INTERNET-DRAFT Gary Bajaj May 2003 Use of SRV records for POP3, POP3S, IMAP and IMAPS. Status of this Memo This document is an Internet-Draft and is subject to all provisions of Section 10 of RFC2026. 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 Comments should be sent to the author. This draft expires in November 2003. 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. 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. Expires November 2003 [Page 1] 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. Transitioning Considerations When transitioning from using a non-SRV solution to using an SRV based solution, old non-SRV aware clients (mail user agents) will continue to look for A records. These will not benefit from redundancy until updated, but will continue to work. 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. References [RFC2782] A DNS RR for specifying the location of services (DNS SRV). A. Gul brandsen, P. Vixie, L. Esibov. February 2000. RFC 2782. Expres November 2003 [Page 2] Author's Address Gary Bajaj BITNETS 58065-12621 118 Ave NW Edmonton, Alberta T5L 4Z4 Canada +1 (780) 418-4477 BEAMERS-Support@BITNETS.com Expires November 2003 [Page 3]