Network Working Group C. Daboo
Internet-Draft Apple Inc.
Expires: November 22, 2009 May 21, 2009
Use of SRV records for locating CalDAV calendar services
draft-daboo-srv-caldav-00
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Abstract
This specification describes how SRV records can be used to locate a
CalDAV based calendar service.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . . 3
3. CalDAV SRV service types . . . . . . . . . . . . . . . . . . . 3
4. CalDAV Client "Bootstrapping" . . . . . . . . . . . . . . . . . 3
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . . 6
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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.
[RFC4791] defines the CalDAV Calendar Access protocol, based on HTTP
[RFC2616], for accessing calendar data stored on a server. CalDAV
clients need to be able to discover appropriate CalDAV servers within
their local area network and at other domains.
This specification defines new SRV service types for the CalDAV
protocol, and gives an example of how clients can use this together
with other protocol features to enable simple client configuration.
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. CalDAV SRV service types
This specification adds two service types for use with SRV records:
caldav: Identifies a CalDAV server that uses HTTP without transport
layer security ([RFC2818]).
caldavs: Identifies a CalDAV server that uses HTTP with transport
layer security ([RFC2818]).
Clients SHOULD honor "TTL", "Priority" and "Weight" values in the SRV
records, as described by [RFC2782].
Example: service record for server without transport layer security
_caldav._tcp SRV 0 1 80 calendar.example.com.
Example: service record for server with transport layer security
_caldavs._tcp SRV 0 1 443 calendar.example.com.
4. CalDAV Client "Bootstrapping"
This section describes a procedure that CalDAV clients MAY use to do
their initial configuration based on minimal user input.
Minimal input from a user would consist of a calendar user address.
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A calendar user address is defined by iCalendar
[I-D.ietf-calsify-rfc2445bis] to be a URI [RFC3986]. Provided a user
identifier and a domain name can be extracted from the URI, this
simple "bootstrap" configuration can be done.
If the calendar user address is a "mailto:" [RFC2368] URI, the
"mailbox" portion of the URI is examined and the "local-part" and
"domain" portions extracted. The "local-part" is used for the user
identifier and the "domain" is used as the service domain.
If the calendar user address is an "http:" [RFC2616] or "https:"
[RFC2818] URI, the "userinfo" and "host" portion of the URI is
extracted. The "userinfo" is used for the user identifier and the
"host" is used as the service domain.
Once the user identifier and service domain have been extracted, the
following is done:
1. An SRV lookup for _caldavs._tcp is done against the extracted
service domain.
2. If no result is found for _caldavs._tcp, the client can try
_caldav._tcp provided non-SSL connections are appropriate.
3. If an SRV record is returned the client extracts the server host
name and port number
4. The client does an authenticated "PROPFIND" [RFC4791] request
using the user identifier, host name and port number and a
request URI of "/". The body of the request should include the
DAV:current-user-principal [RFC5397] property as one of the
properties to return. Note that clients MUST properly handle
HTTP redirect response statuses for the request.
5. If the DAV:current-user-principal property is returned, the
client uses the value for the principal-URI of the calendar user.
Given that quantity, it can do a "PROPFIND" on the principal-URI
and discover additional properties for configuration.
6. If the DAV:current-user-principal property is not returned, then
the client will need to request the principal-URI path from the
user in order to continue with configuration.
5. Security Considerations
Clients that support transport layer security as defined by [RFC2818]
SHOULD try the "caldavs" service first before trying the "caldav"
service. If a user has explicitly requested a connection with
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transport layer security, the client MUST NOT use any service
information returned for the "caldav" service.
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 CalDAV requests against
the host.
6. IANA Considerations
This document does not require any actions on the part of IANA.
7. Acknowledgments
This specification was suggested by discussion that took place within
the Calendaring and Scheduling Consortium's CalDAV Technical
Committee. The authors thank the participants of that group for
their input. The "bootstrapping" process is based on diagrams
developed by Wilfredo Sanchez.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in
RFCs to Indicate Requirement Levels",
BCP 14, RFC 2119, March 1997.
[RFC2368] Hoffman, P., Masinter, L., and J.
Zawinski, "The mailto URL scheme",
RFC 2368, July 1998.
[RFC2616] Fielding, R., Gettys, J., Mogul, J.,
Frystyk, H., Masinter, L., Leach, P.,
and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1",
RFC 2616, 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.
[RFC2818] Rescorla, E., "HTTP Over TLS",
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RFC 2818, May 2000.
[RFC3986] Berners-Lee, T., Fielding, R., and L.
Masinter, "Uniform Resource Identifier
(URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4791] Daboo, C., Desruisseaux, B., and L.
Dusseault, "Calendaring Extensions to
WebDAV (CalDAV)", RFC 4791,
March 2007.
8.2. Informative References
[I-D.ietf-calsify-rfc2445bis] Desruisseaux, B., "Internet
Calendaring and Scheduling Core Object
Specification (iCalendar)",
draft-ietf-calsify-rfc2445bis-10 (work
in progress), April 2009.
[RFC5397] Sanchez, W. and C. Daboo, "WebDAV
Current Principal Extension",
RFC 5397, December 2008.
Author's Address
Cyrus Daboo
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
USA
EMail: cyrus@daboo.name
URI: http://www.apple.com/
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