Network Working Group C. Daboo
Internet-Draft Apple Inc.
Expires: April 19, 2013 October 18, 2012

Use of SRV records for locating email services


This specification describes how SRV records can be used to locate email services.

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Table of Contents

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 autoconfiguration 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]:

Identifies a message submission server as defined by [RFC4409]. Note that this covers connections both with and without transport layer security [RFC3207].

Example: service record

    _submission._tcp     SRV 0 1 587

3.2. IMAP

This specification adds two SRV service types for IMAP [RFC3501]:

Identifies an IMAP server that includes support for the "STARTTLS" extension to optionally allow transport layer security to be negotiated by the client.
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

3.3. POP3

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

Identifies a POP3 server that includes support for the "STLS" command [RFC2595] to optionally allow transport layer security to be negotiated by the client.
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

4. 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" 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 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.

5. Security Considerations

Clients that support transport layer security SHOULD try the "imaps" or "pop3s" services first before trying the "imap" or "pop3" services. If a user has explicitly requested a connection with transport layer security, the client MUST NOT use any service information returned for the "imap" or "pop3" services.

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.

6. IANA Considerations

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

7. References

[RFC1939] Myers, J.G. and M.T. 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.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, October 2008.
[RFC5322] Resnick, P., "Internet Message Format", RFC 5322, October 2008.

Author's Address

Cyrus Daboo Apple Inc. 1 Infinite Loop Cupertino , CA 95014 USA EMail: URI: