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 © 2011 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must 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.

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.

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.

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 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.

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.

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.

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.

Author's Address

   Cyrus Daboo
   Apple Inc.
   1 Infinite Loop
   Cupertino, CA  95014
   USA
   
   EMail: cyrus@daboo.name
   URI:   http://www.apple.com/