Internet Engineering Task Force (IETF) C. Daboo
Request for Comments: 6186 Apple Inc.
Updates: 1939, 3501 March 2011
Category: Standards Track
ISSN: 2070-1721
Use of SRV Records for Locating Email Submission/Access Services
Abstract
This specification describes how SRV records can be used to locate
email services.
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/rfc6186.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . . 3
3. SRV Service Labels . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Email Submission . . . . . . . . . . . . . . . . . . . . . 3
3.2. IMAP . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.3. POP3 . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.4. Priority for Domain Preferences . . . . . . . . . . . . . . 4
4. Guidance for MUAs . . . . . . . . . . . . . . . . . . . . . . . 5
5. Guidance for Service Providers . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 8
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 that would result in automatic configuration of
appropriate services for that user. The minimal information entered
would be the user's email address.
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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.
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 Transport Layer Security
(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.
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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.
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-numbered 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, 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-numbered 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 whichever 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
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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-numbered priority value (0) than 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.
In addition, with SRV RRs it is possible to indicate that a
particular service is not supported at all at a particular domain by
setting the target of an SRV RR to ".". If such records are present,
clients MUST assume that the specified service is not available, and
instead make use of the other SRV RRs for the purposes of determining
the domain preference.
Example: service records for IMAP and POP3 with both TLS and non-TLS
service types are present. Both IMAP and POP3 non-TLS service types
are marked as not available. IMAP (with TLS) has a lower-numbered
priority value 0 than POP3 (with TLS) at priority 10, indicating to
the MUA that IMAP is preferred over POP3, when the MUA can support
either service, and only the TLS versions of the services are
available.
_imap._tcp SRV 0 0 0 .
_imaps._tcp SRV 0 1 993 imap.example.com.
_pop3._tcp SRV 0 0 0 .
_pop3s._tcp SRV 10 1 995 pop3.example.com.
4. Guidance for MUAs
By using SRV records as above, MUAs need initially only to prompt the
user for their email address [RFC5322]. The "local-part" and
"domain" portions are then extracted from the email address by the
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.
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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 [RFC6066]. Certificate verification MUST
use the procedure outlined in Section 6 of [RFC6125] in regard to
verification with an SRV RR as the starting point.
Once a suitable connection has been made, and any required protection
set up, 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 reuse 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.
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.
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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 6 of [RFC6125] 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 [RFC6066].
c. Install the appropriate SRV records for the offered services.
6. Security Considerations
If a user has explicitly requested a connection with a transport
layer security mechanism (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. For example, 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 6 of
[RFC6125] to verify the service.
Implementations of TLS [RFC5246] typically support multiple versions
of the protocol as well as the older Secure Sockets Layer (SSL)
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. Acknowledgments
Thanks to Tony Finch, Ned Freed, Alfred Hoenes, Suresh Krishnan,
Alexey Melnikov, Chris Newman, and Phil Pennock for feedback and
suggestions. Some of this work is based on a previously drafted
document by John Klensin and Eric Hall.
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8. References
8.1. Normative References
[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.
[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.
[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.
[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.
8.2. Informative References
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
July 2009.
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Author's Address
Cyrus Daboo
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
USA
EMail: cyrus@daboo.name
URI: http://www.apple.com/
Daboo Standards Track [Page 9]
