INTERNET-DRAFT Gary Bajaj <draft-bajaj-mail-srv-03.txt> November 2004 Request for Comments: Category: Use of SRV records for POP3, POP3S, IMAP and IMAPS. Status of this Memo This memo defines an Experimental Protocol for the Internet community. This memo does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited. This document is an Internet-Draft and is subject to all provisions of Section 10 of RFC2026. By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, or will be disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Copyright Notice Copyright (C) The Internet Society (2004). Abstract DNS records for the mail services POP3, POP3S, IMAP and IMAPS do not currently provide failover switching as do the DNS MX records for SMTP. This document looks at the issues involved and recommends a solution using SRV records. Expires May 2005 [Page 1]
Introduction Mail servers that require high availability might be multi-homed with upstream connectivity to two or more ISPs. This is traditionally accomplished by running BGP4 such that each upstream provider would route to the site's own IP block. If connectivity to one ISP fails, incoming connections would seamlessly be routed through the other ISP. For various reasons including IP allocation constraints, cost and networking expertise, running BGP4 is impratical for most small ISPs. If using DNS only, SMTP can be made fault tolerant by using multiple MX records, one for each IP serviced by the mail server so that each MX record is tried in turn until an IP responds. Such failover protection using DNS is not currently possible for POP3 and IMAP connections. Using SRV [RFC2782] records seems to be the obvious solution to making POP3, POP3S, IMAP and IMAPS redundant. SRV records also provide for load balancing when using multiple servers that have access to mail spool on a shared mass storage device such as NAS. Examples: SRV RRs: _pop3._tcp SRV 1 0 110 host1.example.com. _pop3._tcp SRV 1 0 110 host2.example.com. _pop3._tcp SRV 0 0 110 host3.example.com. _imap._tcp SRV 1 0 143 host1.example.com. _imap._tcp SRV 1 0 143 host2.example.com. _imap._tcp SRV 0 0 143 host3.example.com. _pop3s._tcp SRV 0 3 995 host1.example.com. _pop3s._tcp SRV 0 1 995 host2.example.com. _imaps._tcp SRV 0 3 993 host1.example.com. _imaps._tcp SRV 0 1 993 host2.example.com. A RRs: host1 A 10.0.0.2 host2 A 172.16.1.2 host3 A 172.16.1.3 host1 and host2 are the same multi-homed host that can accept both insecure (pop3, imap) and secure (pop3s, imaps) connections. host3 is a separate host that is not multi-homed and does not accept secure connections. Connect to either 10.0.0.2 or 172.16.1.2 if either is available (the probability of being selected is 75% for 10.0.0.2 and 25% for 172.16.1.2) to download mail over a secure POP3 or IMAP connection Connect to 172.16.1.3 and if not available connect to either of 10.0.0.2 or 172.16.1.2 to download mail over an insecure POP3 or IMAP connection. Expires May 2005 [Page 2]
Transitioning Considerations When transitioning from using a non-SRV solution to using an SRV based solution, old non-SRV aware clients will continue to look for A records. These will not benefit from redundancy until updated, but will continue to work. A DNS proxy solution that returns the IPs of hosts found within SRV records to MUAs querying A records for that host might be possible for old clients. IANA Considerations Well known labels have to be allocated for the first label of the SRV records. This document has used _pop3, _imap, _pop3s and _imaps. Security Considerations None. References [RFC 2026] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. [RFC2782] A DNS RR for specifying the location of services (DNS SRV). A. Gul brandsen, P. Vixie, L. Esibov. February 2000. RFC 2782. Author's Address Gary Bajaj BITNETS Inc. PO Box 58065 RPO Inglewood Edmonton, Alberta T5L 4Z4 Canada +1 (780) 418-5151 netguru@bitnets.com Acknowledgements Mark Andrews (ISC) Gerrit Schunk Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78 and at www.rfc-editor.org, and except as set forth therein, the authors retain all their rights. Expires May 2005 [Page 3]
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