wdiff draft-ietf-dane-smtp-with-dane-00.txt draft-ietf-dane-smtp-with-dane-01.txt

DANE V. Dukhovni
Internet-Draft Unaffiliated
Intended status: Experimental W. W.H. Hardaker
Expires: April 11, 24, 2014 Parsons
October 08, 21, 2013

SMTP security via opportunistic DANE TLS
draft-ietf-dane-smtp-with-dane-00
draft-ietf-dane-smtp-with-dane-01

Abstract

This memo describes a protocol for opportunistic TLS security based
on the DANE TLSA DNS record. The protocol is downgrade resistant
when the SMTP client supports DANE TLSA and the server domain
publishes TLSA records for its MX hosts. This enables an incremental
transition of the Internet email backbone (MTA to MTA SMTP traffic)
to TLS encrypted and authenticated delivery.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on April 11, 24, 2014.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. SMTP Channel Security . . . . . . . . . . . . . . . . . . 3
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Hardening Opportunistic TLS . . . . . . . . . . . . . . . . . 5
2.1. TLS discovery . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1. Non-MX destinations . . . . . . . . . . . . . . . . . 6
2.1.2. MX resolution . . . . . . . . . . . . . . . . . . . . 6
2.1.2. 7
2.1.3. TLSA record lookup . . . . . . . . . . . . . . . . . 7 9
2.2. DANE authentication . . . . . . . . . . . . . . . . . . . 8 10
2.2.1. TLSA certificate usages . . . . . . . . . . . . . . . 8 11
2.2.2. Certificate matching . . . . . . . . . . . . . . . . 11 12
3. Opportunistic TLS for Submission . . . . . . . . . . . . . . 13 14
4. Mandatory TLS Security . . . . . . . . . . . . . . . . . . . 14 15
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 15 17
7. Normative References . . . . . . . . . . . . . . . . . . . . 16 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17 19

1. Introduction

Lacking verified DNS and "Server Name Indication" (SNI), there has
historically been no scalable way for SMTP server operators to
provide deploy
certificates which match with a trustable identifier. client-trusted subject name. It's only with the
deployment of DNSSEC and DANE that authenticated TLS for SMTP to MX
becomes possible between parties that have not already established an
identity convention out-of-band.

1.1. Background

The Domain Name System Security Extensions (DNSSEC) add data origin
authentication and data integrity to the Domain Name System. DNSSEC
is defined in [RFC4033], [RFC4034] and [RFC4035].

As described in the introduction of [RFC6698], TLS authentication via
the existing public Certificate Authority (CA) Public Key
Infrastructure (PKI) suffers from an over-abundance of trusted
certificate authorities capable of issuing certificates for any
domain of their choice. DNS-Based Authentication of Named Entities
(DANE) leverages the DNSSEC infrastructure to publish trusted keys
and certificates for use with TLS via a new TLSA record type. With
DANE, the public CA PKI can be augmented or replaced by DNSSEC
validated TLSA records.

The Transport Layer Security (TLS [RFC5246]) protocol enables secure
TCP communication. In the context of this memo, channel security is
assumed to be provided by TLS. Used without authentication, TLS
protects only against eavesdropping. With authentication, TLS also
protects against man-in-the-middle (MITM) attacks.

1.2. SMTP Channel Security

The Simple Mail Transport Protocol (SMTP) ([RFC5321]) is multi-hop
store & forward, while TLS security is hop-by-hop. The number of
hops from the sender's Mail User Agent to the recipient mailbox is
rarely less than 2 and is often higher. Some hops may be TLS
protected, some may not. The same SMTP TCP endpoint can serve both
TLS and non-TLS clients, with TLS negotiated via the SMTP STARTTLS
command ([RFC3207]). DNS MX records abstract the next hop next-hop transport
end-point. SMTP addresses are not transport addresses and are
security agnostic. Unlike HTTP, there is no URI scheme for email
addresses to designate whether the SMTP server should be contacted
with or without security.

A Mail Transport Agent (MTA) may need to forward a message to a
particular email recipient <user@example.com>. To deliver the
message, the MTA needs to retrieve the MX hosts of example.com from
DNS, and then deliver the message to one of them. Absent DNSSEC, the
MX lookup is vulnerable to man-in-the-middle and cache poisoning
attacks. As a result, secure verification of MX host certificates is
not possible without DNSSEC, as an An active attacker can forge DNS replies with fake MX
records, and can direct traffic to a server of
their his choice.
Therefore, secure verification of MX host certificates is not
possible without DNSSEC. A man-in-the-middle man in the middle can also suppress the
MX host's STARTTLS EHLO response, convincing the client that
communication over TLS is unavailable.

One might try to harden STARTTLS with SMTP against DNS attacks by
requiring each MX host to posess an X.509 certificate for the
recipient domain that is obtained from the message envelope and is
not subject to DNS reply forgery. Unfortunately, this is
impractical, as email for many domains is handled by third parties,
which are not in a position to obtain certificates for all the
domains they serve. Deployment of SNI (see [RFC6066] Section 3.1) is
no panacea, since the SNI key management is operationally challenging at
large scale unless
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
authentication identity, nor and neither can the MX hostname without
DNSSEC, large scale deployment of authenticated TLS for SMTP requires
secure DNS. At this time, DNSSEC is not yet widely deployed and MTA
to MTA traffic between Internet connected organizations rarely uses
TLS at all, or simply uses TLS opportunistically without
authentication and protects against only passive eavesdropping
attacks.

The only exceptions are cases in which the sending MTA is statically
configured to use TLS for mail sent to specific selected peer domains
and is configured with appropriate subject names (or content digests)
to expect in the presented MX host certificates of those domains.
Such statically configured SMTP secure channels are also used rarely, and
only
generally between domains that make bilateral arrangements with their
business partners. Internet email, on the other hand, requires
contacting many new domains for which security configurations can not
be established in advance.

Note, the above does not apply to mail submission [RFC6409], where a
mail user agent is pre-configured to send all email to a fixed Mail
Submission Agent (MSA). Submission servers usually offer TLS and the
Mail User Agent (MUA) can be statically configured to require TLS
with its chosen MSA. The situation changes when submission servers
are configured dynamically via SRV records (see [RFC6186] Section 6,
although this is not yet widely deployed). 6).
Applications to submission via SRV records will be discussed later in
this memo.

With little opportunity to use TLS authentication, MX hosts that
support STARTTLS often use self-signed or private-CA private CA issued X.509
certificates. Sending systems are rarely configured with a
comprehensive list of trusted CAs and do not check CRLs or implement
OCSP. In essence, they don't and can't reply rely on the existing public
CA PKI. This is not simply a result of complacency on the part SMTP server
administrators and MTA developers. Nor is it just a result consequence of
the relative maturity of the SMTP infrastructure when at the time that TLS
was introduced. Rather, the abstraction of the SMTP transport
endpoint via DNS MX records, often across organization boundaries,
limits the use of public CA PKI with SMTP to a small set of sender-configured sender-
configured peer domains.

This does not mean, however, that the Internet email backbone cannot
benefit from TLS. The fact that transport security is not explicitly
specified in either the recipient address or the MX record means that
new protocols can furnish out-of-band information to SMTP, making it
possible to simultaneously discover both which peer domains support
secure delivery via TLS and how to verify the authenticity of the
associated MX hosts. The first such mechanism that can work an
Internet scale is DANE TLSA, but use of DANE TLSA with MTA to MTA
SMTP must be cognizant of the lack of any realistic role for the
existing public CA PKI.

1.3. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

2. Hardening Opportunistic TLS

This section memo describes opportunistic SMTP over TLS security, where
traffic from DANE TLSA aware SMTP clients to domains that implement
DANE TLSA records in accordance with this memo is secure. Traffic to
other domains continues to be sent in the same manner as before
(either manually configured for security or unencrypted unauthenticated and
unauthenticated). often
unencrypted). It is hoped that, over time, more domains will
implement DNSSEC and publish DANE TLSA records for their MX hosts.
This will enable an incremental transition of the email backbone to
authenticated TLS delivery.

Since email addresses and MX hostnames (or submission SRV records)
neither signal nor deny support for TLS by the receiving domain, it
is possible to use DANE TLSA records to securely signal TLS support
and simultaneously to provide the means by which SMTP clients can
successfully authenticate legitimate SMTP servers.

2.1. TLS discovery

As noted previously (Section 1.2), opportunistic TLS with SMTP
servers that advertise TLS support via STARTTLS is subject to a man
in the middle downgrade attack. Some SMTP servers erroneously
advertise STARTTLS in default configurations that are not not, in fact fact,
TLS capable, and clients need to be prepared to retry plaintext
delivery after STARTTLS fails. A This memo specifies a downgrade
resistant mechanism for that allows a server to advertise TLS support
based on DANE TLSA records is specified
below. records. DNSSEC validated TLSA records are
unlikely to be accidentally published for servers that do not in fact
support TLS, and thus clients can safely interpret their presence as
a commitment by the server operator to implement STARTTLS.

SMTP is a store & forward protocol. An MTA that is not the final
destination for a message recipient forwards the message one hop
closer to the recipient's mailbox. To do so, it must determine the
appropriate next-hop destination. destination, and locate one or more associated
SMTP servers. When DNSSEC validated TLSA records are available for a
given next-hop SMTP server, the TLS connection to that server will be
downgrade resistant. If the records in question are "usable"
([RFC6698], Section 4.1) to authenticate the server, the connection
will also be authenticated and thus immune to eavesdropping or
tampering (unless DNSSEC itself is compromised).

Typically, the next-hop destination defaults to will be the domain part of the
recipient address, which is then subject to MX resolution. The
next-hop next-
hop destination may also be configured by the MTA administrator to be
a next-hop destination host (explicitly exempt from MX resolution),
or a next-hop destination domain (subject to MX resolution) which
takes the place of the domain part of the recipient address. In the language of [RFC5321] Section 5.1, we'll refer to

The protocol in this memo is "opportunistic". Absent "secure"
(DNSSEC validated) TLSA records, mail delivery should generally fall
back to pre-DANE opportunistic TLS. The SMTP client may be
configured to require DANE verified delivery for some or all
destinations, in which case absent "secure" TLSA records delivery
will be deferred.

Below we explain how to determine for a given next-hop destination host or
the associated SMTP servers, the TLSA base domain as "the initial name". and TLSA records.

2.1.1. MX resolution

If Non-MX destinations

As mentioned above, the initial name is a next-hop destination domain subject to may in some cases
be exempt from MX resolution, lookups. In addition, MX lookups for the next-hop
domain may yield no results. In either case, the destination server
for such a
DNSSEC validated "MX" lookup domain is performed, to obtain determined by looking up the list of
associated MX hosts. If no MX corresponding A or
AAAA records.

When "bogus" records are found, encountered either during CNAME expansion,
or when retrieving the associated TLSA RRset, the SMTP client MUST
proceed as if the initial
name is a next-hop host not subject to MX resolution, it is resolved
to one domain were unreachable. Delivery should
either be deferred or more network addresses, may be attempted via any fallback next-hop
(which may also employ opportunistic DANE TLS) configured by performing DNSSEC validated "A"
and/or "AAAA" lookups. the SMTP
client administrator. Proceeding with the original next-hop despite
"bogus" DNS responses would destroy protection against downgrade
attacks.

Following [RFC5321] Section 5.1, if the "A", "AAAA" A or "MX" AAAA lookup of the
initial name yields a CNAME, we replace it with the resulting name as
if it were the initial name and try the same perform a lookup again
with using the new
name. This replacement is performed recursively, although MTAs
typically support only limited recursion in CNAME
expansion so this replacement is performed recursively. If
initially, or at any stage of recursion, the response is "bogus", MX
resolution fails with a temporary error. Mail delivery SHOULD either
be deferred or attempted via any alternative delivery channel
configured by expansion. We
consider the MTA administrator (which may also employ
opportunistic DANE TLS).

With a following cases:

Non-CNAME: The next-hop destination domain subject to MX resolution which has
MX records, if at least one is not a CNAME alias.
The lookup in key for the (possibly empty) chain of
CNAMEs leading DNSSEC validated TLSA records is obtained
by prepending service labels ("_<port-number>._tcp") to the MX RRset
initial next-hop destination domain. If associated "secure" TLSA
records are found (see Section 2.1.3) the TLSA base domain is "insecure", the
next-hop domain. If no secure TLSA records are found,
opportunistic DANE TLS is not applicable, applicable and mail delivery may proceed
proceeds with pre-DANE opportunistic TLS (subject to its various MITM attacks).

With a TLS.

Insecure CNAME: The next-hop destination host not subject to MX resolution or a domain with no MX records, if is a CNAME alias,
but at least one lookup in the (possibly
empty) chain of CNAMEs the CNAME RRs leading to the A or AAAA RRset is "insecure",
the TLSA base domain ultimate target
of this alias (during recursive CNAME expansion) is "insecure".
We treat this case just like the initial non-CNAME case above.

Secure CNAME, no TLSA: The next-hop name, and opportunistic
DANE TLS destination domain is applicable only when a "secure" TLSA RRset is found at
that base domain.

Otherwise, if at each CNAME
alias, and every stage all the CNAME RRs leading to the ultimate target of
this alias (during recursive CNAME expansion expansion) are "secure" (DNSSEC
validated), but no "secure" TLSA RRs are found after prefixing the DNS
response
service labels to the CNAME-expanded next-hop domain. This case
is "secure", and either also treated just like the initial name non-CNAME case.

Secure CNAME, TLSA: The next-hop destination domain is a next-hop host
name not subject CNAME
alias, all the CNAME RRs leading to MX resolution or no MX records the ultimate target of this
alias (during recursive CNAME expansion) are found, "secure", and in
addition "secure" TLSA RRs are found after prefixing the
resulting final name becomes service
labels to the CNAME-expanded next-hop destination and domain. In this case the
CNAME-expanded next-hop domain is taken as the TLSA base domain.
The original next-hop domain for TLSA record lookup. is (see Section 2.2.2) used only as
an alternative name in certificate peername verification if
applicable.

In summary, if it is possible to securely obtain the full, CNAME-
expanded, DNSSEC-validated address records for the non-MX next-hop
domain then that name is the preferred TLSA base domain. If that is
not possible, then the original next-hop domain is used as the TLSA
base domain. When no "secure" TLSA records are found at either the fully
CNAME expanded name that is "secure" or else is original next-hop domain, then opportunistic DANE
TLS does not apply for mail delivery to the initial
name.

Finally, if at each non-MX destination in
question.

2.1.2. MX resolution

In this section we consider next-hop domains that are subject to MX
resolution and every stage have MX records. When DANE TLS is applicable, the DNS response
TLSA base domain will be associated with the MX host selected for
message delivery. Therefore, the MX host names must be determined
securely by performing a DNSSEC validated MX lookup to obtain the
list of associated MX hosts. If the MX RRset is "secure", "insecure", DANE
TLSA does not apply and mail delivery proceeds with pre-DANE
opportunistic TLS (subject to its various MITM attacks and one unecrypted
transmission when STARTTLS is not supported by the destination).

When "bogus" DNSSEC records are encountered during CNAME expansion of
the next-hop domain or more when processing the actual MX RRset, delivery
MUST either be deferred, or MAY be attempted via any fallback next-
hop (which may also employ opportunistic DANE TLS) configured by the
SMTP client administrator. Proceeding with the original next-hop
despite "bogus" DNS responses would destroy protection against
downgrade attacks. When "bogus" DNSSEC records are found, the encountered with
CNAME expansion or TLSA RRset lookup for a particular MX host,
delivery MUST proceed as if MX host in question were unreachable.

MX records MUST be sorted by preference. A better (numerically lower) preference; an MX preference for a host
that does not support TLS with a better
preference and no TLSA records MUST NOT be preempted by a host with a
worse
(numerically higher) MX preference for a host that does support TLS. but with TLSA records. In other words, avoiding
delivery loops trumps any preference for
channel security. by following MX preferences must take place even if it
means insecure delivery.

In each delivery attempt via a candidate accordance with Section 5.1 of [RFC5321], if the MX host, lookup of the
initial name yields a CNAME, we replace the initial name with the
resulting name and perform a new lookup with the new name. MTAs
typically support recursion in CNAME expansion, so this replacement
is performed repeatedly until the ultimate non-CNAME domain is found
(or the limit on the number of CNAMEs to examine is reached). If at
any stage of CNAME expansion the DNS result is "bogus", MX host resolution
fails with a temporary error. In that case, mail delivery MUST
either be treated as though it were deferred, or attempted via any alternative delivery channel
configured by the initial MTA administrator. We consider the following
cases:

Non-CNAME: The next-hop destination host (which is, of course, domain is not subject a CNAME alias,
that is, it resolves directly to further a set of DNSSEC validated
("secure") MX
resolution). The associated TLSA base domain hosts. With each MX host, if MX host CNAME
expansion is equal to supported by the MTA, and the full CNAME
resolved expansion of
the MX exchange host name if is "secure", then the CNAME expansion of expanded MX exchange names host name
is
supported and all CNAMEs encountered the TLSA base domain provided secure TLSA records are "secure". found
there after prefixing service labels ("_<port-number>._tcp").
Otherwise, the
unexpanded initial MX host name of is the TLSA base domain
provided secure TLSA records are found there after prefixing
service labels. With the MX exchange hostname (or its CNAME expansion) as
the TLSA base domain, the original next-hop domain SHOULD be used
only in certificate name checks. If no "secure" TLSA RRs are
found, and no "bogus" records encountered, DANE TLSA is not
applicable with the MX host in question and delivery proceeds as
with pre-DANE opportunistic TLS.

CNAME: The next-hop destination domain is a CNAME alias, and
resolves via a chain of "secure" CNAME records to a final domain
with "secure" MX records. The TLSA base domain. domain for each MX host
in this case is the same as in the "Non-CNAME" case above, but now
both the initial domain and its CNAME-expansion are candidate
names in certificate name checks. If the CNAME chain contains
"insecure" elements, DANE TLSA does not apply to the next-hop
domain, and delivery proceeds via pre-DANE opportunistic TLS.

Note: CNAMEs are not legal in the exchange field of MX records, thus
MTAs
MAY skip over MX records in which the are not obligated to perform MX exchange CNAME expansion. If an
MTA does not perform CNAME expansion, there is a CNAME. There
is some additional potential risk, in this case, that
the MTA may fail to notice that it is one of the MX hosts for the
destination and that it must skip MX records with equal or worse
(numerically higher precedence). If an MTA does allow CNAMEs to be
used in MX records records, it SHOULD process them recursively as described
above to determine
whether opportunistic DANE TLS is applicable and if so the associated most appropriate TLSA RRset base domain.

2.1.2.

2.1.3. TLSA record lookup

When all the DNSSEC lookups, "CNAME", "MX", "A" or "AAAA", used to
obtain a given

Each TLSA base domain (one obtained above (for a non-MX destination, or
for each candidate a particular MX host if
multiple DNSSEC validated of an MX hosts were found) are "secure", and the
SMTP client is configured for opportunistic DANE TLS, it SHOULD
locate the TLSA RRset corresponding destination), when prefixed with
appropriate service labels leads to this base domain. associated "secure" TLSA RRs
(possibly via a chain of "secure" CNAME RRs). If, for example, the
base domain is "mail.example.com", the TLSA RRset is obtained via a
DNSSEC query of the form:

_25._tcp.mail.example.com. IN TLSA ?

Typically, the destination TCP port is 25, but this may be different
with custom routes specified by the MTA administrator or when an MUA
connects to a submission server on port 587. The SMTP client MUST
use the appropriate "_<port>" "_<port-number>" prefix in place of "_25" when
the port number is not equal to 25. The query response may be a
CNAME (or a DNAME + CNAME combination), or the TLSA RRset. DNAME processing with
DNSSEC can be done using standard DNAME resolution techniques and
will not be discussed in detail here. The SMTP client MUST check the
security status of the response.

If the response is "bogus", delivery via the host in question SHOULD
NOT proceed, otherwise the SMTP client is vulnerable to man in the
middle STARTTLS downgrade attacks. If the response is "insecure",
opportunistic DANE TLS is not applicable for the host in question,
and the SMTP client SHOULD proceed with ordinary opportunistic TLS. If the response is "secure" and the
record is a CNAME or DNAME, the SMTP client restarts the TLSA query
at the target domain, following CNAMEs as appropriate (such appropriate.

CNAMEs encountered during TLSA record lookups can be used to share a
single TLSA RRset specifying a common certificate authority or a
common leaf certificate for multiple TLS services. Such CNAME
expansion does not change the SMTP client's notion of the TLSA base domain).
domain, thus when _25._tcp.mail.example.com is a CNAME the base
domain remains mail.example.com and is still used in peer certificate
name checks. For example:

example.com. IN MX 0 mail.example.com.
example.com. IN MX 0 mail2.example.com.
_25._tcp.mail.example.com. IN CNAME 2.1.1._dane.example.com.
_25._tcp.mail2.example.com. IN CNAME 2.1.1._dane.example.com.
2.1.1._dane.example.com. IN TLSA 2 1 1 e3b0c44298fc1c14
9afbf4c8996fb924
27ae41e4649b934c
a495991b7852b855

Here, mail.example.com and mail2.example.com have certificates issued
under a common trust-anchor, but each MX host's TLSA base domain
remains its hostname and MUST match the subject name (or subject
alternative name) in its certificate.

If, after possible CNAME indirection, the response is "secure" and at least one "secure" TLSA
record is found (even if not usable because it is unsupported by the
implementation or administratively disabled) the next-hop host has
committed to TLS support. The SMTP client SHOULD NOT deliver mail
via such a next-hop host unless a TLS session is negotiated via
STARTTLS. This avoids man in the middle STARTTLS downgrade attacks.

When

As noted previously (Section 2.1.1, Section 2.1.2), when no TLSA
records are found at a CNAME-expanded initial name
(insecure (due to an insecure
response or no records), a lack of TLSA records verified by DNSSEC's proof-of-non-
existence), the unexpanded initial name MUST be tried instead. This supports
clients of hosting providers where the provider provider's zone is not DNSSEC
validated, but the client has shared appropriate key material with
the hosting provider to enable TLS via SNI.

When usable TLSA records are available, a client SHOULD NOT deliver
mail via a server that fails to match at least one TLSA record. This
is not a "must" because

SMTP clients may incrementally deploy opportunistic DANE TLS incrementally by
enabling it only for selected peer domains. At times,
clients 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. For Unless
local policy specifies that opportunistic DANE TLS to be robust (resistant to
failures), servers MUST live up to the promises stated by the
existence of the TLSA record, but it is not always possible to compel
clients to use be used
for a security policy chosen by the server. Given particular destination, client MUST NOT deliver mail via a
robust security protocol, clients will hopefully, over time,
willingly choose
server whose certificate chain fails to adopt it. match at least one TLSA
record when usable TLSA records are available.

SMTP clients employing opportunistic DANE TLS and TLSA record
publishers for SMTP servers need to follow the guidance outlined in
[I-D.dukhovni-dane-ops]'s
[I-D.ietf-dane-ops]'s "Certificate Name Check Conventions", "Service
Provider and TLSA Publisher Synchronization" and "TLSA Base Domain
and CNAMEs" sections.

2.2. DANE authentication
2.2.1. TLSA certificate usages

As noted in the introduction, the existing public CA PKI is not
viable for the Internet email backbone. TLSA records for MX hosts or
submission servers that are to be found via SRV records SHOULD NOT
include certificate usage "0" or "1", as in both cases SMTP clients
cannot be expected to perform [RFC5280] PKIX validation or [RFC6125]
identity verification. Clients MAY treat such TLSA records as
unusable.

SMTP clients may also to the extent possible map these usages to the
corresponding non-PKIX certificate usages (0 to 2 and 1 to 3).
Servers publishing these certificate usages hoping to be protected by
both the public CA PKI and by DNSSEC will typically be protected by
neither.

TLSA Publishers should follow the TLSA publication size guidance
found in [I-D.dukhovni-dane-ops] [I-D.ietf-dane-ops] about "DANE DNS Record Size Guidelines".

2.2.1.1. Certificate usage 3

Since opportunistic DANE TLS will be used by non-interactive MTAs,
with no user to "press OK" when authentication fails, reliability of
peer authentication is paramount. TLSA records published for SMTP
servers SHOULD be "3 1 1" records to support opportunistic SMTP over
TLS with DANE. This record specifies the SHA-256 digest of the
server's public key. Since all DANE implementations are required to
support SHA-256, this record works for all clients and need not
change across certificate renewals with the same key.

Authentication via certificate usage "3" TLSA records involves simply
checking that the server's leaf certificate matches the TLSA record.
Other than extracting the relevant certificate elements for
comparison, no other use is made of the certificate content.
Authentication via certificate usage "3" TLSA records involves no
certificate authority signature checks. It also involves no server
name checks, and thus does not impose any new requirements on the
names contained in the server certificate (SNI is not required when
the TLSA record matches the public key of the server's default certificate). It uses the SHA-256 digest which all clients are
obligated to support, and works across certificate renewals with the
same key.

Two TLSA records will need to be published before updating a server's
public key, 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 time out the previous TLSA RRset,
which contains only the old key, the server may be reconfigured to
use the new private key and associated public key certificate. The
amount of time a server should wait before using a new key that is
referenced by new TLSA records should be at least twice the TTL of
the previously published TLSA records. Once the server is using a
new key, the obsolete TLSA RR can be removed from DNS, leaving only
the RR that matches the new key.

2.2.1.2. Certificate usage 2

Some domains may prefer to reduce the operational complexity of
publishing unique TLSA RRs for each TLS service. If the domain
employs a common issuing certificate authority to create certificates
for multiple TLS services, it may be simpler to publish the issuing
authority's public key as a trust-anchor for the certificate chains
of all relevant services. The TLSA RRs for each service issued by
the same TA may then be CNAMEs to a common TLSA RRset that matches
the TA. In this case, the certificate chain presented in the TLS
handshake of each service SHOULD include the TA certificate, as SMTP
clients cannot generally be expected to have domain-issued trust-
anchor certificates in their trusted certificate store. TLSA
Publishers should publish either "2 1 1" or "2 0 1" TLSA parameters,
which specify the SHA-256 digest of the trust-anchor public key or
certificate respectively. As with regular leaf certificate rollover
discussed in Section 2.2.1.1, two such TLSA RRs need to be published
to facilitate TA certificate rollover.

The usability of "2 1 1" or "2 0 1" TLSA RRs with SMTP is not
assured. If server operators employing these RRs universally ensure
that the corresponding TA certificate is included in the SMTP
server's TLS handshake trust certificate chain, clients can safely enable
support for these RRs. If sufficiently many server administrators are
negligent in deploying these RRs,
negligently omit the TA certificate from the server's TLS certificate
chain, SMTP clients will be hesitant to support them, usage "2" TLSA RRs,
since mail delivery will not work to many destination domains if they
do. Server operators are encouraged to implement these RRs, if they
are operationally a better fit for their organization, provided they
do so with care. It is critical to never not forget to include trust-anchor trust-
anchor certificates in server trust chains. SMTP client
implementations SHOULD support these TLSA RRs, unless unless, despite the
above warning, a non-trivial fraction of server operators fail to
publish certificate chains that include the required TA certificate.

2.2.1.3. Certificate usage usages 0 and 1

SMTP servers SHOULD NOT publish TLSA RRs with certificate usage "0"
or "1".
Clients MAY treat such TLSA records as unusable. Alternatively, SMTP
clients that implement this specification MAY ignore the PKIX
validation requirement when they encounter certificate usage "1", and
authenticate the server per the content of the TLSA record alone.
That is, SMTP clients may treat certificate usage "1" as certificate
usage "3".

2.2.1.4. Certificate usage 0

SMTP servers SHOULD NOT publish TLSA RRs with certificate usage "0".
Clients MAY treat such TLSA records as unusable. Alternatively,
since PKIX validation is not possible with opportunistic DANE TLS, SMTP clients MAY treat certificate usage "0" RRs as though they were
certificate usage "2" RRs. But, cannot be expected to be configured with certificate usage "0" the
usability of the TLSA record depends more strongly on its matching
type.

If the matching type is "0" (the server should also avoid this
matching type and should publish usage "3" or "2" public key or
certificate digests), the TLSA record contains the full certificate
or full public key a
suitably complete set of the trusted certificate authority. In this
case the client has all the information it needs to match the server
trust-chain to the TLSA record. The client SHOULD ignore the PKIX
validation requirement, and verify the server's trust chain via its
DANE TLSA records only (name checks still apply as public CAs. Even with usage "2").

If the matching type is not "0", the TLSA record contains only a
digest full set of the trust certificate authority certificate or
public key.
The server operator publishing usage 0 TLSA records may expect that CAs, SMTP clients already have the issuing authority certificate cannot (without relying on hand, and
may omit it from the server's certificate chain. As a result, the
client may not be able to match the DNSSEC for secure
MX records) perform [RFC6125] server trust chain against the
TLSA record if it, in fact, does not have a copy identity verification.

SMTP client treatment of the certificate
authority TLSA RRs with certificate usages "0" or public key.

SMTP "1"
is undefined. For example, clients that implement this specification SHOULD MAY (will likely) treat such TLSA
records with certificate usage "0" and a digest matching type as
unusable, but MAY be explicitly configured to support them when it is
believed that clients posses a sufficiently complete set of trusted
public CA certificates. This is most plausible with an MUA which
only needs enough CA certificates to authenticate its preferred
submission service. unusable.

2.2.2. Certificate matching

When at least one usable "secure" TLSA record is found, the SMTP
client SHOULD use TLSA records to authenticate the next-hop host,
mail SHOULD not be delivered via this next-hop host if authentication
fails, otherwise the SMTP client is vulnerable to TLS man in the
middle attacks.

To match a server via a TLSA record with certificate usage "2", the
client MUST perform name checks to ensure that it has reached the
correct server. The In all cases the SMTP client MUST accept the TLSA
base domain as a valid DNS name in the server certificate. Clients should also
accept securely looked up

MX: If the TLSA base domain was obtained indirectly via an MX lookup, or a lookup
(it is the name of an MX exchange that may be securely CNAME resolved expansion.
expanded), then the initial query name used in the MX lookup
SHOULD be accepted in the peer certificate. The CNAME-expanded
initial query name SHOULD also be accepted if different from the
initial query name.

Non-MX: If no MX records were found and the TLSA base domain is the
CNAME-expanded initial query name, then the initial query name
SHOULD also be accepted.

Accepting certificates with the next-hop domain in addition to the
next-hop MX host allows a domain with multiple MX hosts to field a
single certificate bearing the email domain name across all the MX
hosts, this is also compatible with pre-DANE SMTP clients that are
configured to look for the email domain name in server certificates.

The SMTP client MUST NOT perform certificate usage name checks with
certificate usage "3", since with usage "3" the server is
authenticated directly by matching the TLSA RRset to its certificate
or public key without resort to any issuing authority. The
certificate content is ignored except in so far as it is used to
match the certificate or public key digest (ASN.1 object or its digest) with
the TLSA RRset.

To ensure that the server sends the right certificate chain, the SMTP
client MUST send the TLS SNI extension containing the TLSA base
domain. Since DANE-aware clients are obligated to send SNI
information, which requires at least TLS 1.0, SMTP servers for which
DANE TLSA records are published MUST support TLS 1.0 or later with
any client authorized to use the service.

Each SMTP server MUST present a certificate trust chain (see [RFC2246]
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 RRset includes records with a matching type
indication
indicating a digest record (i.e., a value other than "0"), the
SHA-256 digest of any object SHOULD be provided along with any other
digest published, since clients may support only SHA-256. Unless
SHA-256 proves vulnerable to a "second preimage" attack, it should be
the only digest algorithm used in TLSA records.

If the server's TLSA records match the server's default certificate
chain, the server need not support SNI. The server need not include
the extension in its TLS HELLO, simply returning a matching
certificate chain is sufficient. Servers MUST NOT enforce the use of
SNI by clients, if the client sends no SNI extension, or sends an SNI
extension for an unsupported domain the server MUST simply use its
default certificate chain. The client may be using unauthenticated
opportunistic TLS and may not expect any particular certificate from
the server.

The client may even offer to use anonymous TLS ciphersuites and
servers SHOULD support these, no these. No security is gained by forcing the
use of sending a
certificate the client will is willing to ignore. Indeed support for
anonymous ciphersuites in the server makes audit trails more useful
if
when the chosen ciphersuite is logged, as this will in many cases
record which clients did not care to authenticate the server. (The
Postfix SMTP server supports anonymous TLS ciphersuites by default,
and the Postfix SMTP client offers these at its highest preference
when server authentication is not applicable).

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,
therefore authentication via this protocol is expected to be less
prone to connection failure caused by incompatible configuration of
the client and server.

3. Opportunistic TLS for Submission

Prior to [RFC6409], the SMTP submission protocol was a poster child poster-child
for PKIX TLS. The MUA typically connects to one or more submission
servers explicitly configured by the user. There is no indirection
via insecure MX records, and unlike web browsers, there is no need to
authenticate a large set of TLS servers. Once TLS is enabled for the
desired submission server or servers, provided the server certificate
is correctly maintained, the MUA is able to reliably use TLS to
authenticate the submission server.

[RFC6186] aims to simplify the configuration of the MUA submission
service by dynamically deriving the submission service from the
user's email address. This is done via SRV records, but at the cost
of introducing the same TLS security problems faced by MTA to MTA
SMTP. Prompting the user when the SRV record domain is different
from the email domain is not a robust solution.

The protocol defined in this memo can also be used to
opportunistically secure the
submission service association. discovery. If the email domain is DNSSEC signed,
the SRV records are "secure" and the SRV host publishes secure TLSA
records for submission, then the MUA can safely auto-configure to
authenticate the submission server via DANE. When DANE TLSA records
are not available, the client SHOULD fall back to legacy behavior. behavior
(this may involve prompting the user to accept the resulting server
and perhaps "pin" its certificate).

Specifically, MUAs that dynamically determine the submission server
via SRV records SHOULD support DNSSEC and DANE TLSA records. They
SHOULD use TLSA records to authenticate the server. The processing
of usage 2 and 3 TLSA associations by an MUA is the same as by an MTA
with SRV records replaced by corresponding MX records.

Just as with MX service on port 25, SMTP submission servers SHOULD
NOT publish usage 0 or 1 TLSA associations, and MUAs that support
DANE TLSA are not expected to trust a full list of public CAs.
Server certificate subjectAltNames should include at least the server
name. When the server administrator is also authorized able to obtain certificates a certificate
for the email domain, the server certificate should also include the
email domain name. MUAs that are not able to support DNSSEC may then
be able to authenticate the server domain. If it is practical to
field additional certificates for hosted domains, SNI may be used by
the server to select the appropriate domain's certificate.

4. Mandatory TLS Security

An MTA implementing this protocol may require a stronger security
assurance when sending email to selected destinations to which the
sending organization sends sensitive email and may have regulatory
obligations to protect its content. This protocol is not in conflict
with such a requirement, and in fact it can often simplify
authenticated delivery to such destinations.

Specifically, with domains that publish DANE TLSA records for their
MX hosts a sending MTA can be configured to use the receiving
domains's DANE TLSA records to authenticate the corresponding MX
hosts, thereby obviating the complex manual provisioning process. In
anticipation of, or in response to, a failure to obtain the expected
TLSA records, the sending system's administrator may choose from a
selection of fallback options, if supported by the sending MTA:

o Defer mail if no usable TLSA records are found. This is useful
when the destination is known to publish TLSA records, and lack of
TLSA records is most likely a transient misconfiguration.

o Authenticate the peer via a manually configured certificate
digest. This may be obtained, for example, after a problem is
detected and confirmed to be valid by some out-of-band mechanism.

o Authenticate the peer via the existing public CA PKI, if the peer
server has usable CA issued certificates. In many cases the
sending MTA will need custom certificate name matching rules to
match the destination's gateways. And the sending server must
explicitly configure policy for the destination to always require
TLS to prevent MITM attacks.

o Send via unauthenticated mandatory TLS. This is useful if the
requirement is merely to always encrypt transmissions to protect
against only eavesdropping, and the possibility of MITM attacks is
less of a concern than timely email delivery.

It should be noted that barring administrator intervention, email
SHOULD be deferred when DNSSEC lookups fail, (as distinct from
"secure" non-existence of TLSA records, or secure evidence that the
domain is no longer signed). In addition to configuring fallback
strategies when TLSA records are unexpectedly absent, administrators
may, in hopefully rare cases, need to disable DNSSEC lookups for a
destination to work around a DNSSEC outage.

5. 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.

Acknowledgments from Viktor: Thanks to Tony Finch who finally prodded
me into participating in DANE working group discussions. Thanks to
Paul Hoffman who motivated me to produce this memo and provided
feedback on early drafts. Thanks also to Wietse Venema who created
Postfix, and patiently guided the Postfix DANE implementation to
production quality.

6. Security Considerations

This protocol leverages DANE TLSA records to implement MITM resistant
opportunistic channel security for SMTP. For destination domains
that sign their MX records and publish signed TLSA records for their
MX hosts, this protocol allows sending MTAs (and perhaps dynamically
configured MUAs) 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 and 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 bypass any security benefit the legitimate domain owner might
hope to gain by publishing usage 0 or 1 TLSA RRs. Given the lack of
public CA PKI support in existing MTA deployments, deprecating
certificate usages 0 and 1 in this specifications improves
interoperability without degrading security.

7. Normative References

[I-D.dukhovni-dane-ops]

[I-D.ietf-dane-ops]
Dukhovni, V., V. and W. Hardaker, "DANE TLSA implementation
and operational guidance", draft-dukhovni-dane-ops-00 draft-ietf-dane-ops-00 (work in
progress),
May October 2013.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.

[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.

[RFC3546] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 3546, June 2003.

[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.

[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.

[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.

[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.

[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, May 2008.

[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.

[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.

[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, March 2011.

[RFC6186] Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186, March 2011.

[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, November 2011.

[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.

Authors' Addresses

Viktor Dukhovni
Unaffiliated

Email: ietf-dane@dukhovni.org

Wes Hardaker
Parsons
P.O. Box 382
Davis, CA 95617
US

Email: ietf@hardakers.net