Internet DRAFT - draft-friedl-uta-smtp-mta-certs
draft-friedl-uta-smtp-mta-certs
Using TLS in Applications S. Friedl
Internet-Draft Cisco Systems, Inc.
Intended status: Standards Track T. Kaupe
Expires: September 14, 2014 Microsoft Corp.
S. Gorti
Cisco Systems, Inc.
March 13, 2014
TLS Certificate Identity Verification Procedure for SMTP MTA to MTA
Connections
draft-friedl-uta-smtp-mta-certs-00
Abstract
This document describes TLS server identity verification procedure
for Message Transfer Agent (MTA) to Message Transfer Agent
connections in an SMTP email network, with specific guidance on
identity verification steps associated with delegated email services.
This document is intended to supplement the identify verification
procedures described in [RFC6125].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 14, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Procedure for TLS Server Identity Verification for MTA to
Email Server Connections . . . . . . . . . . . . . . . . . . 4
4.1. Server Identification and Validation . . . . . . . . . . 4
4.1.1. Reference Identity . . . . . . . . . . . . . . . . . 4
4.1.2. Presented Identity . . . . . . . . . . . . . . . . . 5
4.1.3. Wildcards . . . . . . . . . . . . . . . . . . . . . . 5
4.1.4. Server Identity Validation . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
8. Normative References . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
The TLS security protocol [RFC5246] is typically used to encapsulate
application protocols to provide privacy, data integrity and server
identity verification between two application servers which would
otherwise be conducting their communication in plain text over
potentially insecure network links. There are two layers:
o The TLS Record protocol provides for private communication and
message integrity using negotiated encryption and hashing
algorithms.
o The TLS Handshake protocol provides for secure and reliable
sharing of secrets used for encryption, together with
identification and authentication of one or both peers.
There are various RFCs, such as [RFC6125] which describe TLS in the
context of more general domain based applications, with the canonical
example being a user-agent (e.g. browser) securely accessing content
on a server (e.g. web server) [RFC2818]. RFCs also describe TLS in
the more specific context of SMTP, however these RFCs focus on a
user-agent (e.g. mobile or PC mail client) submitting email to a mail
server using the STARTTLS SMTP extension[RFC2595] [RFC3207]. No RFCs
explicitly describe TLS in the context of SMTP communication between
two MTAs in an email network. There exist sufficient differences in
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the MTA-to-MTA scenario, particularly with respect to server
identification, which warrant an explicit set of recommendations.
This document discusses various strategies that a sending MTA can use
for the identification and authentication of a destination MTA when
transferring a message within an email network. It should be noted
that an email network could be defined with MTA to Message Delivery
Agent (MDA) connections, in which case the same verification and
authentication rules should apply to the MTA to MDA scenario.
2. Requirements Language
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].
3. Terminology
o Email Domain: @example.net, found in the [RFC5321] domain section
of the RCPT TO command forward path
o Email Server Hostname: mail.example.net
o Email Server IP Address: the IP address (or addresses) for the
email server.
o MX Record: A type of DNS record that associates an email domain to
one or more email server hostnames. @example.net -->
mail1.example.net, mail2.example.net.
o MTA (Message Transfer Agent). An application that sends messages
on behalf of one or more users to a destination email server over
SMTP. The destination email server will also be an MTA or a
Message Delivery Agent (MDA) though this document will discuss MTA
to MTA connections as the difference between MTA to MTA and MTA to
MDA connections is immaterial with regard to TLS verification and
authentication.
o Self-Hosting: The owner of the email domain also owns and manages
the email server. In this case, it's common for the email server
host name to be rooted to the email domain (e.g. @example.net -->
mail.example.net), but it's not required. Many organizations have
multiple email domains (e.g. @example.net, @sales.example.net,
@ejemplo.es) all self-hosted on a shared email server (e.g.
mail.example.net).
o Delegated Hosting: The owner of the email domain does not own or
manage the email server. Typically, a third-party owns and
manages an email server for a multiplicity of Email Domain owners.
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Delegation is achieved through the MX record (e.g. @example.net
--> mail.delegatedexample.net).
o Reference Identity: An instance of a reference identifier,
constructed from a source domain and optionally a service type,
used by the client for matching purposes when examining presented
identifiers [RFC6125].
o Presented Identity: An instance of a presented identifier that is
presented by a server to a client within a PKIX certificate when
the client attempts to establish secure communication with the
server; the certificate can include one or more presented
identifiers of different types, and if the server hosts more than
one domain then the certificate might present distinct identifiers
for each domain [RFC6125].
4. Procedure for TLS Server Identity Verification for MTA to Email
Server Connections
4.1. Server Identification and Validation
Two fundamental aspects govern how an MTA validates the identity of
an email server when establishing a TLS session:
o How the reference identity of the server is determined
o How the presented identity of the server is validated against the
reference identity
The destination MTA identity is verified through a process of ordered
comparison of reference and presented identity pairs in conformance
to the rules defined in [RFC6125].
4.1.1. Reference Identity
In the case of an MTA, the reference identity of the destination
email server is typically expressed via the MX record for the
recipient email domain. A user addresses email to a recipient at a
specific email domain (e.g. recipient@example.net) and the MTA then
performs a DNS MX query using the [RFC5321] domain section of the
RCPT TO command forward path to determine the email server hostname
(e.g. mail.example.net) for the recipient email domain. It is
important to note that the email server name will not necessarily be
a subdomain of the recipient email domain; this will never be the
case with delegated email hosting.
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4.1.2. Presented Identity
The server presented identity SHOULD be a SubjectAltName (SAN) of
type DNSName of a X.509 public key certificate. In keeping with
[RFC6125], SAN entries SHOULD be used for the presented identity, but
the CN entry of a X.509 public key certificate MAY be used for
backwards compatibility with deployed infrastructure if no SAN
entries exist in the certificate.
4.1.3. Wildcards
[RFC6125] recommends deprecating support for SAN entries that include
wildcards for two primary reasons: (1) the lack of clarity in
existing specification as to the allowable locations of wildcard
characters and (2) the fact that a wildcard SAN entries vouches for
all servers in a domain, including possibly rogue or buggy servers.
In the case of MTA to delegated email service connections, this
document proposes continued support for DNSNames containing wildcards
as wildcard DNSNames are needed to support some delegated email
hosting scenarios.
For delegated hosting, some third parties may use the same email
server hostname(s) for all the domains that they host:
@example1.net --> mail.delegatedexample.net
@example2.net --> mail.delegatedexample.net
Alternatively, a third party email service might use a unique
hostname for each email domain:
@example1.net --> example1net.mail.delegatedexample.net
@example2.net --> example2net.mail.delegatedexample.net
If unique hostnames are associated with each email domain, then there
will be as many host names as email domains and it will not be
possible to include all hostnames as SAN DNSName entries in a
certificate. Wildcarded SAN entries would then be the only mechanism
by which a single certificate may be used for all email server
hostname reference identities. If all the email server hostnames are
of the form [domainidentifier].[hostnameroot] (e.g.
example1net.mail.delegatedexample.net), then the SAN SHOULD include a
DNSName entry of the form *.[hostnameroot] (e.g.
*.mail.delegatedexample.net). The email server certificate SHOULD
NOT include a wildcard character in the presented identity in any
position other than the left-most label.
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4.1.4. Server Identity Validation
Validation of the server identity entails a comparison of the
reference identity to the identity presented in the server
certificate. There are several approaches for validation, given the
various reference identifiers that may be used and the fundamental
difference between the self-hosted and delegated hosting models.
4.1.4.1. Determination of Reference Identity
The reference identity for an email server SHOULD be determined using
the following methods:
1. The [RFC5321] domain section of the RCPT TO command forward
path
2. The email server hostname explicitly configured in the MTA by
an administrator.
3. The email server hostname derived via a DNS/MX query against
the email domain name determined as the [RFC5321] domain section
of the RCPT TO command forward path.
4.1.4.2. Exact Match Between SAN and Reference Identity
An MTA SHOULD validate an email server identity when an exact match
exists between the presented identity as a SAN DNSName entry in the
email server's certificate and the email server's reference identity.
4.1.4.3. Wildcard Match Between SAN and Reference Identity
An MTA SHOULD validate an email server identity when a [RFC6125]
Section 6.4.3 compliant wildcard match exists between the presented
identity as a SAN DNSName entry with wildcards in the email server's
certificate and the email server's reference identity.
4.1.4.4. Exact Match Between CN and Reference Identity
For backward compatibility, if no SAN DNSName entries exist in an
email server's certificate but a CN entry exists then an MTA SHOULD
validate an email server identity if an exact match exists between
the CN and the email server's reference identity.
4.1.4.5. Wildcard Match Between CN and Reference Identity
For backward compatibility, if no SAN DNSName entries exist in an
email server's certificate but a CN entry exists then an MTA SHOULD
validate an email server identity when a [RFC6125] Section 6.4.3
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compliant wildcard match exists between the presented identity as a
CN entry with wildcards in the email server's certificate and the
email server's reference identity.
4.1.4.6. Matching Process
The order of evaluation of the different methods for an MTA to
validate and email server identity are important and an MTA SHOULD
use the following ordering of matching tests:
4.1.4.6.1. SAN Present
If one or more SAN DNSName entries are present in the email server's
certificate the following matching tests SHOULD be used in the order
specified:
1. Exact match of SAN entry to [RFC5321] domain section of the
RCPT TO command forward path
2. Wildcard match of SAN entry to [RFC5321] domain section of the
RCPT TO command forward path
3. Exact match of SAN entry to email server hostname explicitly
configured in the MTA by an administrator.
4. Wildcard match of SAN entry to email server hostname
explicitly configured in the MTA by an administrator.
5. Exact match of SAN entry email server hostname derived via a
DNS/MX query against the [RFC5321] domain section of the RCPT TO
command forward path.
6. Wildcard match of SAN entry email server hostname derived via
a DNS/MX query against the [RFC5321] domain section of the RCPT TO
command forward path.
If one or more SAN DNSName entries are present in the email server's
certificate and none of the matching tests specified above pass, then
the MTA will have failed to validate the email server's identity. and
the MTA SHOULD log an error indicating that the validation process
failed.
4.1.4.6.2. No SAN Entries Present but a CN Entry is Present
An MTA MUST NOT validate an email server identity against the CN
entry of an email server's certificate if there exist one or more SAN
DNSName entries in the certificate. If and only if no SAN DNSName
entries exist in the email server certificate and a CN is present in
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the email server's certificate the same matching process detailed in
4.1.4.6.1 above MUST be used with the CN as the presented identity
instead of the SAN.
5. Security Considerations
This document addresses only the procedure by which an MTA should
verify the identity of an email server with which it wishes to
establish a TLS connection. The procedures described in this
document do nothing to address or ameliorate the fundamental security
risk associated with obtaining an email server name through an
insecure MX query. In the abscence of DNSSEC there exist a large
number of techniques whereby a false email server name could be
returned to the MTA through an insecure MX query. It should be noted
that in the absence of DNSSEC, the MX query is no more risk prone
than a browser A lookup. Use of TLS eliminates risks of passive
listening and imposes a requirement that an attacker to obtain a
certificate that will be trusted by the sending MTA and actively
participate in the session by terminating the TLS session requested
by the sending MTA.
Risks associated with insecure DNS MX lookups may be ameliorated by
explicit association of the email server name to an email domain in
the MTA configuration.
6. IANA Considerations
This document includes no request to IANA.
7. Acknowledgements
Thanks to Dan Wing for multiple reviews of this draft and valuable
suggestions for improving it.
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
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[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
2595, June 1999.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[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
Stephan Friedl
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
Phone: (720)562-6785
EMail: sfriedl@cisco.com
Tom Kaupe
Microsoft Corp.
One Microsoft Way
Redmond, WA 98052
USA
EMail: Tom.Kaupe@microsoft.com
Sriram Gorti
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
Phone: +91 80 4365 7100
EMail: sgorti@cisco.com
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