Internet DRAFT - draft-bajaj-mail-srv
INTERNET-DRAFT Gary Bajaj
<draft-bajaj-mail-srv-03.txt> November 2004
Request for Comments:
Use of SRV records for POP3, POP3S, IMAP and IMAPS.
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Copyright (C) The Internet Society (2004).
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.
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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.
_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.
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
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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.
Well known labels have to be allocated for the first label of the
SRV records. This document has used _pop3, _imap, _pop3s and
Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
A DNS RR for specifying the location of services (DNS SRV). A. Gul
brandsen, P. Vixie, L. Esibov. February 2000. RFC 2782.
PO Box 58065 RPO Inglewood
Edmonton, Alberta T5L 4Z4
+1 (780) 418-5151
Mark Andrews (ISC)
Copyright (C) The Internet Society (2004).
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