Network Working Group C. Daboo
Internet-Draft Apple Inc.
Intended status: Standards Track August 19, 2009
Expires: February 20, 2010
Use of SRV records for locating email services
draft-daboo-srv-email-02
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Abstract
This specification describes how SRV records can be used to locate
email services.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . . 3
3. SRV service types . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Email Submission . . . . . . . . . . . . . . . . . . . . . 3
3.2. IMAP . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.3. POP3 . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Guidance for mail access clients . . . . . . . . . . . . . . . 4
5. Guidance for Service Providers . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
9. Normative References . . . . . . . . . . . . . . . . . . . . . 6
Appendix A. Change History (to be removed prior to
publication as an RFC) . . . . . . . . . . . . . . . . 7
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1. Introduction
[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 SRV RR records.
Internet Email protocols include SMTP [RFC5321], IMAP [RFC3501] and
POP3 [RFC1939].
[RFC5321] defines the MX RR record type to locate SMTP services for a
domain. However, [RFC4409] defines a "profile" of the SMTP service
that is specifically used for message submission - which is of direct
relevance to email clients which typically don't use MX records.
Both IMAP and POP3 are mail access protocols used by email clients to
manipulate email messages after delivery.
Typically email clients have required users to enter host name and
port information for the services they need. This is not ideal as
the way in which server information is specified can differ from
client to client, and can be confusing to users, leading to errors
when inputting the details. A better approach would be to require
miniml 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 email clients.
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].
3. SRV service types
3.1. Email Submission
This specification adds one SRV service type for message submission
[RFC4409]:
submission: Identifies a message submission server as defined by
[RFC4409]. Note that this covers connections both with and
without transport layer security [RFC3207].
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Example: service record
_submission._tcp SRV 0 1 587 mail.example.com.
3.2. IMAP
This specification adds two SRV service types for IMAP [RFC3501]:
imap: Identifies an IMAP server that MAY advertise the
"LOGINDISABLED" capability and MAY require the client to use the
"STARTTLS" command prior to authentication. Although these two
extensions are mandatory-to-implement for both clients and
servers, they are not mandatory-to-use by service providers.
imaps: Identifies an IMAP server where transport layer security is
initiated directly upon connection to the 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 types for POP3 [RFC1939]:
pop3: Identifies a POP3 server that MAY require the client to use
the "STLS" extension command [RFC2595] prior to authentication.
pop3s: Identifies a POP3 server where transport layer security is
initiated directly upon connection to the 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.
4. Guidance for mail access clients
By using SRV records as above, clients need initially only prompt the
user for their email address [RFC5322]. "local-part" and "domain"
portions are then extracted from the email address by the client.
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The client 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 host name and port for the service can be
determined and used to complete client configuration. If an SRV
record is not found, the client will need to prompt the user to enter
host name and port information directly.
Clients that support both POP3 and IMAP SHOULD check for both
services using SRV lookups, and for both SSL and non-SSL variants.
If both POP3 and IMAP services are returned the choice of which to
configure is up to the client - it could prompt the user to make a
choice, or pick one based on local policy.
Subsequent to configuration, the client will connect to the service.
When using "imaps" or "pop3s" services, a transport layer security
negotiation is done immediately upon connection. With "imap" and
"pop3" services, the "STARTTLS" or "STLS" commands respectively are
used to initiate a protected connection. When using transport layer
security in this way, clients SHOULD use the TLS Server Name
Indication [RFC4366] and include the service domain name used in the
SRV record lookup as the name.
Once a suitable connection has been made, and any required protection
setup, the client will typically need to authenticate with the
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 (such as the SASL "PLAIN"
mechanism). When a user identifier is required, clients 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.
5. Guidance for Service Providers
Service providers wanting to offer a POP3 or IMAP service that can be
configured by clients using SRV records need to follow certain
guidelines to ensure proper operation.
a. 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 not conflict with any login name on
the server.
b. If the service provider uses transport layer security, the
service provider MUST ensure a certificate is installed that
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matches the email domain name used to configure clients. 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 client MUST successfully
negotiate transport layer security prior to sending an authentication
command. The client 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 can
potentially cause clients to connect to any server chosen by the
attacker. In the absence of a secure DNS option, clients SHOULD
check that the host name returned in the SRV record matches the
original service domain that was queried. If the host is not in the
queried domain, clients SHOULD verify with the user that the SRV host
name is suitable for use before executing any connections to the
host. Alternatively, if transport layer security is being used,
clients MUST use the service domain used in the SRV record lookup as
the server name for certificate verification pruposes.
7. IANA Considerations
This document does not require any actions on the part of IANA.
8. Acknowledgments
Thanks to Alexey Melnikov, and Chris Newman for feedback and
suggestions.
9. 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.
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[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.
[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.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
Appendix A. Change History (to be removed prior to publication as an
RFC)
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.
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2. Added text to explain that the email address, as well as the
local-part, may be used as the user identifier.
3. Teaked 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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