Internet DRAFT - draft-daboo-srv-email
draft-daboo-srv-email
Network Working Group C. Daboo
Internet-Draft Apple Inc.
Updates: 1939,3501 May 12, 2010
(if approved)
Intended status: Standards Track
Expires: November 13, 2010
Use of SRV Records for Locating Email Submission/Access services
draft-daboo-srv-email-05
Abstract
This specification describes how SRV records can be used to locate
email services.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . . 3
3. SRV Service Labels . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Email Submission . . . . . . . . . . . . . . . . . . . . . 4
3.2. IMAP . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.3. POP3 . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.4. Priority for Domain Preferences . . . . . . . . . . . . . . 5
4. Guidance for MUAs . . . . . . . . . . . . . . . . . . . . . . . 5
5. Guidance for Service Providers . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. Change History (to be removed prior to
publication as an RFC) . . . . . . . . . . . . . . . . 9
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1. Introduction
Internet Email protocols include SMTP [RFC5321], IMAP [RFC3501] and
POP3 [RFC1939]. IMAP and POP3 are both message store access
protocols used by message store user agents (MUAs) to manipulate
email messages after delivery. [RFC4409] defines a "profile" of the
SMTP service that is specifically used for message submission. MUAs
are expected to submit messages to mail submission agents (MSAs)
using this approach.
[RFC2782] defines a DNS-based service discovery protocol that has
been widely adopted as a means of locating particular services within
a local area network and beyond, using DNS SRV Resource Records
(RRs).
[RFC5321] specifies how to use DNS MX RRs to locate SMTP services for
a domain. However, MUAs are expected to use the submission protocol
defined in [RFC4409] which does not use MX records.
Typically MUAs have required users to enter a fully qualified domain
name (FQDN) and port information for the services they need. This is
not ideal as the way in which server configuration information is
specified can differ from MUA to MUA, and can be confusing to users,
leading to errors when inputting the details. Alternatively, some
MUAs have adopted a complex "auto-discovery" process involving
probing a domain to see what services might be available. A better
approach to all this would be to require minimal information to be
entered by a user which would result in automatic configuration of
appropriate services for that user. The minimal information entered
would be the user's email address.
This specification defines new SRV service types for the message
submission, IMAP and POP3 services, to enable simple auto-
configuration of MUAs. The priority field of the SRV record can also
be used to indicate a preference for one message store access
protocol over another.
2. Conventions Used in This Document
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].
Email-related terminology from [RFC5598] is used.
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3. SRV Service Labels
3.1. Email Submission
This specification adds one SRV service label for message submission
[RFC4409]:
submission: Identifies an MSA using [RFC4409]. Note that this
covers connections both with and without TLS [RFC5246] as defined
for SMTP in [RFC3207].
Example: service record
_submission._tcp SRV 0 1 587 mail.example.com.
3.2. IMAP
This specification adds two SRV service labels for IMAP [RFC3501]:
_imap: Identifies an IMAP server that MAY advertise the
"LOGINDISABLED" capability and MAY require the MUA to use the
"STARTTLS" command prior to authentication. Although these two
extensions are mandatory-to-implement for both MUAs and IMAP
servers, they are not mandatory-to-use by service providers.
_imaps: Identifies an IMAP server where TLS [RFC5246] is initiated
directly upon connection to the IMAP server.
Example: service record
_imap._tcp SRV 0 1 143 imap.example.com.
Example: service record
_imaps._tcp SRV 0 1 993 imap.example.com.
3.3. POP3
This specification adds two SRV service labels for POP3 [RFC1939]:
_pop3: Identifies a POP3 server that MAY require the MUA to use the
"STLS" extension command [RFC2595] prior to authentication.
_pop3s: Identifies a POP3 server where TLS [RFC5246] is initiated
directly upon connection to the POP3 server.
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Example: service record
_pop3._tcp SRV 0 1 110 pop3.example.com.
Example: service record
_pop3s._tcp SRV 0 1 995 pop3.example.com.
3.4. Priority for Domain Preferences
The priority field in the SRV RR allows a domain to indicate that
some records have a higher preference than others in the DNS query
results (determined by those records having a lower priority value).
Typically, this is used for choosing a record from a set for a single
service label, however it is not restricted to choice within only one
service.
Often a site will offer both IMAP and POP3 message store access
services for users. However, the site may have a preference for one
over the other that they want to convey to the user to ensure that,
when the user has an MUA capable of using both IMAP and POP3, that
the preferred choice is used.
To aid with this choice, sites SHOULD offer both sets of IMAP (_imap
and/or _imaps) and POP3 (_pop3 and/or _pop3s) SRV records in their
DNS and set the priority for those sets of records such that the
"preferred" service has a lower priority value than the other. When
an MUA supports both IMAP and POP3 it SHOULD retrieve records for
both services and then use the service with the lowest priority
value. If the priority is the same for both services, MUAs are free
to choose which ever one is appropriate. When considering multiple
records for different protocols at the same priority but with
different weights, the client MUST first select the protocol it
intends to use, then perform the weight selection algorithm given in
[RFC2782] on the records associated with the selected protocol.
Example: service records for both IMAP and POP3, with IMAP having a
lower priority value (0) then POP3 (10), indicating to the MUA that
IMAP is preferred over POP3, when the MUA can support either service.
_imap._tcp SRV 0 1 143 imap.example.com.
_pop3._tcp SRV 10 1 110 pop3.example.com.
4. Guidance for MUAs
By using SRV records as above, MUAs need initially only prompt the
user for their email address [RFC5322]. The "local-part" and
"domain" portions are then extracted from the email address by the
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MUA. The MUA uses the "domain" portion as the service domain to
perform SRV lookups for the services it wants to configure. If the
SRV lookup is successful the target FQDN and port for the service can
be determined and used to complete MUA configuration. If an SRV
record is not found, the MUA will need to prompt the user to enter
the FQDN and port information directly, or use some other heuristic.
In the case of multiple SRV records returned for a particular
service, the MUA MUST use the priority and weight fields in the
record to determine which one to use (as per [RFC2782]).
MUAs that support both POP3 and IMAP use the procedure in Section 3.4
to choose between each service when both are offered.
Subsequent to configuration, the MUA will connect to the service.
When using "imaps" or "pop3s" services, a TLS [RFC5246] negotiation
is done immediately upon connection. With "imap", "pop3" and
"submission" services, the "STARTTLS", "STLS" and "STARTTLS" commands
respectively are used to initiate a protected connection using TLS
[RFC5246]. When using TLS in this way, MUAs SHOULD use the TLS
Server Name Indication [RFC4366]. Certificate verification MUST use
the procedure outlined in Section 4.3 of
[I-D.saintandre-tls-server-id-check] in regard to verification with
an SRV RR as the starting point.
Once a suitable connection has been made, and any required protection
setup, the MUA will typically need to authenticate with the IMAP,
POP3 or SMTP (submission) server. The details of that are governed
by the specific protocols themselves, though often times a "user
identifier" is required for some form of user/password
authentication. When a user identifier is required, MUAs MUST first
use the full email address provided by the user, and if that results
in an authentication failure, SHOULD fall back to using the "local-
part" extracted from the email address. This is in line with the
guidance outlined in Section 5. If both these user identifiers
result in authentication failure, the MUA SHOULD prompt the user for
a valid identifier.
Once a successful connection and authentication have been done, MUAs
SHOULD cache the service details (hostname, port, user identity) that
were successfully used, and re-use those when connecting again at a
later time.
If a subsequent connection attempt fails, or authentication fails,
MUAs SHOULD re-try the SRV lookup to "refresh" the cached data for
the same protocol the client had chosen earlier. i.e., this means
that the client MUST NOT change from IMAP service to POP3 (or vice
versa) due to changes in the corresponding SRV priorities without
user interaction.
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5. Guidance for Service Providers
Service providers wanting to offer IMAP, POP3 or SMTP (submission)
services that can be configured by MUAs using SRV records need to
follow certain guidelines to ensure proper operation.
a. IMAP, POP3 and SMTP (submission) servers SHOULD be configured to
allow authentication with email addresses or email local-parts.
In the former case, the email addresses MUST NOT conflict with
other forms of permitted user login name. In the latter case,
the email local-parts need to be unique across the server and
MUST NOT conflict with any login name on the server.
b. If the service provider uses TLS [RFC5246], the service provider
MUST ensure a certificate is installed that can be verified by
MUAs using the procedure outlined in Section 4.3 of
[I-D.saintandre-tls-server-id-check] in regard to verification
with an SRV RR as the starting point. If the service provider
hosts multiple domains on the same IP address, then the service
provider MUST enable support for the TLS Server Name Indication
[RFC4366].
c. Install the appropriate SRV records for the offered services.
6. Security Considerations
If a user has explicitly requested a connection with transport layer
security (user interfaces sometimes present this choice as a "use
SSL" or "secure connection" checkbox), the MUA MUST successfully
negotiate transport layer security prior to sending an authentication
command. The MUA MAY do this with "imaps", "pop3s", "imap" with
"STARTTLS", or "pop3" with "STLS". Service providers MAY offer any
subset of these four options for the mail service.
A malicious attacker with access to the DNS server data, or able to
get spoofed answers cached in a recursive resolver, can potentially
cause MUAs to connect to any IMAP, POP3 or submission server chosen
by the attacker. In the absence of a secure DNS option, MUAs SHOULD
check that the target FQDN returned in the SRV record matches the
original service domain that was queried. If the target FQDN is not
in the queried domain, MUAs SHOULD verify with the user that the SRV
target FQDN is suitable for use before executing any connections to
the host. Alternatively, if TLS [RFC5246] is being used for the
email service, MUAs MUST use the procedure outlined in Section 4.3 of
[I-D.saintandre-tls-server-id-check] to verify the service.
Implementations of TLS [RFC5246] typically support multiple versions
of the protocol as well as the older Secure Sockets Layer (SSL)
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protocol. Because of known security vulnerabilities, email clients
and email servers MUST NOT request, offer, or use SSL 2.0. See
Appendix E.2 of [RFC5246] for further details.
7. IANA Considerations
This document does not require any actions on the part of IANA.
8. Acknowledgments
Thanks to Tony Finch, Ned Freed, Alfred Hoenes, Suresh Krishnan,
Alexey Melnikov, and Chris Newman for feedback and suggestions. Some
of this work is based on a previous internet draft by John Klensin
and Eric Hall.
9. References
9.1. Normative References
[I-D.saintandre-tls-server-id-check] Saint-Andre, P. and J. Hodges,
"Representation and
Verification of Application
Server Identity in Certificates
Used with Transport Layer
Security (TLS)", draft-
saintandre-tls-server-id-check-
04 (work in progress),
April 2010.
[RFC1939] Myers, J. and M. Rose, "Post
Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
[RFC2119] Bradner, S., "Key words for use
in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119,
March 1997.
[RFC2595] Newman, C., "Using TLS with
IMAP, POP3 and ACAP", RFC 2595,
June 1999.
[RFC2782] Gulbrandsen, A., Vixie, P., and
L. Esibov, "A DNS RR for
specifying the location of
services (DNS SRV)", RFC 2782,
February 2000.
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[RFC3207] Hoffman, P., "SMTP Service
Extension for Secure SMTP over
Transport Layer Security",
RFC 3207, February 2002.
[RFC3501] Crispin, M., "INTERNET MESSAGE
ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[RFC4366] Blake-Wilson, S., Nystrom, M.,
Hopwood, D., Mikkelsen, J., and
T. Wright, "Transport Layer
Security (TLS) Extensions",
RFC 4366, April 2006.
[RFC4409] Gellens, R. and J. Klensin,
"Message Submission for Mail",
RFC 4409, April 2006.
[RFC5246] Dierks, T. and E. Rescorla,
"The Transport Layer Security
(TLS) Protocol Version 1.2",
RFC 5246, August 2008.
[RFC5321] Klensin, J., "Simple Mail
Transfer Protocol", RFC 5321,
October 2008.
[RFC5322] Resnick, P., Ed., "Internet
Message Format", RFC 5322,
October 2008.
9.2. Informative References
[RFC5598] Crocker, D., "Internet Mail
Architecture", RFC 5598,
July 2009.
Appendix A. Change History (to be removed prior to publication as an
RFC)
Changes in -05:
1. IESG review: Indicate that this spec updates 1937 and 3501.
2. IESG review: Fixed minor typos found in IESG review.
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3. IESG review: Added text explaining how to deal with both SRV
priority and weight.
4. IESG review: Added text to explain client methodology when
dealing with a failed connection to a service and how they re-do
SRV lookup without changing the service.
5. IESG review: Added statement that SSL v2 is not allowed.
6. IESG review: Changed TLS server name indication reference back to
RFC4366.
7. Changing "transport layer security" to TLS when it specifically
refers to RFC5246.
Changes in -04:
1. Updated reference to draft-saintandre-tls-server-id-check.
2. Tweaked 3.4 to indicate that the _XXXs variants of service type
are also included in the "weighting" approach.
3. Tweaked Acknowledgments.
Changes in -03:
1. Added ability to use priority to select one access protocol over
another.
2. Added statement that clients should retry SRV on subsequent
connection failure.
3. Added statement about handling multiple records for the same
service.
4. Stronger use of MUST NOT in Section 5(a).
5. GENART: Added statement that clients should prompt the user if
both email and local-part authentication fail.
6. Tweaked title.
7. "Service type" -> "Service label"
8. "Host name" -> "target FQDN"
9. Improvements to security considerations wrt DNS attacks.
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10. MUA and service provider guidance now includes submission
service.
11. Added References to draft-saintandre-tls-server-id-check that
should define the proper cert validation procedures.
12. SECDIR: reworked introduction.
13. Switched to using terminology from RFC5598.
Changes in -02:
1. Tweaked text for imap to better describe mandatory-to-implement
behavior from base spec.
2. Tweaked text for pop3 along similar lines as imap.
3. Teaked security considerations to account for use of STARTTLS and
STLS.
4. Added examples for imaps and pop3s.
5. Re-worked client guidelines.
6. Added service provider guidelines.
Changes in -01:
1. Tweaked text for pop3 to make it clearer that STLS is an
extension.
2. Added text to explain that the email address, as well as the
local-part, may be used as the user identifier.
3. Tweaked security considerations to account for use of STARTTLS
and STLS.
Author's Address
Cyrus Daboo
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
USA
EMail: cyrus@daboo.name
URI: http://www.apple.com/
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