt-rua record is
; required.
tlsrpt-version = %s"v=TLSRPTv1"
tlsrpt-rua = %s"rua="
tlsrpt-uri *(*WSP "," *WSP tlsrpt-uri)
tlsrpt-uri = URI
; "URI" is imported from [RFC3986];
; commas (ASCII 0x2C), exclamation
; points (ASCII 0x21), and semicolons
; (ASCII 0x3B) MUST be encoded
tlsrpt-extension = tlsrpt-ext-name "=" tlsrpt-ext-value
tlsrpt-ext-name = (ALPHA / DIGIT) *31(ALPHA /
DIGIT / "_" / "-" / ".")
tlsrpt-ext-value = 1*(%x21-3A / %x3C / %x3E-7E)
; chars excluding "=", ";", SP, and control
; chars
If multiple TXT records for "_smtp._tls" are returned by the
resolver, records that do not begin with "v=TLSRPTv1;" are discarded.
If the number of resulting records is not one, senders MUST assume
the recipient domain does not implement TLSRPT. If the resulting TXT
record contains multiple strings (as described in Section 3.3 of
[RFC7208]), then the record MUST be treated as if those strings are
concatenated without adding spaces.
The record supports the ability to declare more than one rua, and if
there exists more than one, the reporter MAY attempt to deliver to
each of the supported rua destinations. A receiver MAY opt to only
attempt delivery to one of the endpoints; however, the report SHOULD
NOT be considered successfully delivered until one of the endpoints
accepts delivery of the report.
Margolis, et al. Standards Track [Page 7]RFC 8460 SMTP TLS Reporting September 2018
Parsers MUST accept TXT records that are syntactically valid (i.e.,
valid key/value pairs separated by semicolons) and implement a
superset of this specification, in which case unknown fields SHALL be
ignored.
3.1. Example Reporting Policy
3.1.1. Report Using MAILTO
_smtp._tls.example.com. IN TXT \
"v=TLSRPTv1;rua=mailto:reports@example.com"
3.1.2. Report Using HTTPS
_smtp._tls.example.com. IN TXT \
"v=TLSRPTv1; \
rua=https://reporting.example.com/v1/tlsrpt"
4. Reporting Schema
The report is composed as a plaintext file encoded in the Internet
JSON (I-JSON) format [RFC7493].
Aggregate reports contain the following fields:
o Report metadata:
* The organization responsible for the report
* Contact information for one or more responsible parties for the
contents of the report
* A unique identifier for the report
* The reporting date range for the report
o Policy, consisting of:
* One of the following policy types: (1) the MTA-STS Policy
applied (as a string), (2) the DANE TLSA record applied (as a
string, with each RR entry of the RRset listed and separated by
a semicolon), and (3) the literal string "no-policy-found", if
neither a DANE nor MTA-STS Policy could be found.
* The domain for which the policy is applied
* The MX host
Margolis, et al. Standards Track [Page 8]RFC 8460 SMTP TLS Reporting September 2018
o Aggregate counts, comprising result type, Sending MTA IP,
receiving MTA hostname, session count, and an optional additional
information field containing a URI for recipients to review
further information on a failure type.
Note that the failure types are non-exclusive; an aggregate report
may contain overlapping "counts" of failure types when a single send
attempt encountered multiple errors. Reporters may report multiple
applied policies (for example, an MTA-STS Policy and a DANE TLSA
record for the same domain and MX). Because of this, even in the
case where only a single policy was applied, the "policies" field of
the report body MUST be an array and not a singular value.
In the case of multiple failure types, the "failure-details" array
would contain multiple entries. Each entry would have its own set of
information pertaining to that failure type.
4.1. Report Time Frame
The report SHOULD cover a full day, from 00:00-24:00 UTC. This
should allow for easier correlation of failure events. To avoid
unintentionally overloading the system processing the reports, the
reports should be delivered after some delay, perhaps several hours.
As an example, a sending site might want to introduce a random delay
of up to four hours:
func generate_sleep_delay() {
min_delay = 1
max_delay = 14400
rand = random(min_delay, max_delay)
return rand
}
func generate_report(policy_domain) {
do_rpt_work(policy_domain)
send_rpt(policy_domain)
}
func generate_tlsrpt() {
sleep(generate_sleep_delay())
for policy_domain in list_of_tlsrpt_enabled_domains {
generate_report(policy_domain)
}
}
Margolis, et al. Standards Track [Page 9]RFC 8460 SMTP TLS Reporting September 2018
4.2. Delivery Summary
4.2.1. Success Count
o "total-successful-session-count": This indicates that the Sending
MTA was able to successfully negotiate a policy-compliant TLS
connection and serves to provide a "heartbeat" to receiving
domains that signifies reporting is functional and tabulating
correctly. This field contains an aggregate count of successful
connections for the reporting system.
4.2.2. Failure Count
o "total-failure-session-count": This indicates that the Sending MTA
was unable to successfully establish a connection with the
receiving platform. Section 4.3, "Result Types", will elaborate
on the failed negotiation attempts. This field contains an
aggregate count of failed connections.
4.3. Result Types
The list of result types will start with the minimal set below and is
expected to grow over time based on real-world experience. The
initial set is outlined in Sections 4.3.1 to 4.3.4:
4.3.1. Negotiation Failures
o "starttls-not-supported": This indicates that the recipient MX did
not support STARTTLS.
o "certificate-host-mismatch": This indicates that the certificate
presented did not adhere to the constraints specified in the MTA-
STS or DANE policy, e.g., if the MX hostname does not match any
identities listed in the subject alternative name (SAN) [RFC5280].
o "certificate-expired": This indicates that the certificate has
expired.
o "certificate-not-trusted": This is a label that covers multiple
certificate-related failures that include, but are not limited to,
errors such as untrusted/unknown certification authorities (CAs),
certificate name constraints, certificate chain errors, etc. When
using this declaration, the reporting MTA SHOULD utilize the
"failure-reason-code" to provide more information to the receiving
entity.
Margolis, et al. Standards Track [Page 10]RFC 8460 SMTP TLS Reporting September 2018
o "validation-failure": This indicates a general failure for a
reason not matching a category above. When using this
declaration, the reporting MTA SHOULD utilize the "failure-reason-
code" to provide more information to the receiving entity.
4.3.2. Policy Failures
4.3.2.1. DANE-Specific Policy Failures
o "tlsa-invalid": This indicates a validation error in the TLSA
record associated with a DANE policy. None of the records in the
RRset were found to be valid.
o "dnssec-invalid": This indicates that no valid records were
returned from the recursive resolver.
o "dane-required": This indicates that the sending system is
configured to require DANE TLSA records for all the MX hosts of
the destination domain, but no DNSSEC-validated TLSA records were
present for the MX host that is the subject of the report.
Mandatory DANE for SMTP is described in Section 6 of [RFC7672].
Such policies may be created by mutual agreement between two
organizations that frequently exchange sensitive content via
email.
4.3.2.2. MTA-STS-specific Policy Failures
o "sts-policy-fetch-error": This indicates a failure to retrieve an
MTA-STS policy, for example, because the policy host is
unreachable.
o "sts-policy-invalid": This indicates a validation error for the
overall MTA-STS Policy.
o "sts-webpki-invalid": This indicates that the MTA-STS Policy could
not be authenticated using PKIX validation.
4.3.3. General Failures
When a negotiation failure cannot be categorized into one of the
"Negotiation Failures" stated above, the reporter SHOULD use the
"validation-failure" category. As TLS grows and becomes more
complex, new mechanisms may not be easily categorized. This allows
for a generic feedback category. When this category is used, the
reporter SHOULD also use "failure-reason-code" to give some feedback
to the receiving entity. This is intended to be a short text field,
and the contents of the field should be an error code or error text,
such as "X509_V_ERR_UNHANDLED_CRITICAL_CRL_EXTENSION".
Margolis, et al. Standards Track [Page 11]RFC 8460 SMTP TLS Reporting September 2018
4.3.4. Transient Failures
Transient errors due to too-busy networks, TCP timeouts, etc., are
not required to be reported.
4.4. JSON Report Schema
The JSON schema is derived from the HTTP Public Key Pinning (HPKP)
JSON schema; see Section 3 of [RFC7469].
{
"organization-name": organization-name,
"date-range": {
"start-datetime": date-time,
"end-datetime": date-time
},
"contact-info": email-address,
"report-id": report-id,
"policies": [{
"policy": {
"policy-type": policy-type,
"policy-string": policy-string,
"policy-domain": domain,
"mx-host": mx-host-pattern
},
"summary": {
"total-successful-session-count": total-successful-session-count,
"total-failure-session-count": total-failure-session-count
},
"failure-details": [
{
"result-type": result-type,
"sending-mta-ip": ip-address,
"receiving-mx-hostname": receiving-mx-hostname,
"receiving-mx-helo": receiving-mx-helo,
"receiving-ip": receiving-ip,
"failed-session-count": failed-session-count,
"additional-information": additional-info-uri,
"failure-reason-code": failure-reason-code
}
]
}
]
}
JSON Report Format
Margolis, et al. Standards Track [Page 12]RFC 8460 SMTP TLS Reporting September 2018
o "organization-name": The name of the organization responsible for
the report. It is provided as a string.
o "date-time": The date-time indicates the start and end times for
the report range. It is provided as a string formatted according
to "Internet Date/Time Format", Section 5.6 of [RFC3339]. The
report should be for a full UTC day, 00:00-24:00.
o "email-address": The contact information for the party responsible
for the report. It is provided as a string formatted according to
"Addr-Spec Specification", Section 3.4.1 of [RFC5322].
o "report-id": A unique identifier for the report. Report authors
may use whatever scheme they prefer to generate a unique
identifier. It is provided as a string.
o "policy-type": The type of policy that was applied by the sending
domain. Presently, the only three valid choices are "tlsa",
"sts", and the literal string "no-policy-found". It is provided
as a string.
o "policy-string": An encoding of the applied policy as a JSON array
of strings, whether it's a TLSA record ([RFC6698], Section 2.3) or
an MTA-STS Policy. Examples follow in the next section.
o "domain": The Policy Domain against which the MTA-STS or DANE
policy is defined. In the case of Internationalized Domain Names
[RFC5891], the domain MUST consist of the Punycode-encoded
A-labels [RFC3492] and not the U-labels.
o "mx-host-pattern": In the case where "policy-type" is "sts", it's
the pattern of MX hostnames from the applied policy. It is
provided as a JSON array of strings and is interpreted in the same
manner as the rules in "MX Host Validation"; see Section 4.1 of
[RFC8461]. In the case of Internationalized Domain Names
[RFC5891], the domain MUST consist of the Punycode-encoded
A-labels [RFC3492] and not the U-labels.
o "result-type": A value from Section 4.3, "Result Types", above.
o "ip-address": The IP address of the Sending MTA that attempted the
STARTTLS connection. It is provided as a string representation of
an IPv4 (see below) or IPv6 [RFC5952] address in dot-decimal or
colon-hexadecimal notation.
o "receiving-mx-hostname": The hostname of the receiving MTA MX
record with which the Sending MTA attempted to negotiate a
STARTTLS connection.
Margolis, et al. Standards Track [Page 13]RFC 8460 SMTP TLS Reporting September 2018
o "receiving-mx-helo" (optional): The HELLO (HELO) or Extended HELLO
(EHLO) string from the banner announced during the reported
session.
o "receiving-ip": The destination IP address that was used when
creating the outbound session. It is provided as a string
representation of an IPv4 (see below) or IPv6 [RFC5952] address in
dot-decimal or colon-hexadecimal notation.
o "total-successful-session-count": The aggregate count (an integer,
encoded as a JSON number) of successfully negotiated TLS-enabled
connections to the receiving site.
o "total-failure-session-count": The aggregate count (an integer,
encoded as a JSON number) of failures to negotiate a TLS-enabled
connection to the receiving site.
o "failed-session-count": The number of (attempted) sessions that
match the relevant "result-type" for this section (an integer,
encoded as a JSON number).
o "additional-info-uri" (optional): A URI [RFC3986] that points to
additional information around the relevant "result-type". For
example, this URI might host the complete certificate chain
presented during an attempted STARTTLS session.
o "failure-reason-code": A text field to include a TLS-related error
code or error message.
For report purposes, an IPv4 address is defined via the following
ABNF:
IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
dec-octet = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255
And an IPv6 address is defined via the following ABNF:
IPv6address = <as defined in [RFC5954]>
Margolis, et al. Standards Track [Page 14]RFC 8460 SMTP TLS Reporting September 2018
4.5. Policy Samples
Part of the report body includes the policy that is applied when
attempting relay to the destination.
For DANE TLSA policies, this is a JSON array of strings each
representing the RDATA of a single TLSA resource record as a space-
separated list of its four TLSA fields; the fields are in
presentation format (defined in [RFC6698], Section 2.2) with no
internal spaces or grouping parentheses:
[
"3 0 1 1F850A337E6DB9C609C522D136A475638CC43E1ED424F8EEC8513
D747D1D085D",
"3 0 1 12350A337E6DB9C6123522D136A475638CC43E1ED424F8EEC8513
D747D1D1234"
]
For MTA-STS policies, this is an array of JSON strings that
represents the policy that is declared by the receiving site,
including any errors that may be present. Note that where there are
multiple "mx" values, they must be listed as separate "mx" elements
in the policy array rather than as a single nested "mx" sub-array.
[
"version: STSv1",
"mode: testing",
"mx: mx1.example.com",
"mx: mx2.example.com",
"mx: mx.backup-example.com",
"max_age: 604800"
]
5. Report Delivery
Reports can be delivered either via SMTP (as an email message) or via
HTTP POST.
Margolis, et al. Standards Track [Page 15]RFC 8460 SMTP TLS Reporting September 2018
5.1. Report Filename
The filename is RECOMMENDED to be constructed using the following
ABNF:
filename = sender "!" policy-domain "!" begin-timestamp
"!" end-timestamp [ "!" unique-id ] "." extension
unique-id = 1*(ALPHA / DIGIT)
sender = domain ; from [RFC5321] -- this is used
; as the domain for the `contact-info`
; address in the report body.
; In the case of Internationalized Domain
; Names [RFC5891], the domain MUST consist of
; the Punycode-encoded A-labels [RFC3492] and
; not the U-labels.
policy-domain = domain
; In the case of Internationalized Domain
; Names [RFC5891], the domain MUST consist of
; the Punycode-encoded A-labels [RFC3492] and
; not the U-labels.
begin-timestamp = 1*DIGIT
; seconds since 00:00:00 UTC January 1, 1970
; indicating start of the time range contained
; in the report
end-timestamp = 1*DIGIT
; seconds since 00:00:00 UTC January 1, 1970
; indicating end of the time range contained
; in the report
extension = "json" / "json.gz"
The extension MUST be "json" for a plain JSON file or "json.gz" for a
JSON file compressed using gzip.
"unique-id" allows an optional unique ID generated by the Sending MTA
to distinguish among multiple reports generated simultaneously by
different sources for the same Policy Domain. For example, this is a
possible filename for a compressed report to the Policy Domain
"example.net" from the Sending MTA "mail.sndr.example.com":
"mail.sndr.example.com!example.net!1470013207!1470186007!001.json.gz"
Margolis, et al. Standards Track [Page 16]RFC 8460 SMTP TLS Reporting September 2018
5.2. Compression
The report SHOULD be subjected to gzip [RFC1952] compression for both
email and HTTPS transport. Declining to apply compression can cause
the report to be too large for a receiver to process (a commonly
observed receiver limit is ten megabytes); compressing the file
increases the chances of acceptance of the report at some
computational cost.
5.3. Email Transport
The report MAY be delivered by email. To make the reports machine-
parsable for the receivers, we define a top-level media type
"multipart/report" with a new parameter "report-type="tlsrpt"".
Inside it, there are two parts: The first part is human readable,
typically "text/plain", and the second part is machine readable with
a new media type defined called "application/tlsrpt+json". If
compressed, the report should use the media type "application/
tlsrpt+gzip".
In addition, the following two new top-level message header fields
are defined:
"TLS-Report-Domain: Receiver-Domain"
"TLS-Report-Submitter: Sender-Domain"
The "TLS-Report-Submitter" value MUST match the value found in the
domain [RFC5321] of the "contact-info" from the report body. These
message header fields MUST be included and should allow for easy
searching for all reports submitted by a reporting domain or a
particular submitter, for example, in IMAP [RFC3501]:
"s SEARCH HEADER "TLS-Report-Domain" "example.com""
It is presumed that the aggregate reporting address will be equipped
to process new message header fields and extract MIME parts with the
prescribed media type and filename, and ignore the rest. These
additional headers SHOULD be included in the DKIM [RFC6376] signature
for the message.
Margolis, et al. Standards Track [Page 17]
RFC 8460 SMTP TLS Reporting September 2018
The RFC5322.Subject field for report submissions SHOULD conform to
the following ABNF:
tlsrpt-subject = %s"Report" FWS ; "Report"
%s"Domain:" FWS ; "Domain:"
domain-name FWS ; per [RFC6376]
%s"Submitter:" FWS ; "Submitter:"
domain-name FWS ; per [RFC6376]
%s"Report-ID:" FWS ; "Report-ID:
"<" id-left "@" id-right ">" ; per [RFC5322]
[CFWS] ; per [RFC5322]
; (as with FWS)
The first domain-name indicates the DNS domain name about which the
report was generated. The second domain-name indicates the DNS
domain name representing the Sending MTA generating the report. The
purpose of the "Report-ID:" portion of the field is to enable the
Policy Domain to identify and ignore duplicate reports that might be
sent by a Sending MTA.
For instance, this is a possible Subject field for a report to the
Policy Domain "example.net" from the Sending MTA
"mail.sender.example.com". It is line-wrapped as allowed by
[RFC5322]:
Subject: Report Domain: example.net
Submitter: mail.sender.example.com
Report-ID: <735ff.e317+bf22029@mailexample.net>
Margolis, et al. Standards Track [Page 18]RFC 8460 SMTP TLS Reporting September 2018
5.3.1. Example Report
From: tlsrpt@mail.sender.example.com
Date: Fri, May 09 2017 16:54:30 -0800
To: mts-sts-tlsrpt@example.net
Subject: Report Domain: example.net
Submitter: mail.sender.example.com
Report-ID: <735ff.e317+bf22029@example.net>
TLS-Report-Domain: example.net
TLS-Report-Submitter: mail.sender.example.com
MIME-Version: 1.0
Content-Type: multipart/report; report-type="tlsrpt";
boundary="----=_NextPart_000_024E_01CC9B0A.AFE54C00"
Content-Language: en-us
This is a multipart message in MIME format.
------=_NextPart_000_024E_01CC9B0A.AFE54C00
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
This is an aggregate TLS report from mail.sender.example.com
------=_NextPart_000_024E_01CC9B0A.AFE54C00
Content-Type: application/tlsrpt+gzip
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
filename="mail.sender.example!example.com!
1013662812!1013749130.json.gz"
<gzipped content of report>
------=_NextPart_000_024E_01CC9B0A.AFE54C00--
...
Note that, when sending failure reports via SMTP, Sending MTAs MUST
NOT honor MTA-STS or DANE TLSA failures.
5.4. HTTPS Transport
The report MAY be delivered by POST to HTTPS. If compressed, the
report SHOULD use the media type "application/tlsrpt+gzip"; otherwise
it SHOULD use the media type "application/tlsrpt+json" (see
Section 6, "IANA Considerations").
The receiving system MUST return a "successful" response from its
HTTPS server, typically a 200 or 201 HTTP code [RFC7231]. Other
codes could indicate a delivery failure and may be retried as per
Margolis, et al. Standards Track [Page 19]RFC 8460 SMTP TLS Reporting September 2018
local sender policy. The receiving system is not expected to process
reports at receipt time and MAY store them for processing at a later
time.
5.5. Delivery Retry
In the event of a delivery failure, regardless of the delivery
method, a sender SHOULD attempt redelivery for up to 24 hours after
the initial attempt. As previously stated, the reports are optional,
so while it is ideal to attempt redelivery, it is not required. If
multiple retries are attempted, ideally they SHOULD be done with
exponential backoff.
5.6. Metadata Variances
As stated above, there are a variable number of ways to declare
information about the data therein. If any of the items declared via
subject or filename disagree with the report, the report MUST be
considered the authoritative source.
6. IANA Considerations
The following are the IANA considerations discussed in this document.
6.1. Message Headers
Below is the Internet Assigned Numbers Authority (IANA) Permanent
Message Header Field registration information per [RFC3864].
Header field name: TLS-Report-Domain
Applicable protocol: mail
Status: standard
Author/Change controller: IETF
Specification document(s): RFC 8460
Header field name: TLS-Report-Submitter
Applicable protocol: mail
Status: standard
Author/Change controller: IETF
Specification document(s): RFC 8460
Margolis, et al. Standards Track [Page 20]RFC 8460 SMTP TLS Reporting September 2018
6.2. Report Type
This document creates a new registry for the "report-type" parameter
to the Content-Type header field for the "multipart/report" top-level
media type defined in [RFC6522].
The registry name is "Report Type Registry", and the procedure for
updating the registry will be "Specification Required" [RFC8126].
An entry in this registry should contain:
o the report-type being registered
o one or more registered media types that can be used with this
report-type
o the document containing the registration action
o an optional comment
The initial entries are:
Report-Type: tlsrpt
Media Type: application/tlsrpt+gzip, application/tlsrpt+json
Registered By: [RFC8460]
Comment: Media types suitable for use with this report-type are
defined in Sections 6.4 and 6.5 of [RFC8460]
Report-Type: disposition-notification
Media Type: message/disposition-notification
Registered By: [RFC8098], Section 10
Report-Type: disposition-notification
Media Type: message/global-disposition-notification
Registered By: [RFC6533], Section 6
Report-Type: delivery-status
Media Type: message/delivery-status
Registered By: [RFC3464], Section 6.2
Report-Type: delivery-status
Media Type: message/global-delivery-status
Registered By: [RFC6533], Section 6
Margolis, et al. Standards Track [Page 21]RFC 8460 SMTP TLS Reporting September 2018
6.3. +gzip Media Type Suffix
This document registers a new media type suffix "+gzip". The gzip
format is a public domain, cross-platform, interoperable file storage
and transfer format, specified in [RFC1952]; it supports compression
and is used as the underlying representation by a variety of file
formats. The media type "application/gzip" has been registered for
such files. The suffix "+gzip" MAY be used with any media type whose
representation follows that established for "application/gzip". The
registration form for the structured syntax suffix for use with media
types is as follows:
Type name: gzip file storage and transfer format.
+suffix: +gzip
References: [RFC1952] [RFC6713]
Encoding considerations: gzip is a binary encoding.
Fragment identifier considerations: The syntax and semantics of
fragment identifiers specified for +gzip SHOULD be as specified for
"application/gzip". (At publication of this document, there is no
fragment identification syntax defined for "application/gzip".) The
syntax and semantics for fragment identifiers for a specific "xxx/
yyy+gzip" SHOULD be processed as follows:
For cases defined in +gzip, where the fragment identifier
resolves per the +gzip rules, process as specified in
+gzip.
For cases defined in +gzip, where the fragment identifier does
not resolve per the +gzip rules, process as specified in
"xxx/yyy+gzip".
For cases not defined in +gzip, process as specified in
"xxx/yyy+gzip".
Interoperability considerations: N/A
Security considerations: gzip format doesn't provide confidentiality
protection. Integrity protection is provided by an Adler-32
checksum, which is not cryptographically strong. See also the
security considerations of [RFC6713]. Each individual media type
registered with a +gzip suffix can have additional security
considerations. Additionally, gzip objects can contain multiple
Margolis, et al. Standards Track [Page 22]RFC 8460 SMTP TLS Reporting September 2018
files and associated paths. File paths must be validated when the
files are extracted; a malicious file path could otherwise cause the
extractor to overwrite application or system files.
Contact: art@ietf.org
Author/Change controller: Internet Engineering Task Force
(iesg@ietf.org).
6.4. application/tlsrpt+json Media Type
This document registers multiple media types, beginning with Table 1
below.
+-------------+----------------+-------------+-------------------+
| Type | Subtype | File Ext | Specification |
+-------------+----------------+-------------+-------------------+
| application | tlsrpt+json | .json | Section 5.3 |
+-------------+----------------+-------------+-------------------+
Table 1: SMTP TLS Reporting Media Type
Type name: application
Subtype name: tlsrpt+json
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: Encoding considerations are identical to
those specified for the "application/json" media type. See
[RFC7493].
Security considerations: Security considerations relating to SMTP TLS
Reporting are discussed in Section 7.
Interoperability considerations: This document specifies the format
of conforming messages and the interpretation thereof.
Published specification: Section 5.3 of RFC 8460.
Applications that use this media type: Mail User Agents (MUAs) and
Mail Transfer Agents.
Margolis, et al. Standards Track [Page 23]RFC 8460 SMTP TLS Reporting September 2018
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): ".json"
Macintosh file type code(s): N/A
Person & email address to contact for further information:
See the Authors' Addresses section.
Intended usage: COMMON
Restrictions on usage: N/A
Author: See the Authors' Addresses section.
Change controller: Internet Engineering Task Force (iesg@ietf.org).
6.5. application/tlsrpt+gzip Media Type
+-------------+----------------+-------------+-------------------+
| Type | Subtype | File Ext | Specification |
+-------------+----------------+-------------+-------------------+
| application | tlsrpt+gzip | .gz | Section 5.3 |
+-------------+----------------+-------------+-------------------+
Table 2: SMTP TLS Reporting Media Type
Type name: application
Subtype name: tlsrpt+gzip
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: Binary
Security considerations: Security considerations relating to SMTP TLS
Reporting are discussed in Section 7. Security considerations
related to gzip compression are discussed in RFC 6713.
Interoperability considerations: This document specifies the format
of conforming messages and the interpretation thereof.
Margolis, et al. Standards Track [Page 24]RFC 8460 SMTP TLS Reporting September 2018
Published specification: Section 5.3 of RFC 8460.
Applications that use this media type: Mail User Agents (MUAs) and
Mail Transfer Agents.
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): The first two bytes are 0x1f, 0x8b.
File extension(s): ".gz"
Macintosh file type code(s): N/A
Person & email address to contact for further information:
See the Authors' Addresses section.
Intended usage: COMMON
Restrictions on usage: N/A
Author: See the Authors' Addresses section.
Change controller: Internet Engineering Task Force (iesg@ietf.org).
6.6. STARTTLS Validation Result Types
This document creates a new registry, "STARTTLS Validation Result
Types". The initial entries in the registry are:
+-----------------------------+--------------+
| Result Type | Description |
+-----------------------------+--------------+
| starttls-not-supported | Section 4.3 |
| certificate-host-mismatch | Section 4.3 |
| certificate-expired | Section 4.3 |
| tlsa-invalid | Section 4.3 |
| dnssec-invalid | Section 4.3 |
| dane-required | Section 4.3 |
| certificate-not-trusted | Section 4.3 |
| sts-policy-invalid | Section 4.3 |
| sts-webpki-invalid | Section 4.3 |
| validation-failure | Section 4.3 |
| sts-policy-fetch-error | Section 4.3 |
+-----------------------------+--------------+
Margolis, et al. Standards Track [Page 25]RFC 8460 SMTP TLS Reporting September 2018
The above entries are described in Section 4.3, "Result Types". New
result types can be added to this registry using the "Expert Review"
IANA registration policy.
7. Security Considerations
SMTP TLS Reporting provides visibility into misconfigurations or
attempts to intercept or tamper with mail between hosts who support
STARTTLS. There are several security risks presented by the
existence of this reporting channel:
o Flooding of the Aggregate Report URI (rua) endpoint: An attacker
could flood the endpoint with excessive reporting traffic and
prevent the receiving domain from accepting additional reports.
This type of Denial-of-Service attack would limit visibility into
STARTTLS failures, leaving the receiving domain blind to an
ongoing attack.
o Untrusted content: An attacker could inject malicious code into
the report, exploiting any vulnerabilities in the report-handling
systems of the receiving domain. Implementers are advised to take
precautions against evaluating the contents of the report.
o Report snooping: An attacker could create a bogus TLSRPT record to
receive statistics about a domain the attacker does not own.
Since an attacker that is able to poison DNS is already able to
receive counts of SMTP connections (and, absent DANE or MTA-STS
policies, actual SMTP message payloads), this does not present a
significant new vulnerability.
o Ignoring HTTPS validation when submitting reports: When reporting
benign misconfigurations, it is likely that a misconfigured SMTP
server may also mean a misconfigured HTTPS server; as a result,
reporters who require HTTPS validity on the reporting endpoint may
fail to alert administrators about such misconfigurations.
Conversely, in the event of an actual attack, an attacker who
wishes to create a gap in reporting and could intercept HTTPS
reports could, just as easily, simply thwart the resolution of the
TLSRPT TXT record or establishment of the TCP session to the HTTPS
endpoint. Furthermore, such a man-in-the-middle attacker could
discover most or all of the metadata exposed in a report merely
through passive observation. As a result, we consider the risks
of failure to deliver reports on misconfigurations to outweigh
those of attackers intercepting reports.
Margolis, et al. Standards Track [Page 26]RFC 8460 SMTP TLS Reporting September 2018
o Reports as DDoS: TLSRPT allows specifying destinations for the
reports that are outside the authority of the Policy Domain, which
allows domains to delegate processing of reports to a partner
organization. However, an attacker who controls the Policy Domain
DNS could also use this mechanism to direct the reports to an
unwitting victim, flooding that victim with excessive reports.
DMARC [RFC7489] defines a solution for verifying delegation to
avoid such attacks; the need for this is greater with DMARC,
however, because DMARC allows an attacker to trigger reports to a
target from an innocent third party by sending mail to that third
party (which triggers a report from the third party to the
target). In the case of TLSRPT, the attacker would have to induce
the third party to send mail to the attacker in order to trigger
reports from the third party to the victim; this reduces the risk
of such an attack and the need for a verification mechanism.
Finally, because TLSRPT is intended to help administrators discover
man-in-the-middle attacks against transport-layer encryption,
including attacks designed to thwart negotiation of encrypted
connections (by downgrading opportunistic encryption or, in the case
of MTA-STS, preventing discovery of a new MTA-STS Policy), we must
also consider the risk that an adversary who can induce such a
downgrade attack can also prevent discovery of the TLSRPT TXT record
(and thus prevent discovery of the successful downgrade attack).
Administrators are thus encouraged to deploy TLSRPT TXT records with
a large TTL (reducing the window for successful application of
transient attacks against DNS resolution of the record) or to deploy
DNSSEC on the deploying zone.
8. Privacy Considerations
MTAs are generally considered public knowledge; however, the
internals of how those MTAs are configured and the users of those
MTAs may not be as public. It should be noted that providing a
receiving site with information about TLS failures may reveal
information about the sender's configuration or even information
about the senders themselves. For example, sending a report may
disclose what TLS implementation the sender uses, as the inability to
negotiate a session may be a known incompatibility between two
implementations. This may, indirectly, leak information on the
reporter's operating system or even region, if, for example, a rare
TLS implementation is popular among certain users or in certain
locations.
Margolis, et al. Standards Track [Page 27]RFC 8460 SMTP TLS Reporting September 2018
9. References
9.1. Normative References
[RFC1952] Deutsch, P., "GZIP file format specification version 4.3",
RFC 1952, DOI 10.17487/RFC1952, May 1996,
<https://www.rfc-editor.org/info/rfc1952>.
[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>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/info/rfc3339>.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
<https://www.rfc-editor.org/info/rfc3492>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
Margolis, et al. Standards Track [Page 28]RFC 8460 SMTP TLS Reporting September 2018
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891,
DOI 10.17487/RFC5891, August 2010,
<https://www.rfc-editor.org/info/rfc5891>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010,
<https://www.rfc-editor.org/info/rfc5952>.
[RFC6068] Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto'
URI Scheme", RFC 6068, DOI 10.17487/RFC6068, October 2010,
<https://www.rfc-editor.org/info/rfc6068>.
[RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, DOI 10.17487/RFC6376, September 2011,
<https://www.rfc-editor.org/info/rfc6376>.
[RFC6522] Kucherawy, M., Ed., "The Multipart/Report Media Type for
the Reporting of Mail System Administrative Messages",
STD 73, RFC 6522, DOI 10.17487/RFC6522, January 2012,
<https://www.rfc-editor.org/info/rfc6522>.
[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>.
[RFC6713] Levine, J., "The 'application/zlib' and 'application/gzip'
Media Types", RFC 6713, DOI 10.17487/RFC6713, August 2012,
<https://www.rfc-editor.org/info/rfc6713>.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014,
<https://www.rfc-editor.org/info/rfc7208>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>.
Margolis, et al. Standards Track [Page 29]RFC 8460 SMTP TLS Reporting September 2018
[RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
DOI 10.17487/RFC7493, March 2015,
<https://www.rfc-editor.org/info/rfc7493>.
[RFC7671] Dukhovni, V. and W. Hardaker, "The DNS-Based
Authentication of Named Entities (DANE) Protocol: Updates
and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671,
October 2015, <https://www.rfc-editor.org/info/rfc7671>.
[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>.
[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>.
[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>.
9.2. Informative References
[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>.
[RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464,
DOI 10.17487/RFC3464, January 2003,
<https://www.rfc-editor.org/info/rfc3464>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<https://www.rfc-editor.org/info/rfc3501>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.
[RFC6533] Hansen, T., Ed., Newman, C., and A. Melnikov,
"Internationalized Delivery Status and Disposition
Notifications", RFC 6533, DOI 10.17487/RFC6533, February
2012, <https://www.rfc-editor.org/info/rfc6533>.
Margolis, et al. Standards Track [Page 30]RFC 8460 SMTP TLS Reporting September 2018
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <https://www.rfc-editor.org/info/rfc7435>.
[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
2015, <https://www.rfc-editor.org/info/rfc7469>.
[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<https://www.rfc-editor.org/info/rfc7489>.
[RFC8098] Hansen, T., Ed. and A. Melnikov, Ed., "Message Disposition
Notification", STD 85, RFC 8098, DOI 10.17487/RFC8098,
February 2017, <https://www.rfc-editor.org/info/rfc8098>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
Margolis, et al. Standards Track [Page 31]RFC 8460 SMTP TLS Reporting September 2018
Appendix A. Example Reporting Policy
A.1. Report Using MAILTO
_smtp._tls.mail.example.com. IN TXT \
"v=TLSRPTv1;rua=mailto:reports@example.com"
A.2. Report Using HTTPS
_smtp._tls.mail.example.com. IN TXT \
"v=TLSRPTv1; \
rua=https://reporting.example.com/v1/tlsrpt"
Appendix B. Example JSON Report
Below is an example JSON report for messages from Company-X to
Company-Y, where 100 sessions were attempted to Company-Y servers
with an expired certificate, and 200 sessions were attempted to
Company-Y servers that did not successfully respond to the "STARTTLS"
command. Additionally, 3 sessions failed due to
"X509_V_ERR_PROXY_PATH_LENGTH_EXCEEDED".
{
"organization-name": "Company-X",
"date-range": {
"start-datetime": "2016-04-01T00:00:00Z",
"end-datetime": "2016-04-01T23:59:59Z"
},
"contact-info": "sts-reporting@company-x.example",
"report-id": "5065427c-23d3-47ca-b6e0-946ea0e8c4be",
"policies": [{
"policy": {
"policy-type": "sts",
"policy-string": ["version: STSv1","mode: testing",
"mx: *.mail.company-y.example","max_age: 86400"],
"policy-domain": "company-y.example",
"mx-host": "*.mail.company-y.example"
},
"summary": {
"total-successful-session-count": 5326,
"total-failure-session-count": 303
},
"failure-details": [{
"result-type": "certificate-expired",
"sending-mta-ip": "2001:db8:abcd:0012::1",
"receiving-mx-hostname": "mx1.mail.company-y.example",
"failed-session-count": 100
}, {
Margolis, et al. Standards Track [Page 32]RFC 8460 SMTP TLS Reporting September 2018
"result-type": "starttls-not-supported",
"sending-mta-ip": "2001:db8:abcd:0013::1",
"receiving-mx-hostname": "mx2.mail.company-y.example",
"receiving-ip": "203.0.113.56",
"failed-session-count": 200,
"additional-information": "https://reports.company-x.example/
report_info ? id = 5065427 c - 23 d3# StarttlsNotSupported "
}, {
"result-type": "validation-failure",
"sending-mta-ip": "198.51.100.62",
"receiving-ip": "203.0.113.58",
"receiving-mx-hostname": "mx-backup.mail.company-y.example",
"failed-session-count": 3,
"failure-reason-code": "X509_V_ERR_PROXY_PATH_LENGTH_EXCEEDED"
}]
}]
}
Margolis, et al. Standards Track [Page 33]RFC 8460 SMTP TLS Reporting September 2018
Contributors
Laetitia Baudoin
Google, Inc.
lbaudoin@google.com
Authors' Addresses
Daniel Margolis
Google, Inc.
Email: dmargolis@google.com
Alexander Brotman
Comcast, Inc.
Email: alex_brotman@comcast.com
Binu Ramakrishnan
Oath, Inc.
Email: prbinu@yahoo.com
Janet Jones
Microsoft, Inc.
Email: janet.jones@microsoft.com
Mark Risher
Google, Inc.
Email: risher@google.com
Margolis, et al. Standards Track [Page 34]
