SMTP Security via Opportunistic DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS)
RFC 7672
| Document | Type | RFC - Proposed Standard (October 2015) Errata | |
|---|---|---|---|
| Authors | Viktor Dukhovni , Wes Hardaker | ||
| Last updated | 2020-01-21 | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Last Call review
(of
-16)
Ready with Nits
|
||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Ólafur Guðmundsson | ||
| Shepherd write-up | Show Last changed 2015-03-23 | ||
| IESG | IESG state | RFC 7672 (Proposed Standard) | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | Stephen Farrell | ||
| Send notices to | (None) | ||
| IANA | IANA review state | Version Changed - Review Needed | |
| IANA action state | No IANA Actions |
RFC 7672
Internet Engineering Task Force (IETF) V. Dukhovni
Request for Comments: 7672 Two Sigma
Category: Standards Track W. Hardaker
ISSN: 2070-1721 Parsons
October 2015
SMTP Security via Opportunistic DNS-Based Authentication of Named
Entities (DANE) Transport Layer Security (TLS)
Abstract
This memo describes a downgrade-resistant protocol for SMTP transport
security between Message Transfer Agents (MTAs), based on the DNS-
Based Authentication of Named Entities (DANE) TLSA DNS record.
Adoption of this protocol enables an incremental transition of the
Internet email backbone to one using encrypted and authenticated
Transport Layer Security (TLS).
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7672.
Copyright Notice
Copyright (c) 2015 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
(http://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.
Dukhovni & Hardaker Standards Track [Page 1]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
Table of Contents
1. Introduction ....................................................3
1.1. Terminology ................................................4
1.2. Background .................................................6
1.3. SMTP Channel Security ......................................6
1.3.1. STARTTLS Downgrade Attack ...........................7
1.3.2. Insecure Server Name without DNSSEC .................7
1.3.3. Sender Policy Does Not Scale ........................8
1.3.4. Too Many Certification Authorities ..................9
2. Identifying Applicable TLSA Records .............................9
2.1. DNS Considerations .........................................9
2.1.1. DNS Errors, "Bogus" Responses, and
"Indeterminate" Responses ...........................9
2.1.2. DNS Error Handling .................................11
2.1.3. Stub Resolver Considerations .......................12
2.2. TLS Discovery .............................................13
2.2.1. MX Resolution ......................................14
2.2.2. Non-MX Destinations ................................16
2.2.3. TLSA Record Lookup .................................18
3. DANE Authentication ............................................20
3.1. TLSA Certificate Usages ...................................20
3.1.1. Certificate Usage DANE-EE(3) .......................21
3.1.2. Certificate Usage DANE-TA(2) .......................22
3.1.3. Certificate Usages PKIX-TA(0) and PKIX-EE(1) .......23
3.2. Certificate Matching ......................................24
3.2.1. DANE-EE(3) Name Checks .............................24
3.2.2. DANE-TA(2) Name Checks .............................24
3.2.3. Reference Identifier Matching ......................25
4. Server Key Management ..........................................26
5. Digest Algorithm Agility .......................................27
6. Mandatory TLS Security .........................................27
7. Note on DANE for Message User Agents ...........................28
8. Interoperability Considerations ................................28
8.1. SNI Support ...............................................28
8.2. Anonymous TLS Cipher Suites ...............................29
9. Operational Considerations .....................................29
9.1. Client Operational Considerations .........................29
9.2. Publisher Operational Considerations ......................30
10. Security Considerations .......................................30
11. References ....................................................31
11.1. Normative References .....................................31
11.2. Informative References ...................................33
Acknowledgements ..................................................34
Authors' Addresses ................................................34
Dukhovni & Hardaker Standards Track [Page 2]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
1. Introduction
This memo specifies a new connection security model for Message
Transfer Agents (MTAs). This model is motivated by key features of
inter-domain SMTP delivery, principally, the fact that the
destination server is selected indirectly via DNS Mail Exchange (MX)
records and that neither email addresses nor MX hostnames signal a
requirement for either secure or cleartext transport. Therefore,
aside from a few manually configured exceptions, SMTP transport
security is, by necessity, opportunistic (for a definition of
"Opportunistic Security", see [RFC7435]).
This specification uses the presence of DANE TLSA records to securely
signal TLS support and to publish the means by which SMTP clients can
successfully authenticate legitimate SMTP servers. This becomes
"opportunistic DANE TLS" and is resistant to downgrade and
man-in-the-middle (MITM) attacks. It enables an incremental
transition of the email backbone to authenticated TLS delivery, with
increased global protection as adoption increases.
With opportunistic DANE TLS, traffic from SMTP clients to domains
that publish "usable" DANE TLSA records in accordance with this memo
is authenticated and encrypted. Traffic from legacy clients or to
domains that do not publish TLSA records will continue to be sent in
the same manner as before, via manually configured security,
(pre-DANE) opportunistic TLS, or just cleartext SMTP.
Problems with the existing use of TLS in MTA-to-MTA SMTP that
motivate this specification are described in Section 1.3. The
specification itself follows, in Sections 2 and 3, which describe,
respectively, how to locate and use DANE TLSA records with SMTP. In
Section 6, we discuss the application of DANE TLS to destinations for
which channel integrity and confidentiality are mandatory. In
Section 7, we briefly comment on the potential applicability of this
specification to Message User Agents.
Dukhovni & Hardaker Standards Track [Page 3]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
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
[RFC2119].
The following terms or concepts are used throughout this document:
Man-in-the-middle (MITM) attack: Active modification of network
traffic by an adversary able to thereby compromise the
confidentiality or integrity of the data.
Downgrade attack: (From [RFC4949].) A type of MITM attack in which
the attacker can cause two parties, at the time they negotiate a
security association, to agree on a lower level of protection than
the highest level that could have been supported by both of them.
Downgrade-resistant: A protocol is "downgrade-resistant" if it
employs effective countermeasures against downgrade attacks.
"Secure", "bogus", "insecure", "indeterminate": DNSSEC validation
results, as defined in Section 4.3 of [RFC4035].
Validating security-aware stub resolver and non-validating
security-aware stub resolver:
Capabilities of the stub resolver in use, as defined in [RFC4033];
note that this specification requires the use of a security-aware
stub resolver.
(Pre-DANE) opportunistic TLS: Best-effort use of TLS that is
generally vulnerable to DNS forgery and STARTTLS downgrade
attacks. When a TLS-encrypted communication channel is not
available, message transmission takes place in the clear. MX
record indirection generally precludes authentication even when
TLS is available.
Opportunistic DANE TLS: Best-effort use of TLS that is resistant to
downgrade attacks for destinations with DNSSEC-validated TLSA
records. When opportunistic DANE TLS is determined to be
unavailable, clients should fall back to pre-DANE opportunistic
TLS. Opportunistic DANE TLS requires support for DNSSEC, DANE,
and STARTTLS on the client side, and STARTTLS plus a DNSSEC
published TLSA record on the server side.
Dukhovni & Hardaker Standards Track [Page 4]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
Reference identifier: (Special case of [RFC6125] definition.) One
of the domain names associated by the SMTP client with the
destination SMTP server for performing name checks on the server
certificate. When name checks are applicable, at least one of the
reference identifiers MUST match an [RFC6125] DNS-ID (or, if none
are present, the [RFC6125] CN-ID) of the server certificate (see
Section 3.2.3).
MX hostname: The RRDATA of an MX record consists of a 16 bit
preference followed by a Mail Exchange domain name (see [RFC1035],
Section 3.3.9). We will use the term "MX hostname" to refer to
the latter, that is, the DNS domain name found after the
preference value in an MX record. Thus, an "MX hostname" is
specifically a reference to a DNS domain name rather than any host
that bears that name.
Delayed delivery: Email delivery is a multi-hop store-and-forward
process. When an MTA is unable to forward a message that may
become deliverable later, the message is queued and delivery is
retried periodically. Some MTAs may be configured with a fallback
next-hop destination that handles messages that the MTA would
otherwise queue and retry. When a fallback next-hop destination
is configured, messages that would otherwise have to be delayed
may be sent to the fallback next-hop destination instead. The
fallback destination may itself be subject to opportunistic or
mandatory DANE TLS (Section 6) as though it were the original
message destination.
Original next-hop destination: The logical destination for mail
delivery. By default, this is the domain portion of the recipient
address, but MTAs may be configured to forward mail for some or
all recipients via designated relays. The original next-hop
destination is, respectively, either the recipient domain or the
associated configured relay.
MTA: Message Transfer Agent ([RFC5598], Section 4.3.2).
MSA: Message Submission Agent ([RFC5598], Section 4.3.1).
MUA: Message User Agent ([RFC5598], Section 4.2.1).
RR: A DNS resource record as defined in [RFC1034], Section 3.6.
RRset: An RRset ([RFC2181], Section 5) is a group of DNS resource
records that share the same label, class, and type.
Dukhovni & Hardaker Standards Track [Page 5]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
1.2. Background
The Domain Name System Security Extensions (DNSSEC) add data origin
authentication, data integrity, and data nonexistence proofs to the
Domain Name System (DNS). DNSSEC is defined in [RFC4033], [RFC4034],
and [RFC4035].
As described in the introduction of [RFC6698], TLS authentication via
the existing public Certification Authority (CA) PKI suffers from an
overabundance of trusted parties capable of issuing certificates for
any domain of their choice. DANE leverages the DNSSEC infrastructure
to publish public keys and certificates for use with the Transport
Layer Security (TLS) [RFC5246] protocol via the "TLSA" DNS record
type. With DNSSEC, each domain can only vouch for the keys of its
delegated sub-domains.
The TLS protocol enables secure TCP communication. In the context of
this memo, channel security is assumed to be provided by TLS. Used
without authentication, TLS provides only privacy protection against
eavesdropping attacks. Otherwise, TLS also provides data origin
authentication to guard against MITM attacks.
1.3. SMTP Channel Security
With HTTPS, TLS employs X.509 certificates [RFC5280] issued by one of
the many CAs bundled with popular web browsers to allow users to
authenticate their "secure" websites. Before we specify a new DANE
TLS security model for SMTP, we will explain why a new security model
is needed. In the process, we will explain why the familiar HTTPS
security model is inadequate to protect inter-domain SMTP traffic.
The subsections below outline four key problems with applying
traditional Web PKI [RFC7435] to SMTP; these problems are addressed
by this specification. Since an SMTP channel security policy is not
explicitly specified in either the recipient address or the MX
record, a new signaling mechanism is required to indicate when
channel security is possible and should be used. The publication of
TLSA records allows server operators to securely signal to SMTP
clients that TLS is available and should be used. DANE TLSA makes it
possible to simultaneously discover which destination domains support
secure delivery via TLS and how to verify the authenticity of the
associated SMTP services, providing a path forward to ubiquitous SMTP
channel security.
Dukhovni & Hardaker Standards Track [Page 6]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
1.3.1. STARTTLS Downgrade Attack
SMTP [RFC5321] is a single-hop protocol in a multi-hop store-and-
forward email delivery process. An SMTP envelope recipient address
does not correspond to a specific transport-layer endpoint address;
rather, at each relay hop, the transport-layer endpoint is the
next-hop relay, while the envelope recipient address typically
remains the same. Unlike HTTP and its corresponding secured version,
HTTPS, where the use of TLS is signaled via the URI scheme, email
recipient addresses do not directly signal transport security policy.
Indeed, no such signaling could work well with SMTP, since TLS
encryption of SMTP protects email traffic on a hop-by-hop basis while
email addresses could only express end-to-end policy.
With no mechanism available to signal transport security policy, SMTP
relays employ a best-effort "opportunistic" security model for TLS.
A single SMTP server TCP listening endpoint can serve both TLS and
non-TLS clients; the use of TLS is negotiated via the SMTP STARTTLS
command [RFC3207]. The server signals TLS support to the client over
a cleartext SMTP connection, and, if the client also supports TLS, it
may negotiate a TLS-encrypted channel to use for email transmission.
The server's indication of TLS support can be easily suppressed by an
MITM attacker. Thus, pre-DANE SMTP TLS security can be subverted by
simply downgrading a connection to cleartext. No TLS security
feature can prevent this. The attacker can simply disable TLS.
1.3.2. Insecure Server Name without DNSSEC
With SMTP, DNS MX records abstract the next-hop transport endpoint
and allow administrators to specify a set of target servers to which
SMTP traffic should be directed for a given domain.
A TLS client is vulnerable to MITM attacks unless it verifies that
the server's certificate binds the public key to a name that matches
one of the client's reference identifiers. A natural choice of
reference identifier is the server's domain name. However, with
SMTP, server names are not directly encoded in the recipient address;
instead, they are obtained indirectly via MX records. Without
DNSSEC, the MX lookup is vulnerable to MITM and DNS cache poisoning
attacks. Active attackers can forge DNS replies with fake MX records
and can redirect email to servers with names of their choice.
Therefore, secure verification of SMTP TLS certificates matching the
server name is not possible without DNSSEC.
Dukhovni & Hardaker Standards Track [Page 7]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
One might try to harden TLS for SMTP against DNS attacks by using the
envelope recipient domain as a reference identifier and by requiring
each SMTP server to possess a trusted certificate for the envelope
recipient domain rather than the MX hostname. Unfortunately, this is
impractical, as email for many domains is handled by third parties
that are not in a position to obtain certificates for all the domains
they serve. Deployment of the Server Name Indication (SNI) extension
to TLS (see Section 3 of [RFC6066]) is no panacea, since SNI key
management is operationally challenging except when the email service
provider is also the domain's registrar and its certificate issuer;
this is rarely the case for email.
Since the recipient domain name cannot be used as the SMTP server
reference identifier, and neither can the MX hostname without DNSSEC,
large-scale deployment of authenticated TLS for SMTP requires that
the DNS be secure.
Since SMTP security depends critically on DNSSEC, it is important to
point out that SMTP with DANE is consequently the most conservative
possible trust model. It trusts only what must be trusted and no
more. Adding any other trusted actors to the mix can only reduce
SMTP security. A sender may choose to further harden DNSSEC for
selected high-value receiving domains by configuring explicit trust
anchors for those domains instead of relying on the chain of trust
from the root domain. However, detailed discussion of DNSSEC
security practices is out of scope for this document.
1.3.3. Sender Policy Does Not Scale
Sending systems are in some cases explicitly configured to use TLS
for mail sent to selected peer domains, but this requires configuring
sending MTAs with appropriate subject names or certificate content
digests from their peer domains. Due to the resulting administrative
burden, such statically configured SMTP secure channels are used
rarely (generally only between domains that make bilateral
arrangements with their business partners). Internet email, on the
other hand, requires regularly contacting new domains for which
security configurations cannot be established in advance.
The abstraction of the SMTP transport endpoint via DNS MX records,
often across organizational boundaries, limits the use of public CA
PKI with SMTP to a small set of sender-configured peer domains. With
little opportunity to use TLS authentication, sending MTAs are rarely
configured with a comprehensive list of trusted CAs. SMTP services
that support STARTTLS often deploy X.509 certificates that are
self-signed or issued by a private CA.
Dukhovni & Hardaker Standards Track [Page 8]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
1.3.4. Too Many Certification Authorities
Even if it were generally possible to determine a secure server name,
the SMTP client would still need to verify that the server's
certificate chain is issued by a trusted CA (a trust anchor). MTAs
are not interactive applications where a human operator can make a
decision (wisely or otherwise) to selectively disable TLS security
policy when certificate chain verification fails. With no user to
"click OK", the MTA's list of public CA trust anchors would need to
be comprehensive in order to avoid bouncing mail addressed to sites
that employ unknown CAs.
On the other hand, each trusted CA can issue certificates for any
domain. If even one of the configured CAs is compromised or operated
by an adversary, it can subvert TLS security for all destinations.
Any set of CAs is simultaneously both overly inclusive and not
inclusive enough.
2. Identifying Applicable TLSA Records
2.1. DNS Considerations
2.1.1. DNS Errors, "Bogus" Responses, and "Indeterminate" Responses
An SMTP client that implements opportunistic DANE TLS per this
specification depends critically on the integrity of DNSSEC lookups,
as discussed in Section 1.3.2. This section lists the DNS resolver
requirements needed to avoid downgrade attacks when using
opportunistic DANE TLS.
A DNS lookup may signal an error or return a definitive answer. A
security-aware resolver MUST be used for this specification.
Security-aware resolvers will indicate the security status of a DNS
RRset with one of four possible values defined in Section 4.3 of
[RFC4035]: "secure", "insecure", "bogus", and "indeterminate". In
[RFC4035], the meaning of the "indeterminate" security status is:
An RRset for which the resolver is not able to determine whether
the RRset should be signed, as the resolver is not able to obtain
the necessary DNSSEC RRs. This can occur when the security-aware
resolver is not able to contact security-aware name servers for
the relevant zones.
Note that the "indeterminate" security status has a conflicting
definition in Section 5 of [RFC4033]:
There is no trust anchor that would indicate that a specific
portion of the tree is secure.
Dukhovni & Hardaker Standards Track [Page 9]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
In this document, the term "indeterminate" will be used exclusively
in the [RFC4035] sense. Therefore, obtaining "indeterminate" lookup
results is a (transient) failure condition, namely, the inability to
locate the relevant DNS records. DNS records that would be
classified "indeterminate" in the sense of [RFC4035] are simply
classified as "insecure".
We do not need to distinguish between zones that lack a suitable
ancestor trust anchor, and delegations (ultimately) from a trust
anchor that designate a child zone as being "insecure". All
"insecure" RRsets MUST be handled identically: in either case,
non-validated data for the query domain is all that is and can be
available, and authentication using the data is impossible. As the
DNS root zone has been signed, we expect that validating resolvers
used by Internet-facing MTAs will be configured with trust anchor
data for the root zone and that therefore domains with no ancestor
trust anchor will not be possible in most deployments.
As noted in Section 4.3 of [RFC4035], a security-aware DNS resolver
MUST be able to determine whether a given non-error DNS response is
"secure", "insecure", "bogus", or "indeterminate". It is expected
that most security-aware stub resolvers will not signal an
"indeterminate" security status (in the sense of [RFC4035]) to the
application and will instead signal a "bogus" or error result. If a
resolver does signal an [RFC4035] "indeterminate" security status,
this MUST be treated by the SMTP client as though a "bogus" or error
result had been returned.
An MTA using a non-validating security-aware stub resolver MAY use
the stub resolver's ability, if available, to signal DNSSEC
validation status based on information the stub resolver has learned
from an upstream validating recursive resolver. Security-oblivious
stub resolvers [RFC4033] MUST NOT be used. In accordance with
Section 4.9.3 of [RFC4035]:
... a security-aware stub resolver MUST NOT place any reliance on
signature validation allegedly performed on its behalf, except
when the security-aware stub resolver obtained the data in
question from a trusted security-aware recursive name server via a
secure channel.
To avoid much repetition in the text below, we will pause to explain
the handling of "bogus" or "indeterminate" DNSSEC query responses.
These are not necessarily the result of a malicious actor; they can,
for example, occur when network packets are corrupted or lost in
transit. Therefore, "bogus" or "indeterminate" replies are equated
in this memo with lookup failure.
Dukhovni & Hardaker Standards Track [Page 10]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
There is an important non-failure condition we need to highlight in
addition to the obvious case of the DNS client obtaining a non-empty
"secure" or "insecure" RRset of the requested type. Namely, it is
not an error when either "secure" or "insecure" nonexistence is
determined for the requested data. When a DNSSEC response with a
validation status that is either "secure" or "insecure" reports
either no records of the requested type or nonexistence of the query
domain, the response is not a DNS error condition. The DNS client
has not been left without an answer; it has learned that records of
the requested type do not exist.
Security-aware stub resolvers will, of course, also signal DNS lookup
errors in other cases, for example, when processing a "SERVFAIL"
[RFC2136] response code (RCODE) [RFC1035], which will not have an
associated DNSSEC status. All lookup errors are treated the same way
by this specification, regardless of whether they are from a "bogus"
or "indeterminate" DNSSEC status or from a more generic DNS error:
the information that was requested cannot be obtained by the
security-aware resolver at this time. Thus, a lookup error is either
a failure to obtain the relevant RRset if it exists or a failure to
determine that no such RRset exists when it does not.
In contrast to a "bogus" response or an "indeterminate" response, an
"insecure" DNSSEC response is not an error; rather, as explained
above, it indicates that the target DNS zone is either delegated as
an "insecure" child of a "secure" parent zone or not a descendant of
any of the configured DNSSEC trust anchors in use by the SMTP client.
"Insecure" results will leave the SMTP client with degraded channel
security but do not stand in the way of message delivery. See
Section 2.2 for further details.
2.1.2. DNS Error Handling
When a DNS lookup failure (an error, "bogus", or "indeterminate", as
defined above) prevents an SMTP client from determining which SMTP
server or servers it should connect to, message delivery MUST be
delayed. This naturally includes, for example, the case when a
"bogus" or "indeterminate" response is encountered during MX
resolution. When multiple MX hostnames are obtained from a
successful MX lookup but a later DNS lookup failure prevents network
address resolution for a given MX hostname, delivery may proceed via
any remaining MX hosts.
When a particular SMTP server is securely identified as the delivery
destination, a set of DNS lookups (Section 2.2) MUST be performed to
locate any related TLSA records. If any DNS queries used to locate
TLSA records fail (due to "bogus" or "indeterminate" records,
timeouts, malformed replies, SERVFAIL responses, etc.), then the SMTP
Dukhovni & Hardaker Standards Track [Page 11]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
client MUST treat that server as unreachable and MUST NOT deliver the
message via that server. If no servers are reachable, delivery is
delayed.
In the text that follows, we will only describe what happens when all
relevant DNS queries succeed. If any DNS failure occurs, the SMTP
client MUST behave as described in this section, by "skipping" the
SMTP server or destination that is problematic. Queries for
candidate TLSA records are explicitly part of "all relevant DNS
queries", and SMTP clients MUST NOT continue to connect to an SMTP
server or destination whose TLSA record lookup fails.
2.1.3. Stub Resolver Considerations
A note about DNAME aliases: a query for a domain name whose ancestor
domain is a DNAME alias returns the DNAME RR for the ancestor domain
along with a CNAME that maps the query domain to the corresponding
sub-domain of the target domain of the DNAME alias [RFC6672].
Therefore, whenever we speak of CNAME aliases, we implicitly allow
for the possibility that the alias in question is the result of an
ancestor domain DNAME record. Consequently, no explicit support for
DNAME records is needed in SMTP software; it is sufficient to process
the resulting CNAME aliases. DNAME records only require special
processing in the validating stub resolver library that checks the
integrity of the combined DNAME + CNAME reply. When DNSSEC
validation is handled by a local caching resolver rather than the MTA
itself, even that part of the DNAME support logic is outside the MTA.
When a stub resolver returns a response containing a CNAME alias that
does not also contain the corresponding query results for the target
of the alias, the SMTP client will need to repeat the query at the
target of the alias and should do so recursively up to some
configured or implementation-dependent recursion limit. If at any
stage of CNAME expansion an error is detected, the lookup of the
original requested records MUST be considered to have failed.
Whether a chain of CNAME records was returned in a single stub
resolver response or via explicit recursion by the SMTP client, if at
any stage of recursive expansion an "insecure" CNAME record is
encountered, then it and all subsequent results (in particular, the
final result) MUST be considered "insecure", regardless of whether or
not any earlier CNAME records leading to the "insecure" record were
"secure".
Note that a security-aware non-validating stub resolver may return to
the SMTP client an "insecure" reply received from a validating
recursive resolver that contains a CNAME record along with additional
answers recursively obtained starting at the target of the CNAME. In
Dukhovni & Hardaker Standards Track [Page 12]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
this case, the only possible conclusion is that some record in the
set of records returned is "insecure", and it is, in fact, possible
that the initial CNAME record and a subset of the subsequent records
are "secure".
If the SMTP client needs to determine the security status of the DNS
zone containing the initial CNAME record, it will need to issue a
separate query of type "CNAME" that returns only the initial CNAME
record. Specifically, as discussed in Section 2.2.2, when "insecure"
A or AAAA records are found for an SMTP server via a CNAME alias, the
SMTP client will need to perform an additional CNAME query in order
to determine whether or not the DNS zone in which the alias is
published is DNSSEC signed.
2.2. TLS Discovery
As noted previously (in Section 1.3.1), opportunistic TLS with SMTP
servers that advertise TLS support via STARTTLS is subject to an MITM
downgrade attack. Also, some SMTP servers that are not, in fact, TLS
capable erroneously advertise STARTTLS by default, and clients need
to be prepared to retry cleartext delivery after STARTTLS fails. In
contrast, DNSSEC-validated TLSA records MUST NOT be published for
servers that do not support TLS. Clients can safely interpret their
presence as a commitment by the server operator to implement TLS and
STARTTLS.
This memo defines four actions to be taken after the search for a
TLSA record returns "secure" usable results, "secure" unusable
results, "insecure" or no results, or an error signal. The term
"usable" in this context is in the sense of Section 4.1 of [RFC6698].
Specifically, if the DNS lookup for a TLSA record returns:
A "secure" TLSA RRset with at least one usable record: Any
connection to the MTA MUST employ TLS encryption and MUST
authenticate the SMTP server using the techniques discussed in the
rest of this document. Failure to establish an authenticated TLS
connection MUST result in falling back to the next SMTP server or
delayed delivery.
A "secure" non-empty TLSA RRset where all the records are unusable:
Any connection to the MTA MUST be made via TLS, but authentication
is not required. Failure to establish an encrypted TLS connection
MUST result in falling back to the next SMTP server or delayed
delivery.
Dukhovni & Hardaker Standards Track [Page 13]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
An "insecure" TLSA RRset or DNSSEC-authenticated denial of existence
of the TLSA records:
A connection to the MTA SHOULD be made using (pre-DANE)
opportunistic TLS; this includes using cleartext delivery when the
remote SMTP server does not appear to support TLS. The MTA MAY
retry in cleartext when delivery via TLS fails during the
handshake or even during data transfer.
Any lookup error: Lookup errors, including "bogus" and
"indeterminate" as explained in Section 2.1.1, MUST result in
falling back to the next SMTP server or delayed delivery.
An SMTP client MAY be configured to mandate DANE-verified delivery
for some destinations. With mandatory DANE TLS (Section 6), delivery
proceeds only when "secure" TLSA records are used to establish an
encrypted and authenticated TLS channel with the SMTP server.
When the original next-hop destination is an address literal rather
than a DNS domain, DANE TLS does not apply. Delivery proceeds using
any relevant security policy configured by the MTA administrator.
Similarly, when an MX RRset incorrectly lists a network address in
lieu of an MX hostname, if an MTA chooses to connect to the network
address in the nonconformant MX record, DANE TLSA does not apply for
such a connection.
In the subsections that follow, we explain how to locate the SMTP
servers and the associated TLSA records for a given next-hop
destination domain. We also explain which name or names are to be
used in identity checks of the SMTP server certificate.
2.2.1. MX Resolution
In this section, we consider next-hop domains that are subject to MX
resolution and have MX records. The TLSA records and the associated
base domain are derived separately for each MX hostname that is used
to attempt message delivery. DANE TLS can authenticate message
delivery to the intended next-hop domain only when the MX records are
obtained securely via a DNSSEC-validated lookup.
MX records MUST be sorted by preference; an MX hostname with a worse
(numerically higher) MX preference that has TLSA records MUST NOT
preempt an MX hostname with a better (numerically lower) preference
that has no TLSA records. In other words, prevention of delivery
loops by obeying MX preferences MUST take precedence over channel
security considerations. Even with two equal-preference MX records,
an MTA is not obligated to choose the MX hostname that offers more
Dukhovni & Hardaker Standards Track [Page 14]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
security. Domains that want secure inbound mail delivery need to
ensure that all their SMTP servers and MX records are configured
accordingly.
In the language of [RFC5321], Section 5.1, the original next-hop
domain is the "initial name". If the MX lookup of the initial name
results in a CNAME alias, the MTA replaces the initial name with the
resulting name and performs a new lookup with the new name. MTAs
typically support recursion in CNAME expansion, so this replacement
is performed repeatedly (up to the MTA's recursion limit) until the
ultimate non-CNAME domain is found.
If the MX RRset (or any CNAME leading to it) is "insecure" (see
Section 2.1.1) and DANE TLS for the given destination is mandatory
(Section 6), delivery MUST be delayed. If the MX RRset is "insecure"
and DANE TLS is not mandatory, the SMTP client is free to use
pre-DANE opportunistic TLS (possibly even cleartext).
Since the protocol in this memo is an Opportunistic Security protocol
[RFC7435], the SMTP client MAY elect to use DANE TLS (as described in
Section 2.2.2 below), even with MX hosts obtained via an "insecure"
MX RRset. For example, when a hosting provider has a signed DNS zone
and publishes TLSA records for its SMTP servers, hosted domains that
are not signed may still benefit from the provider's TLSA records.
Deliveries via the provider's SMTP servers will not be subject to
active attacks when sending SMTP clients elect to use the provider's
TLSA records (active attacks that tamper with the "insecure" MX RRset
are of course still possible in this case).
When the MX records are not (DNSSEC) signed, an active attacker can
redirect SMTP clients to MX hosts of his choice. Such redirection is
tamper-evident when SMTP servers found via "insecure" MX records are
recorded as the next-hop relay in the MTA delivery logs in their
original (rather than CNAME-expanded) form. Sending MTAs SHOULD log
unexpanded MX hostnames when these result from "insecure" MX lookups.
Any successful authentication via an insecurely determined MX host
MUST NOT be misrepresented in the mail logs as secure delivery to the
intended next-hop domain.
In the absence of DNS lookup errors (Section 2.1.1), if the MX RRset
is not "insecure", then it is "secure", and the SMTP client MUST
treat each MX hostname as described in Section 2.2.2. When, for a
given MX hostname, no TLSA records are found or only "insecure" TLSA
records are found, DANE TLSA is not applicable with the SMTP server
in question, and delivery proceeds to that host as with pre-DANE
opportunistic TLS. To avoid downgrade attacks, any errors during
TLSA lookups MUST, as explained in Section 2.1.2, cause the SMTP
server in question to be treated as unreachable.
Dukhovni & Hardaker Standards Track [Page 15]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
2.2.2. Non-MX Destinations
This section describes the algorithm used to locate the TLSA records
and associated TLSA base domain for an input domain that is not
subject to MX resolution, that represents a hostname from a "secure"
MX RRset, or that lacks MX records. Such domains include:
o Any host that is configured by the sending MTA administrator as
the next-hop relay for some or all domains and that is not subject
to MX resolution.
o A domain that has MX records. When a domain has MX records, we
treat each MX host listed in those MX records as though it were a
non-MX destination -- that is, in the same way as we would treat
an administrator-configured relay that handles mail for that
domain. (Unlike administrator-specified relays, MTAs are not
required to support CNAME expansion of next-hop names found via MX
lookups.)
o A next-hop destination domain subject to MX resolution that has no
MX records. In this case, the domain's name is implicitly also
its sole SMTP server name.
Note that DNS queries with type TLSA are mishandled by load-balancing
nameservers that serve the MX hostnames of some large email
providers. The DNS zones served by these nameservers are not signed
and contain no TLSA records. These nameservers SHOULD provide
"insecure" negative replies that indicate the nonexistence of the
TLSA records, but instead they fail by not responding at all or by
responding with a DNS RCODE [RFC1035] other than NXDOMAIN, e.g.,
SERVFAIL or NOTIMP [RFC2136].
To avoid problems delivering mail to domains whose SMTP servers are
served by these problematic nameservers, the SMTP client MUST perform
any A and/or AAAA queries for the destination before attempting to
locate the associated TLSA records. This lookup is needed in any
case to determine (1) whether or not the destination domain is
reachable and (2) the DNSSEC validation status of the chain of CNAME
queries required to reach the ultimate address records.
If no address records are found, the destination is unreachable. If
address records are found but the DNSSEC validation status of the
first query response is "insecure" (see Section 2.1.3), the SMTP
client SHOULD NOT proceed to search for any associated TLSA records.
In the case of these problematic domains, TLSA queries would lead to
DNS lookup errors and would cause messages to be consistently delayed
and ultimately returned to the sender. We don't expect to find any
Dukhovni & Hardaker Standards Track [Page 16]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
"secure" TLSA records associated with a TLSA base domain that lies in
an unsigned DNS zone. Therefore, skipping TLSA lookups in this case
will also reduce latency, with no detrimental impact on security.
If the A and/or AAAA lookup of the initial name yields a CNAME, we
replace it with the resulting name as if it were the initial name and
perform a lookup again using the new name. This replacement is
performed recursively (up to the MTA's recursion limit).
We consider the following cases for handling a DNS response for an
A or AAAA DNS lookup:
Not found: When the DNS queries for A and/or AAAA records yield
neither a list of addresses nor a CNAME (or CNAME expansion is not
supported), the destination is unreachable.
Non-CNAME: The answer is not a CNAME alias. If the address RRset is
"secure", TLSA lookups are performed as described in Section 2.2.3
with the initial name as the candidate TLSA base domain. If no
"secure" TLSA records are found, DANE TLS is not applicable and
mail delivery proceeds with pre-DANE opportunistic TLS (which,
being best-effort, degrades to cleartext delivery when STARTTLS is
not available or the TLS handshake fails).
Insecure CNAME: The input domain is a CNAME alias, but the ultimate
network address RRset is "insecure" (see Section 2.1.1). If the
initial CNAME response is also "insecure", DANE TLS does not
apply. Otherwise, this case is treated just like the non-CNAME
case above, where a search is performed for a TLSA record with the
original input domain as the candidate TLSA base domain.
Secure CNAME: The input domain is a CNAME alias, and the ultimate
network address RRset is "secure" (see Section 2.1.1). Two
candidate TLSA base domains are tried: the fully CNAME-expanded
initial name and, failing that, the initial name itself.
In summary, if it is possible to securely obtain the full,
CNAME-expanded, DNSSEC-validated address records for the input
domain, then that name is the preferred TLSA base domain. Otherwise,
the unexpanded input domain is the candidate TLSA base domain. When
no "secure" TLSA records are found at either the CNAME-expanded or
unexpanded domain, then DANE TLS does not apply for mail delivery via
the input domain in question. And, as always, errors, "bogus"
results, or "indeterminate" results for any query in the process MUST
result in delaying or abandoning delivery.
Dukhovni & Hardaker Standards Track [Page 17]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
2.2.3. TLSA Record Lookup
When the SMTP server's hostname is not a CNAME or DNAME alias, the
list of associated candidate TLSA base domains (see below) consists
of just the server hostname.
When the hostname is an alias with a "secure" (at every stage) full
expansion, the list of candidate TLSA base domains (see below) is a
pair of domains: the fully expanded server hostname first, and the
unexpanded server hostname second.
Each candidate TLSA base domain (alias-expanded or original) is in
turn prefixed with service labels of the form "_<port>._tcp". The
resulting domain name is used to issue a DNSSEC query with the query
type set to TLSA ([RFC6698], Section 7.1).
The first of these candidate domains to yield a "secure" TLSA RRset
becomes the actual TLSA base domain.
For SMTP, the destination TCP port is typically 25, but this may be
different with custom routes specified by the MTA administrator, in
which case the SMTP client MUST use the appropriate number in the
"_<port>" prefix in place of "_25". If, for example, the candidate
base domain is "mx.example.com" and the SMTP connection is to port
25, the TLSA RRset is obtained via a DNSSEC query of the form:
_25._tcp.mx.example.com. IN TLSA ?
The query response may be a CNAME or the actual TLSA RRset. If the
response is a CNAME, the SMTP client (through the use of its
security-aware stub resolver) restarts the TLSA query at the target
domain, following CNAMEs as appropriate, and keeps track of whether
or not the entire chain is "secure". If any "insecure" records are
encountered or the TLSA records don't exist, the next candidate TLSA
base domain is tried instead.
If the ultimate response is a "secure" TLSA RRset, then the candidate
TLSA base domain will be the actual TLSA base domain, and the TLSA
RRset will constitute the TLSA records for the destination. If none
of the candidate TLSA base domains yield "secure" TLSA records, then
the SMTP client is free to use pre-DANE opportunistic TLS (possibly
even cleartext).
TLSA record publishers may leverage CNAMEs to reference a single
authoritative TLSA RRset specifying a common CA or a common
end-entity certificate to be used with multiple TLS services. Such
CNAME expansion does not change the SMTP client's notion of the TLSA
base domain; thus, when _25._tcp.mx.example.com is a CNAME, the base
Dukhovni & Hardaker Standards Track [Page 18]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
domain remains mx.example.com, and this is still the reference
identifier used together with the next-hop domain in peer certificate
name checks.
Note that shared end-entity certificate associations expose the
publishing domain to substitution attacks, where an MITM attacker can
reroute traffic to a different server that shares the same end-entity
certificate. Such shared end-entity TLSA records SHOULD be avoided
unless the servers in question are functionally equivalent or employ
mutually incompatible protocols (an active attacker gains nothing by
diverting client traffic from one such server to another).
A better example, employing a shared trust anchor rather than shared
end-entity certificates, is illustrated by the DNSSEC-validated
records below:
example.com. IN MX 0 mx1.example.com.
example.com. IN MX 0 mx2.example.com.
_25._tcp.mx1.example.com. IN CNAME tlsa201._dane.example.com.
_25._tcp.mx2.example.com. IN CNAME tlsa201._dane.example.com.
tlsa201._dane.example.com. IN TLSA 2 0 1 e3b0c44298fc1c149a...
The SMTP servers mx1.example.com and mx2.example.com will be expected
to have certificates issued under a common trust anchor, but each MX
hostname's TLSA base domain remains unchanged despite the above CNAME
records. Correspondingly, each SMTP server will be associated with a
pair of reference identifiers consisting of its hostname plus the
next-hop domain "example.com".
If, during TLSA resolution (including possible CNAME indirection), at
least one "secure" TLSA record is found (even if not usable because
it is unsupported by the implementation or support is
administratively disabled), then the corresponding host has signaled
its commitment to implement TLS. The SMTP client MUST NOT deliver
mail via the corresponding host unless a TLS session is negotiated
via STARTTLS. This is required to avoid MITM STARTTLS downgrade
attacks.
As noted previously (in Section 2.2.2), when no "secure" TLSA records
are found at the fully CNAME-expanded name, the original unexpanded
name MUST be tried instead. This supports customers of hosting
providers where the provider's zone cannot be validated with DNSSEC
but the customer has shared appropriate key material with the hosting
provider to enable TLS via SNI. Intermediate names that arise during
CNAME expansion that are neither the original name nor the final name
are never candidate TLSA base domains, even if "secure".
Dukhovni & Hardaker Standards Track [Page 19]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
3. DANE Authentication
This section describes which TLSA records are applicable to SMTP
opportunistic DANE TLS and how to apply such records to authenticate
the SMTP server. With opportunistic DANE TLS, both the TLS support
implied by the presence of DANE TLSA records and the verification
parameters necessary to authenticate the TLS peer are obtained
together. In contrast to protocols where channel security policy is
set exclusively by the client, authentication via this protocol is
expected to be less prone to connection failure caused by
incompatible configuration of the client and server.
3.1. TLSA Certificate Usages
The DANE TLSA specification [RFC6698] defines multiple TLSA RR types
via combinations of three numeric parameters. The numeric values of
these parameters were later given symbolic names in [RFC7218]. The
rest of the TLSA record is the "certificate association data field",
which specifies the full or digest value of a certificate or
public key.
Since opportunistic DANE TLS will be used by non-interactive MTAs,
with no user to "click OK" when authentication fails, reliability of
peer authentication is paramount. Server operators are advised to
publish TLSA records that are least likely to fail authentication due
to interoperability or operational problems. Because DANE TLS relies
on coordinated changes to DNS and SMTP server settings, the best
choice of records to publish will depend on site-specific practices.
The certificate usage element of a TLSA record plays a critical role
in determining how the corresponding certificate association data
field is used to authenticate a server's certificate chain.
Sections 3.1.1 and 3.1.2 explain the process for certificate usages
DANE-EE(3) and DANE-TA(2), respectively. Section 3.1.3 briefly
explains why certificate usages PKIX-TA(0) and PKIX-EE(1) are not
applicable with opportunistic DANE TLS.
In summary, we RECOMMEND the use of "DANE-EE(3) SPKI(1) SHA2-256(1)",
with "DANE-TA(2) Cert(0) SHA2-256(1)" TLSA records as a second
choice, depending on site needs. See Sections 3.1.1 and 3.1.2 for
more details. Other combinations of TLSA parameters either (1) are
explicitly unsupported or (2) offer little to recommend them over
these two.
Dukhovni & Hardaker Standards Track [Page 20]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
3.1.1. Certificate Usage DANE-EE(3)
Authentication via certificate usage DANE-EE(3) TLSA records involves
simply checking that the server's leaf certificate matches the TLSA
record. In particular, the binding of the server public key to its
name is based entirely on the TLSA record association. The server
MUST be considered authenticated even if none of the names in the
certificate match the client's reference identity for the server.
The expiration date of the server certificate MUST be ignored: the
validity period of the TLSA record key binding is determined by the
validity interval of the TLSA record DNSSEC signature.
With DANE-EE(3), servers need not employ SNI (they may ignore the
client's SNI message) even when the server is known under independent
names that would otherwise require separate certificates. It is
instead sufficient for the TLSA RRsets for all the domains in
question to match the server's default certificate. Of course, with
SMTP servers it is simpler still to publish the same MX hostname for
all the hosted domains.
For domains where it is practical to make coordinated changes in DNS
TLSA records during SMTP server key rotation, it is often best to
publish end-entity DANE-EE(3) certificate associations. DANE-EE(3)
certificates don't suddenly stop working when leaf or intermediate
certificates expire, nor do they fail when the server operator
neglects to configure all the required issuer certificates in the
server certificate chain.
TLSA records published for SMTP servers SHOULD, in most cases, be
"DANE-EE(3) SPKI(1) SHA2-256(1)" records. Since all DANE
implementations are required to support SHA2-256, this record type
works for all clients and need not change across certificate renewals
with the same key.
Dukhovni & Hardaker Standards Track [Page 21]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
3.1.2. Certificate Usage DANE-TA(2)
Some domains may prefer to avoid the operational complexity of
publishing unique TLSA RRs for each TLS service. If the domain
employs a common issuing CA to create certificates for multiple TLS
services, it may be simpler to publish the issuing authority as a
trust anchor (TA) for the certificate chains of all relevant
services. The TLSA query domain (TLSA base domain with port and
protocol prefix labels) for each service issued by the same TA may
then be set to a CNAME alias that points to a common TLSA RRset that
matches the TA. For example:
example.com. IN MX 0 mx1.example.com.
example.com. IN MX 0 mx2.example.com.
_25._tcp.mx1.example.com. IN CNAME tlsa201._dane.example.com.
_25._tcp.mx2.example.com. IN CNAME tlsa201._dane.example.com.
tlsa201._dane.example.com. IN TLSA 2 0 1 e3b0c44298fc1c14....
With usage DANE-TA(2), the server certificates will need to have
names that match one of the client's reference identifiers (see
[RFC6125]). The server MAY employ SNI to select the appropriate
certificate to present to the client.
SMTP servers that rely on certificate usage DANE-TA(2) TLSA records
for TLS authentication MUST include the TA certificate as part of the
certificate chain presented in the TLS handshake server certificate
message even when it is a self-signed root certificate. Many SMTP
servers are not configured with a comprehensive list of trust
anchors, nor are they expected to be at any point in the future.
Some MTAs will ignore all locally trusted certificates when
processing usage DANE-TA(2) TLSA records. Thus, even when the TA
happens to be a public CA known to the SMTP client, authentication is
likely to fail unless the TA certificate is included in the TLS
server certificate message.
With some SMTP server software, it is not possible to configure the
server to include self-signed (root) CA certificates in the server
certificate chain. Such servers either MUST publish DANE-TA(2)
records for an intermediate certificate or MUST instead use
DANE-EE(3) TLSA records.
TLSA records with a matching type of Full(0) are discouraged. While
these potentially obviate the need to transmit the TA certificate in
the TLS server certificate message, client implementations may not be
able to augment the server certificate chain with the data obtained
from DNS, especially when the TLSA record supplies a bare key
(selector SPKI(1)). Since the server will need to transmit the TA
certificate in any case, server operators SHOULD publish TLSA records
Dukhovni & Hardaker Standards Track [Page 22]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
with a matching type other than Full(0) and avoid potential
interoperability issues with large TLSA records containing full
certificates or keys.
TLSA Publishers employing DANE-TA(2) records SHOULD publish records
with a selector of Cert(0). Such TLSA records are associated with
the whole trust anchor certificate, not just with the trust anchor
public key. In particular, the SMTP client SHOULD then apply any
relevant constraints from the trust anchor certificate, such as, for
example, path length constraints.
While a selector of SPKI(1) may also be employed, the resulting TLSA
record will not specify the full trust anchor certificate content,
and elements of the trust anchor certificate other than the public
key become mutable. This may, for example, allow a subsidiary CA to
issue a chain that violates the trust anchor's path length or name
constraints.
3.1.3. Certificate Usages PKIX-TA(0) and PKIX-EE(1)
Note that this section applies to MTA-to-MTA SMTP, which is normally
on port 25 -- that is, to servers that are the SMTP servers for one
or more destination domains. Other uses of SMTP, such as in
MUA-to-MSA submission on ports 587 or 465, are out of scope for this
document. Where those other uses also employ TLS opportunistically
and/or depend on DNSSEC as a result of DNS-based discovery of service
location, the relevant specifications should, as appropriate, arrive
at similar conclusions.
As noted in Sections 1.3.1 and 1.3.2, sending MTAs cannot, without
relying on DNSSEC for "secure" MX records and DANE for STARTTLS
support signaling, perform server identity verification or prevent
STARTTLS downgrade attacks. The use of PKIX CAs offers no added
security, since an attacker capable of compromising DNSSEC is free to
replace any PKIX-TA(0) or PKIX-EE(1) TLSA records with records
bearing any convenient non-PKIX certificate usage. Finally, as
explained in Section 1.3.4, there is no list of trusted CAs agreed
upon by all MTAs and no user to "click OK" when a server's CA is not
trusted by a client.
Therefore, TLSA records for the port 25 SMTP service used by client
MTAs SHOULD NOT include TLSA RRs with certificate usage PKIX-TA(0) or
PKIX-EE(1). SMTP client MTAs cannot be expected to be configured
with a suitably complete set of trusted public CAs. Lacking a
complete set of public CAs, MTA clients would not be able to verify
the certificates of SMTP servers whose issuing root CAs are not
trusted by the client.
Dukhovni & Hardaker Standards Track [Page 23]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
Opportunistic DANE TLS needs to interoperate without bilateral
coordination of security settings between client and server systems.
Therefore, parameter choices that are fragile in the absence of
bilateral coordination are unsupported. Nothing is lost; since the
PKIX certificate usages cannot aid SMTP TLS security, they can only
impede SMTP TLS interoperability.
SMTP client treatment of TLSA RRs with certificate usages PKIX-TA(0)
or PKIX-EE(1) is undefined. As with any other unsupported
certificate usage, SMTP clients MAY treat such records as "unusable".
3.2. Certificate Matching
When at least one usable "secure" TLSA record is found, the SMTP
client MUST use TLSA records to authenticate the SMTP server.
Messages MUST NOT be delivered via the SMTP server if authentication
fails; otherwise, the SMTP client is vulnerable to MITM attacks.
3.2.1. DANE-EE(3) Name Checks
The SMTP client MUST NOT perform certificate name checks with
certificate usage DANE-EE(3) (Section 3.1.1).
3.2.2. DANE-TA(2) Name Checks
To match a server via a TLSA record with certificate usage
DANE-TA(2), the client MUST perform name checks to ensure that it has
reached the correct server. In all DANE-TA(2) cases, the SMTP client
MUST employ the TLSA base domain as the primary reference identifier
for matching the server certificate.
TLSA records for MX hostnames: If the TLSA base domain was obtained
indirectly via a "secure" MX lookup (including any CNAME-expanded
name of an MX hostname), then the original next-hop domain used in
the MX lookup MUST be included as a second reference identifier.
The CNAME-expanded original next-hop domain MUST be included as a
third reference identifier if different from the original next-hop
domain. When the client MTA is employing DANE TLS security
despite "insecure" MX redirection, the MX hostname is the only
reference identifier.
TLSA records for non-MX hostnames: If MX records were not used
(e.g., if none exist) and the TLSA base domain is the
CNAME-expanded original next-hop domain, then the original
next-hop domain MUST be included as a second reference identifier.
Dukhovni & Hardaker Standards Track [Page 24]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
Accepting certificates with the original next-hop domain in addition
to the MX hostname allows a domain with multiple MX hostnames to
field a single certificate bearing a single domain name (i.e., the
email domain) across all the SMTP servers. This also aids
interoperability with pre-DANE SMTP clients that are configured to
look for the email domain name in server certificates -- for example,
with "secure" DNS records as shown below:
exchange.example.org. IN CNAME mail.example.org.
mail.example.org. IN CNAME example.com.
example.com. IN MX 10 mx10.example.com.
example.com. IN MX 15 mx15.example.com.
example.com. IN MX 20 mx20.example.com.
;
mx10.example.com. IN A 192.0.2.10
_25._tcp.mx10.example.com. IN TLSA 2 0 1 ...
;
mx15.example.com. IN CNAME mxbackup.example.com.
mxbackup.example.com. IN A 192.0.2.15
; _25._tcp.mxbackup.example.com. IN TLSA ? (NXDOMAIN)
_25._tcp.mx15.example.com. IN TLSA 2 0 1 ...
;
mx20.example.com. IN CNAME mxbackup.example.net.
mxbackup.example.net. IN A 198.51.100.20
_25._tcp.mxbackup.example.net. IN TLSA 2 0 1 ...
Certificate name checks for delivery of mail to exchange.example.org
via any of the associated SMTP servers MUST accept at least the names
"exchange.example.org" and "example.com", which are, respectively,
the original and fully expanded next-hop domain. When the SMTP
server is mx10.example.com, name checks MUST accept the TLSA base
domain "mx10.example.com". If, despite the fact that MX hostnames
are required to not be aliases, the MTA supports delivery via
"mx15.example.com" or "mx20.example.com", then name checks MUST
accept the respective TLSA base domains "mx15.example.com" and
"mxbackup.example.net".
3.2.3. Reference Identifier Matching
When name checks are applicable (certificate usage DANE-TA(2)), if
the server certificate contains a Subject Alternative Name extension
[RFC5280] with at least one DNS-ID [RFC6125], then only the DNS-IDs
are matched against the client's reference identifiers. The CN-ID
[RFC6125] is only considered when no DNS-IDs are present. The server
certificate is considered matched when one of its presented
identifiers [RFC5280] matches any of the client's reference
identifiers.
Dukhovni & Hardaker Standards Track [Page 25]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
Wildcards are valid in either DNS-IDs or the CN-ID when applicable.
The wildcard character must be the entire first label of the DNS-ID
or CN-ID. Thus, "*.example.com" is valid, while "smtp*.example.com"
and "*smtp.example.com" are not. SMTP clients MUST support wildcards
that match the first label of the reference identifier, with the
remaining labels matching verbatim. For example, the DNS-ID
"*.example.com" matches the reference identifier "mx1.example.com".
SMTP clients MAY, subject to local policy, allow wildcards to match
multiple reference identifier labels, but servers cannot expect broad
support for such a policy. Therefore, any wildcards in server
certificates SHOULD match exactly one label in either the TLSA base
domain or the next-hop domain.
4. Server Key Management
Two TLSA records MUST be published before employing a new EE or TA
public key or certificate: one matching the currently deployed key
and the other matching the new key scheduled to replace it. Once
sufficient time has elapsed for all DNS caches to expire the previous
TLSA RRset and related signature RRsets, servers may be configured to
use the new EE private key and associated public key certificate or
may employ certificates signed by the new trust anchor.
Once the new public key or certificate is in use, the TLSA RR that
matches the retired key can be removed from DNS, leaving only RRs
that match keys or certificates in active use.
As described in Section 3.1.2, when server certificates are validated
via a DANE-TA(2) trust anchor and CNAME records are employed to store
the TA association data at a single location, the responsibility of
updating the TLSA RRset shifts to the operator of the trust anchor.
Before a new trust anchor is used to sign any new server
certificates, its certificate (digest) is added to the relevant TLSA
RRset. After enough time elapses for the original TLSA RRset to age
out of DNS caches, the new trust anchor can start issuing new server
certificates. Once all certificates issued under the previous trust
anchor have expired, its associated RRs can be removed from the TLSA
RRset.
In the DANE-TA(2) key management model, server operators do not
generally need to update DNS TLSA records after initially creating a
CNAME record that references the centrally operated DANE-TA(2) RRset.
If a particular server's key is compromised, its TLSA CNAME SHOULD be
replaced with a DANE-EE(3) association until the certificate for the
compromised key expires, at which point it can return to using a
CNAME record. If the central trust anchor is compromised, all
Dukhovni & Hardaker Standards Track [Page 26]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
servers need to be issued new keys by a new TA, and an updated
DANE-TA(2) TLSA RRset needs to be published containing just the
new TA.
SMTP servers cannot expect broad Certificate Revocation List (CRL) or
Online Certificate Status Protocol (OCSP) support from SMTP clients.
As outlined above, with DANE, compromised server or trust anchor keys
can be "revoked" by removing them from the DNS without the need for
client-side support for OCSP or CRLs.
5. Digest Algorithm Agility
While [RFC6698] specifies multiple digest algorithms, it does not
specify a protocol by which the SMTP client and TLSA record publisher
can agree on the strongest shared algorithm. Such a protocol would
allow the client and server to avoid exposure to deprecated weaker
algorithms that are published for compatibility with less capable
clients. When stronger algorithms are an option, deprecated
algorithms SHOULD be avoided. Such a protocol is specified in
[RFC7671]. SMTP clients and servers that implement this
specification MUST comply with the requirements outlined in Section 9
of [RFC7671].
6. Mandatory TLS Security
An MTA implementing this protocol may require a stronger security
assurance when sending email to selected destinations. The sending
organization may need to send sensitive email and/or may have
regulatory obligations to protect its content. This protocol is not
in conflict with such a requirement and, in fact, can often simplify
authenticated delivery to such destinations.
Specifically, with domains that publish DANE TLSA records for their
MX hostnames, a sending MTA can be configured to use the receiving
domain's DANE TLSA records to authenticate the corresponding SMTP
server. Authentication via DANE TLSA records is easier to manage, as
changes in the receiver's expected certificate properties are made on
the receiver end and don't require manually communicated
configuration changes. With mandatory DANE TLS, when no usable TLSA
records are found, message delivery is delayed. Thus, mail is only
sent when an authenticated TLS channel is established to the remote
SMTP server.
Administrators of mail servers that employ mandatory DANE TLS need to
carefully monitor their mail logs and queues. If a partner domain
unwittingly misconfigures its TLSA records, disables DNSSEC, or
misconfigures SMTP server certificate chains, mail will be delayed
and may bounce if the issue is not resolved in a timely manner.
Dukhovni & Hardaker Standards Track [Page 27]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
7. Note on DANE for Message User Agents
We note that SMTP is also used between Message User Agents (MUAs) and
Message Submission Agents (MSAs) [RFC6409]. In [RFC6186], a protocol
is specified that enables an MUA to dynamically locate the MSA based
on the user's email address. SMTP connection security considerations
for MUAs implementing [RFC6186] are largely analogous to connection
security requirements for MTAs, and this specification could be
applied largely verbatim with DNS MX records replaced by
corresponding DNS Service (SRV) records [RFC7673].
However, until MUAs begin to adopt the dynamic configuration
mechanisms of [RFC6186], they are adequately served by more
traditional static TLS security policies. Specification of DANE TLS
for MUA-to-MSA SMTP is left to future documents that focus
specifically on SMTP security between MUAs and MSAs.
8. Interoperability Considerations
8.1. SNI Support
To ensure that the server sends the right certificate chain, the SMTP
client MUST send the TLS SNI extension containing the TLSA base
domain. This precludes the use of the Secure Socket Layer (SSL)
HELLO that is SSL 2.0 compatible by the SMTP client.
Each SMTP server MUST present a certificate chain (see [RFC5246],
Section 7.4.2) that matches at least one of the TLSA records. The
server MAY rely on SNI to determine which certificate chain to
present to the client. Clients that don't send SNI information may
not see the expected certificate chain.
If the server's TLSA records match the server's default certificate
chain, the server need not support SNI. In either case, the server
need not include the SNI extension in its TLS HELLO, as simply
returning a matching certificate chain is sufficient. Servers
MUST NOT enforce the use of SNI by clients, as the client may be
using unauthenticated opportunistic TLS and may not expect any
particular certificate from the server. If the client sends no SNI
extension or sends an SNI extension for an unsupported domain, the
server MUST simply send some fallback certificate chain of its
choice. The reason for not enforcing strict matching of the
requested SNI hostname is that DANE TLS clients are typically willing
to accept multiple server names but can only send one name in the SNI
extension. The server's fallback certificate may match a different
name acceptable to the client, e.g., the original next-hop domain.
Dukhovni & Hardaker Standards Track [Page 28]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
8.2. Anonymous TLS Cipher Suites
Since many SMTP servers either do not support or do not enable any
anonymous TLS cipher suites, SMTP client TLS HELLO messages SHOULD
offer to negotiate a typical set of non-anonymous cipher suites
required for interoperability with such servers. An SMTP client
employing pre-DANE opportunistic TLS MAY also include one or more
anonymous TLS cipher suites in its TLS HELLO. SMTP servers that need
to interoperate with opportunistic TLS clients SHOULD be prepared to
interoperate with such clients by either always selecting a mutually
supported non-anonymous cipher suite or correctly handling client
connections that negotiate anonymous cipher suites.
Note that while SMTP server operators are under no obligation to
enable anonymous cipher suites, no security is gained by sending
certificates to clients that will ignore them. Indeed, support for
anonymous cipher suites in the server makes audit trails more
informative. Log entries that record connections that employed an
anonymous cipher suite record the fact that the clients did not care
to authenticate the server.
9. Operational Considerations
9.1. Client Operational Considerations
An operational error on the sending or receiving side that cannot be
corrected in a timely manner may, at times, lead to consistent
failure to deliver time-sensitive email. The sending MTA
administrator may have to choose between allowing email to queue
until the error is resolved and disabling opportunistic or mandatory
DANE TLS (Section 6) for one or more destinations. The choice to
disable DANE TLS security should not be made lightly. Every
reasonable effort should be made to determine that problems with mail
delivery are the result of an operational error and not an attack. A
fallback strategy may be to configure explicit out-of-band TLS
security settings if supported by the sending MTA.
SMTP clients may deploy opportunistic DANE TLS incrementally by
enabling it only for selected sites or may occasionally need to
disable opportunistic DANE TLS for peers that fail to interoperate
due to misconfiguration or software defects on either end. Some
implementations MAY support DANE TLS in an "audit only" mode in which
failure to achieve the requisite security level is logged as a
warning and delivery proceeds at a reduced security level. Unless
local policy specifies "audit only" or specifies that opportunistic
DANE TLS is not to be used for a particular destination, an SMTP
Dukhovni & Hardaker Standards Track [Page 29]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
client MUST NOT deliver mail via a server whose certificate chain
fails to match at least one TLSA record when usable TLSA records are
found for that server.
9.2. Publisher Operational Considerations
Some MTAs enable STARTTLS selectively. For example, they might only
support STARTTLS with clients that have previously demonstrated
"proper MTA behavior", e.g., by retrying the delivery of deferred
messages (greylisting). If such an MTA publishes DANE TLSA records,
sending MTAs that implement this specification will not attempt the
initial cleartext SMTP transaction needed to establish the "proper
MTA behavior", because they cannot establish the required channel
security. Server operators MUST NOT implement selective STARTTLS if
they also want to support DANE TLSA.
TLSA Publishers MUST follow the guidelines in Section 8 of [RFC7671].
TLSA Publishers SHOULD follow the TLSA publication size guidance
found in Section 10.1 of [RFC7671].
TLSA Publishers SHOULD follow the TLSA record TTL and signature
lifetime recommendations found in Section 13 of [RFC7671].
10. Security Considerations
This protocol leverages DANE TLSA records to implement MITM-resistant
Opportunistic Security [RFC7435] for SMTP. For destination domains
that sign their MX records and publish signed TLSA records for their
MX hostnames, this protocol allows sending MTAs to securely discover
both the availability of TLS and how to authenticate the destination.
This protocol does not aim to secure all SMTP traffic, as that is not
practical until DNSSEC and DANE adoption are universal. The
incremental deployment provided by following this specification is a
best possible path for securing SMTP. This protocol coexists and
interoperates with the existing insecure Internet email backbone.
The protocol does not preclude existing non-opportunistic SMTP TLS
security arrangements, which can continue to be used as before via
manual configuration with negotiated out-of-band key and TLS
configuration exchanges.
Opportunistic SMTP TLS depends critically on DNSSEC for downgrade
resistance and secure resolution of the destination name. If DNSSEC
is compromised, it is not possible to fall back on the public CA PKI
to prevent MITM attacks. A successful breach of DNSSEC enables the
attacker to publish TLSA usage 3 certificate associations and thereby
Dukhovni & Hardaker Standards Track [Page 30]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
bypass any security benefit the legitimate domain owner might hope to
gain by publishing usage 0 or usage 1 TLSA RRs. Given the lack of
public CA PKI support in existing MTA deployments, avoiding
certificate usages 0 and 1 simplifies implementation and deployment
with no adverse security consequences.
Implementations must strictly follow Sections 2.1.2, 2.1.3, 2.2,
2.2.1, 2.2.2, 2.2.3, 3.2, and 9.1 of this specification; these
sections indicate when it is appropriate to initiate a
non-authenticated connection or cleartext connection to an SMTP
server. Specifically, in order to prevent downgrade attacks on this
protocol, implementations must not initiate a connection when this
specification indicates that a particular SMTP server must be
considered unreachable.
11. References
11.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <http://www.rfc-editor.org/info/rfc3207>.
[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,
<http://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,
<http://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,
<http://www.rfc-editor.org/info/rfc4035>.
Dukhovni & Hardaker Standards Track [Page 31]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[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,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<http://www.rfc-editor.org/info/rfc5321>.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
DOI 10.17487/RFC5598, July 2009,
<http://www.rfc-editor.org/info/rfc5598>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<http://www.rfc-editor.org/info/rfc6066>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125,
March 2011, <http://www.rfc-editor.org/info/rfc6125>.
[RFC6186] Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186,
DOI 10.17487/RFC6186, March 2011,
<http://www.rfc-editor.org/info/rfc6186>.
[RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the
DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,
<http://www.rfc-editor.org/info/rfc6672>.
[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, <http://www.rfc-editor.org/info/rfc6698>.
Dukhovni & Hardaker Standards Track [Page 32]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
[RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify
Conversations about DNS-Based Authentication of Named
Entities (DANE)", RFC 7218, DOI 10.17487/RFC7218,
April 2014, <http://www.rfc-editor.org/info/rfc7218>.
[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, <http://www.rfc-editor.org/info/rfc7671>.
11.2. Informative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, DOI 10.17487/RFC2136, April 1997,
<http://www.rfc-editor.org/info/rfc2136>.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<http://www.rfc-editor.org/info/rfc2181>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<http://www.rfc-editor.org/info/rfc4949>.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
<http://www.rfc-editor.org/info/rfc6409>.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <http://www.rfc-editor.org/info/rfc7435>.
[RFC7673] Finch, T., Miller, M., and P. Saint-Andre, "Using
DNS-Based Authentication of Named Entities (DANE) TLSA
Records with SRV Records", RFC 7673, DOI 10.17487/RFC7673,
October 2015, <http://www.rfc-editor.org/info/rfc7673>.
Dukhovni & Hardaker Standards Track [Page 33]
RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015
Acknowledgements
The authors would like to extend great thanks to Tony Finch, who
started the original version of a DANE SMTP document. His work is
greatly appreciated and has been incorporated into this document.
The authors would like to additionally thank Phil Pennock for his
comments and advice on this document.
Acknowledgements from Viktor: Thanks to Paul Hoffman, who motivated
me to begin work on this memo and provided feedback on early draft
versions of this document. Thanks to Patrick Koetter, Perry Metzger,
and Nico Williams for valuable review comments. Thanks also to
Wietse Venema, who created Postfix, and whose advice and feedback
were essential to the development of the Postfix DANE implementation.
Authors' Addresses
Viktor Dukhovni
Two Sigma
Email: ietf-dane@dukhovni.org
Wes Hardaker
Parsons
P.O. Box 382
Davis, CA 95617
United States
Email: ietf@hardakers.net
Dukhovni & Hardaker Standards Track [Page 34]