The RFC Archive
 The RFC Archive   RFC 6186   « Jump to any RFC number directly 
 RFC Home
Full RFC Index
Recent RFCs
RFC Standards
Best Current Practice
RFC Errata
1 April RFC



IETF RFC 6186

Use of SRV Records for Locating Email Submission/Access Services

Last modified on Thursday, March 31st, 2011

Permanent link to RFC 6186
Search GitHub Wiki for RFC 6186
Show other RFCs mentioning RFC 6186







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/RFC 6186.

 Copyright Notice

   Copyright (c) 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.









Daboo                        Standards Track                 PAGE 1 top


RFC 6186 SRV for Email March 2011 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 [RFC 5321], IMAP [RFC 3501], and POP3 [RFC 1939]. IMAP and POP3 are both message store access protocols used by message store user agents (MUAs) to manipulate email messages after delivery. [RFC 4409] 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. [RFC 2782] 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). [RFC 5321] 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 [RFC 4409], 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. Daboo Standards Track PAGE 2 top

RFC 6186 SRV for Email March 2011 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 [RFC 2119]. Email-related terminology from [RFC 5598] is used. 3. SRV Service Labels 3.1. Email Submission This specification adds one SRV service label for message submission [RFC 4409]: submission: Identifies an MSA using [RFC 4409]. Note that this covers connections both with and without Transport Layer Security (TLS) [RFC 5246] as defined for SMTP in [RFC 3207]. Example: service record _submission._tcp SRV 0 1 587 mail.example.com. 3.2. IMAP This specification adds two SRV service labels for IMAP [RFC 3501]: _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 [RFC 5246] is initiated directly upon connection to the IMAP server. Example: service record _imap._tcp SRV 0 1 143 imap.example.com. Daboo Standards Track PAGE 3 top

RFC 6186 SRV for Email March 2011 Example: service record _imaps._tcp SRV 0 1 993 imap.example.com. 3.3. POP3 This specification adds two SRV service labels for POP3 [RFC 1939]: _pop3: Identifies a POP3 server that MAY require the MUA to use the "STLS" extension command [RFC 2595] prior to authentication. _pop3s: Identifies a POP3 server where TLS [RFC 5246] 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 Daboo Standards Track PAGE 4 top

RFC 6186 SRV for Email March 2011 intends to use, then perform the weight selection algorithm given in [RFC 2782] 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 [RFC 5322]. 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 [RFC 2782]). MUAs that support both POP3 and IMAP use the procedure in Section 3.4 to choose between each service when both are offered. Daboo Standards Track PAGE 5 top

RFC 6186 SRV for Email March 2011 Subsequent to configuration, the MUA will connect to the service. When using "imaps" or "pop3s" services, a TLS [RFC 5246] 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 [RFC 5246]. When using TLS in this way, MUAs SHOULD use the TLS Server Name Indication [RFC 6066]. Certificate verification MUST use the procedure outlined in Section 6 of [RFC 6125] 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. Daboo Standards Track PAGE 6 top

RFC 6186 SRV for Email March 2011 b. If the service provider uses TLS [RFC 5246], the service provider MUST ensure a certificate is installed that can be verified by MUAs using the procedure outlined in Section 6 of [RFC 6125] 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 [RFC 6066]. 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 [RFC 5246] is being used for the email service, MUAs MUST use the procedure outlined in Section 6 of [RFC 6125] to verify the service. Implementations of TLS [RFC 5246] 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 [RFC 5246] 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. Daboo Standards Track PAGE 7 top

RFC 6186 SRV for Email March 2011 8. References 8.1. Normative References [RFC 1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, May 1996. [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC 2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC 2595, June 1999. [RFC 2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000. [RFC 3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over Transport Layer Security", RFC 3207, February 2002. [RFC 3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003. [RFC 4409] Gellens, R. and J. Klensin, "Message Submission for Mail", RFC 4409, April 2006. [RFC 5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. [RFC 5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, October 2008. [RFC 5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, October 2008. [RFC 6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, January 2011. [RFC 6125] 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 [RFC 5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July 2009. Daboo Standards Track PAGE 8 top

RFC 6186 SRV for Email March 2011 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 top

Use of SRV Records for Locating Email Submission/Access Services RFC TOTAL SIZE: 18895 bytes PUBLICATION DATE: Thursday, March 31st, 2011 LEGAL RIGHTS: The IETF Trust (see BCP 78)


RFC-ARCHIVE.ORG

© RFC 6186: The IETF Trust, Thursday, March 31st, 2011
© the RFC Archive, 2024, RFC-Archive.org
Maintainer: J. Tunnissen

Privacy Statement