Network Working Group C. Daboo
Internet-Draft Apple Inc.
Intended status: Standards Track October 4, 2009
Expires: April 7, 2010
Use of SRV records for locating CalDAV and CardDAV services
draft-daboo-srv-caldav-01
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Abstract
This specification describes how SRV records and well-known URIs can
be used to locate Calendaring Extensions to WebDAV (CalDAV) or vCard
Extensions to WebDAV (CardDAV) services.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . . 3
3. CalDAV SRV Service Types . . . . . . . . . . . . . . . . . . . 4
4. CalDAV and CardDAV Service Well-Known URI . . . . . . . . . . . 4
4.1. Example: well-known URI as context path . . . . . . . . . . 4
4.2. Example: well-known URI redirects to actual context
path . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Client "Bootstrapping" Guidelines . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7.1. caldav Well-Known URI Registration . . . . . . . . . . . . 7
7.2. carddav Well-Known URI Registration . . . . . . . . . . . . 7
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . . 8
Appendix A. Change History (to be removed prior to
publication as an RFC) . . . . . . . . . . . . . . . . 9
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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, e.g., to minimize the
need for end users to know specific details such as hostname and port
for their servers.
[I-D.ietf-vcarddav-carddav] defines the CardDAV vCard Access protocol
based on HTTP [RFC2616], for accessing contact data stored on a
server. As with CalDAV, clients also need to be able to discover
CardDAV servers.
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.
SRV service types for CardDAV are already defined in Section 11 of
[I-D.ietf-vcarddav-carddav].
Another issue with CalDAV or CardDAV service discovery is that the
service may not be located at the "root" URI of the HTTP server
hosting it. For example, if CalDAV is implemented as a "servlet" in
a web server "container", the servlet "context path" might be
"/caldav/". So the URI for the CalDAV service would be, e.g.,
"http://caldav.example.com/caldav/" rather than
"http://caldav.example.com/". SRV records by themselves only provide
a hostname and port for the service, not a path. Since the client
"bootstrapping" process requires initial access to the "context path"
of the service, there needs to be a simple way for clients to also
discover what that path is.
This specification makes use of the "well known URI" feature
[I-D.nottingham-site-meta] of HTTP servers to provide a well known
URI for CalDAV or CardDAV services that clients can make use of. The
well known URI will point to a resource on the server that might be
the actual "context path" of the CalDAV or CardDAV service, or it
might simply be a "stub" resource that provides a redirect to the
actual "context path".
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
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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 and CardDAV Service Well-Known URI
Two "well-known" URIs are registered by this specification for CalDAV
and CardDAV services, "caldav" and "carddav" respectively (see
Section 7). These URIs point to a resource that the client can use
as the "context path" for the service they are trying to use. The
actual service could be located at that specific path. Alternatively
the server MAY redirect HTTP requests for that resource (using the
"301 Moved Permanently" status response) to the actual "context
path". Clients MUST handle HTTP redirects on the well-known URI.
4.1. Example: well-known URI as context path
A CalDAV server has a "context path" that is the same as the well-
known URI, so the client will use "/.well-known/caldav" as the path
for its "bootstrapping" process after it has first found the hostname
and port via an SRV lookup.
4.2. Example: well-known URI redirects to actual context path
A CalDAV server has a "context path" that is "/servlet/caldav". The
client will use "/.well-known/caldav" as the path for its
"bootstrapping" process after it has first found the hostname and
port via an SRV lookup. When the client makes its initial HTTP
request against "/.well-known/caldav", the server would issue an HTTP
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301 redirect response with a Location response header using the path
"/servlet/caldav". The client would then "follow" this redirect to
the new resource and continue making HTTP requests there to complete
its "bootstrapping" process.
5. Client "Bootstrapping" Guidelines
This section describes a procedure that CalDAV or CardDAV clients MAY
use to do their initial configuration based on minimal user input.
For a CalDAV server, minimal input from a user would consist of a
calendar user address. A calendar user address is defined by
iCalendar [RFC5545] 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.
For a CardDAV server, minimal input from a user would consist of
their email address [RFC5322] for the domain where the CardDAV
service is hosted. The "mailbox" portion of the email address 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.
Once the user identifier and service domain have been extracted, the
following is done:
1. An SRV lookup for _caldavs._tcp (for CalDAV) or _carddavs._tcp
(for CardDAV) is done against the extracted service domain.
2. If no result is found for that, the client can try _caldav._tcp
(for CalDAV) or _carddav._tcp (for CardDAV) 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
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request URI of "/.well-known/caldav" (for CalDAV) or "/.well-
known/carddav" (for CardDAV). 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 responses for the request.
5. If the server returns a 404 Not Found HTTP status response to the
request on the well-known URI, clients MAY try repeating the
request on the "root" URI "/".
6. If the DAV:current-user-principal property is returned on the
initial request, the client uses that value for the principal-URI
of the authenticated user. With that, it can do a "PROPFIND" on
the principal-URI and discover additional properties for
configuration.
7. 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.
6. Security Considerations
Clients that support transport layer security as defined by [RFC2818]
SHOULD try the "caldavs" or "carddavs" services first before trying
the "caldav" or "carddav" services respectively. If a user has
explicitly requested a connection with transport layer security, the
client MUST NOT use any service information returned for the "caldav"
or "carddav" services. Clients MUST follow the certificate
verification process specified in
[I-D.saintandre-tls-server-id-check].
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 or CardDAV
requests against the host.
7. IANA Considerations
This document defines two "well-known" URIs using the registration
procedure and template from Section 5.1 of
[I-D.nottingham-site-meta].
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7.1. caldav Well-Known URI Registration
URI suffix: caldav
Change controller: IETF.
Specification document(s): This RFC.
Related information: See also [RFC4791].
7.2. carddav Well-Known URI Registration
URI suffix: carddav
Change controller: IETF.
Specification document(s): This RFC.
Related information: See also [I-D.ietf-vcarddav-carddav].
8. 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.
9. References
9.1. Normative References
[I-D.ietf-vcarddav-carddav] Daboo, C., "vCard Extensions to
WebDAV (CardDAV)",
draft-ietf-vcarddav-carddav-09
(work in progress),
September 2009.
[I-D.nottingham-site-meta] Nottingham, M. and E. Hammer-
Lahav, "Defining Well-Known
URIs",
draft-nottingham-site-meta-03
(work in progress),
September 2009.
[I-D.saintandre-tls-server-id-check] Saint-Andre, P., Zeilenga, K.,
and J. Hodges, "Server Identity
Verification in Application
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Protocols", draft-saintandre-
tls-server-id-check-02 (work in
progress), October 2009.
[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",
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.
9.2. Informative References
[RFC5322] Resnick, P., Ed., "Internet
Message Format", RFC 5322,
October 2008.
[RFC5397] Sanchez, W. and C. Daboo,
"WebDAV Current Principal
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Extension", RFC 5397,
December 2008.
[RFC5545] Desruisseaux, B., "Internet
Calendaring and Scheduling Core
Object Specification
(iCalendar)", RFC 5545,
September 2009.
Appendix A. Change History (to be removed prior to publication as an
RFC)
Changes in -01:
1. Added discovery of CardDAV service.
2. Now makes use of well-known URIs for the service "context path".
3. Updated to RFC 5545 reference.
4. Added reference to certificate verification spec.
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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