Network Working Group                                           C. Daboo
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                            July 7, 2009
Expires: January 8, 2010


             Use of SRV records for locating email services
                        draft-daboo-srv-email-01

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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.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
   8.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5
   Appendix A.  Change History (to be removed prior to
                publication as an RFC) . . . . . . . . . . . . . . . . 6




































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.  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.

.  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].

.  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 includes support for the
      "STARTTLS" extension to optionally allow transport layer security
      to be negotiated by the client.

   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.

3.3.  POP3

   This specification adds two SRV service types for POP3 [RFC1939]:

   pop3:  Identifies a POP3 server that might also include support for
      the optional "STLS" extension command [RFC2595] to allow transport
      layer security to be negotiated by the client.

   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.

.  Guidance for mail access clients

   By using SRV records as above, clients need only prompt the user for
   their email address [RFC5322]. "local-part" and "domain" portions are
   extracted from the email address by the client.  The "local-part" or
   the full email address is used for the user identifier (used for
   authentication with the appropriate service) and the "domain" is used
   as the service domain.

   The client uses 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



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   information directly.

   Clients that support both POP3 and IMAP SHOULD check for both
   services using SRV lookups.  If both 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.

   Servers typically use just the "local-part" as the user identifier or
   the full email address.  Clients should attempt to authenticate with
   either the "local-part" or email address first, and if that fails,
   try the other next.

.  Security Considerations

   If a user has explicitly requested a connection with transport layer
   security, the client MUST either use service information returned for
   the "imaps" or "pop3s" services, or it can use service information
   returned for the "imap" or "pop3" services, but only if the
   "STARTTLS" or "STLS" commands are available and complete
   successfully.

   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.

.  IANA Considerations

   This document does not require any actions on the part of IANA.

.  Acknowledgments

   Thanks to Alexey Melnikov for feedback and suggestions.

.  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.

   [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 -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.  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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