Internet Engineering Task Force (IETF) C. Daboo

Internet Engineering Task Force (IETF) C. Daboo
Request for Comments: 6764 Apple Inc.
Updates: 4791, 6352 February 2013
Category: Standards Track
ISSN: 2070-1721

Locating Services for Calendaring Extensions to
WebDAV (CalDAV) and vCard Extensions to WebDAV (CardDAV)

Abstract

This specification describes how DNS SRV records, DNS TXT records,
and well-known URIs can be used together or separately to locate
CalDAV (Calendaring Extensions to Web Distributed Authoring and
Versioning (WebDAV)) or CardDAV (vCard Extensions to WebDAV)
services.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6764.

Copyright Notice

Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

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Table of Contents

1. Introduction ....................................................2
2. Conventions Used in This Document ...............................3
3. CalDAV SRV Service Labels .......................................3
4. CalDAV and CardDAV Service TXT Records ..........................4
5. CalDAV and CardDAV Service Well-Known URI .......................4
5.1. Example: Well-Known URI Redirects to Actual
"Context Path" .............................................5
6. Client "Bootstrapping" Procedures ...............................5
7. Guidance for Service Providers ..................................8
8. Security Considerations .........................................9
9. IANA Considerations .............................................9
9.1. Well-Known URI Registrations ...............................9
9.1.1. caldav Well-Known URI Registration .................10
9.1.2. carddav Well-Known URI Registration ................10
9.2. Service Name Registrations ................................10
9.2.1. caldav Service Name Registration ...................10
9.2.2. caldavs Service Name Registration ..................11
9.2.3. carddav Service Name Registration ..................11
9.2.4. carddavs Service Name Registration .................12
10. Acknowledgments ...............................................12
11. References ....................................................12
11.1. Normative References .....................................12
11.2. Informative References ...................................14

1. Introduction

[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 the fully
qualified domain name (FQDN) and port number for their servers.

[RFC6352] defines the CardDAV address book 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.

[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 DNS SRV Resource Records
(RRs). This has been enhanced to provide additional service meta-
data by use of DNS TXT RRs as per [RFC6763].

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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
[RFC6352].

Another issue with CalDAV or CardDAV service discovery is that the
service might not be located at the "root" URI of the HTTP server
hosting it. Thus, a client needs to be able to determine the
complete path component of the Request-URI to use in HTTP requests:
the "context path". 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 RRs by themselves only provide an
FQDN and port number 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
[RFC5785] of HTTP servers to provide a well-known URI for CalDAV or
CardDAV services that clients can use. The well-known URI will point
to a resource on the server that is simply a "stub" resource that
provides a redirect to the actual "context path" resource
representing the service endpoint.

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 Labels

This specification adds two SRV service labels for use with CalDAV:

_caldav: Identifies a CalDAV server that uses HTTP without
Transport Layer Security (TLS) [RFC2818].

_caldavs: Identifies a CalDAV server that uses HTTP with TLS
[RFC2818].

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Clients MUST honor Priority and Weight values in the SRV RRs, as
described by [RFC2782].

Example: service record for server without TLS

_caldav._tcp SRV 0 1 80 calendar.example.com.

Example: service record for server with TLS

_caldavs._tcp SRV 0 1 443 calendar.example.com.

4. CalDAV and CardDAV Service TXT Records

When SRV RRs are used to advertise CalDAV and CardDAV services, it is
also convenient to be able to specify a "context path" in the DNS to
be retrieved at the same time. To enable that, this specification
uses a TXT RR that follows the syntax defined in Section 6 of
[RFC6763] and defines a "path" key for use in that record. The value
of the key MUST be the actual "context path" to the corresponding
service on the server.

A site might provide TXT records in addition to SRV records for each
service. When present, clients MUST use the "path" value as the
"context path" for the service in HTTP requests. When not present,
clients use the ".well-known" URI approach described next.

Example: text record for service with TLS

_caldavs._tcp TXT path=/caldav

5. 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 9). These URIs point to a resource that the client can use
as the initial "context path" for the service they are trying to
connect to. The server MUST redirect HTTP requests for that resource
to the actual "context path" using one of the available mechanisms
provided by HTTP (e.g., using a 301, 303, or 307 response). Clients
MUST handle HTTP redirects on the ".well-known" URI. Servers MUST
NOT locate the actual CalDAV or CardDAV service endpoint at the
".well-known" URI as per Section 1.1 of [RFC5785].

Servers SHOULD set an appropriate Cache-Control header value (as per
Section 14.9 of [RFC2616]) in the redirect response to ensure caching
occurs or does not occur as needed or as required by the type of
response generated. For example, if it is anticipated that the

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location of the redirect might change over time, then a "no-cache"
value would be used.

To facilitate "context paths" that might differ from user to user,
the server MAY require authentication when a client tries to access
the ".well-known" URI (i.e., the server would return a 401 status
response to the unauthenticated request from the client, then return
the redirect response only after a successful authentication by the
client).

5.1. 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 FQDN and port
number via an SRV lookup or via manual entry of information by the
user, from which the client can parse suitable information. When the
client makes an HTTP request against "/.well-known/caldav", the
server would issue an HTTP 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.

6. Client "Bootstrapping" Procedures

This section describes a procedure that CalDAV or CardDAV clients
SHOULD use to do their initial configuration based on minimal user
input. The goal is to determine an http: or https: URI that
describes the full path to the user's principal-URL [RFC3744].

1. Processing user input:

* For a CalDAV server:

+ Minimal input from a user would consist of a calendar user
address and a password. 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 "bootstrapping" configuration can be done.

+ If the calendar user address is a "mailto:" [RFC6068] URI,
the "mailbox" portion of the URI is examined, and the
"local-part" and "domain" portions are extracted.

+ If the calendar user address is an "http:" [RFC2616] or
"https:" [RFC2818] URI, the "userinfo" and "host" portion
of the URI [RFC3986] is extracted.

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* 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, and a password. The "mailbox" portion of the
email address is examined, and the "local-part" and
"domain" portions are extracted.

2. Determination of service FQDN and port number:

* An SRV lookup for _caldavs._tcp (for CalDAV) or _carddavs._tcp
(for CardDAV) is done with the extracted "domain" as the
service domain.

* If no result is found, the client can try _caldav._tcp (for
CalDAV) or _carddav._tcp (for CardDAV) provided non-TLS
connections are appropriate.

* If an SRV record is returned, the client extracts the target
FQDN and port number. If multiple SRV records are returned,
the client MUST use the Priority and Weight fields in the
record to determine which one to pick (as per [RFC2782]).

* If an SRV record is not found, the client will need to prompt
the user to enter the FQDN and port number information
directly or use some other heuristic, for example, using the
extracted "domain" as the FQDN and default HTTPS or HTTP port
numbers. In this situation, clients MUST first attempt an
HTTP connection with TLS.

3. Determination of initial "context path":

* When an SRV lookup is done and a valid SRV record returned,
the client MUST also query for a corresponding TXT record and
check for the presence of a "path" key in its response. If
present, the value of the "path" key is used for the initial
"context path".

* When an initial "context path" has not been determined from a
TXT record, the initial "context path" is taken to be
"/.well-known/caldav" (for CalDAV) or "/.well-known/carddav"
(for CardDAV).

* If the initial "context path" derived from a TXT record
generates HTTP errors when targeted by requests, the client
SHOULD repeat its "bootstrapping" procedure using the
appropriate ".well-known" URI instead.

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4. Determination of user identifier:

* The client will need to make authenticated HTTP requests to
the service. Typically, a "user identifier" is required for
some form of user/password authentication. When a user
identifier is required, clients MUST first use the "mailbox"
portion of the calendar user address provided by the user in
the case of a "mailto:" address and, if that results in an
authentication failure, SHOULD fall back to using the "local-
part" extracted from the "mailto:" address. For an "http:" or
"https:" calendar user address, the "userinfo" portion is used
as the user identifier for authentication. This is in line
with the guidance outlined in Section 7. If these user
identifiers result in authentication failure, the client
SHOULD prompt the user for a valid identifier.

5. Connecting to the service:

* Subsequent to configuration, the client will make HTTP
requests to the service. When using "_caldavs" or "_carddavs"
services, a TLS negotiation is done immediately upon
connection. The client MUST do certificate verification using
the procedure outlined in Section 6 of [RFC6125] in regard to
verification with an SRV RR as the starting point.

* The client does a "PROPFIND" [RFC4918] request with the
request URI set to the initial "context path". 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. The server will use the HTTP authentication
procedure outlined in [RFC2617] or use some other appropriate
authentication schemes to authenticate the user.

* If the server returns a 404 ("Not Found") HTTP status response
to the request on the initial "context path", clients MAY try
repeating the request on the "root" URI "/" or prompt the user
for a suitable path.

* If the DAV:current-user-principal property is returned on the
request, the client uses that value for the principal-URL of
the authenticated user. With that, it can execute a
"PROPFIND" request on the principal-URL and discover
additional properties for configuration (e.g., calendar or
address book "home" collections).

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* If the DAV:current-user-principal property is not returned,
then the client will need to request the principal-URL path
from the user in order to continue with configuration.

Once a successful account discovery step has been done, clients
SHOULD cache the service details that were successfully used (user
identity, principal-URL with full scheme/host/port details) and reuse
those when connecting again at a later time.

If a subsequent connection attempt fails, or authentication fails
persistently, clients SHOULD retry the SRV lookup and account
discovery to "refresh" the cached data.

7. Guidance for Service Providers

Service providers wanting to offer CalDAV or CardDAV services that
can be configured by clients using SRV records need to follow certain
procedures to ensure proper operation.

o CalDAV or CardDAV servers SHOULD be configured to allow
authentication with calendar user addresses (just taking the
"mailbox" portion of any "mailto:" URI) or email addresses
respectively, or with "user identifiers" extracted from them. In
the former case, the addresses MUST NOT conflict with other forms
of a permitted user login name. In the latter case, the extracted
"user identifiers" need to be unique across the server and MUST
NOT conflict with any login name on the server.

o Servers MUST force authentication for "PROPFIND" requests that
retrieve the DAV:current-user-principal property to ensure that
the value of the DAV:current-user-principal property returned
corresponds to the principal-URL of the user making the request.

o If the service provider uses TLS, the service provider MUST ensure
a certificate is installed that can be verified by clients using
the procedure outlined in Section 6 of [RFC6125] in regard to
verification with an SRV RR as the starting point. In particular,
certificates SHOULD include SRV-ID and DNS-ID identifiers as
appropriate, as described in Section 8.

o Service providers should install the appropriate SRV records for
the offered services and optionally include TXT records.

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8. Security Considerations

Clients that support TLS 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 TLS, the client MUST NOT use any service
information returned for the "_caldav" or "_carddav" services.
Clients MUST follow the certificate-verification process specified in
[RFC6125].

A malicious attacker with access to the DNS server data, or that is
able to get spoofed answers cached in a recursive resolver, 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 target FQDN returned in the SRV record matches the
original service domain that was queried. If the target FQDN is not
in the queried domain, clients SHOULD verify with the user that the
SRV target FQDN is suitable for use before executing any connections
to the host. Alternatively, if TLS is being used for the service,
clients MUST use the procedure outlined in Section 6 of [RFC6125] to
verify the service. When the target FQDN does not match the original
service domain that was queried, clients MUST check the SRV-ID
identifier in the server's certificate. If the FQDN does match,
clients MUST check any SRV-ID identifiers in the server's certificate
or, if no SRV-ID identifiers are present, MUST check the DNS-ID
identifiers in the server's certificate.

Implementations of TLS [RFC5246], used as the basis for TLS
([RFC2818]), typically support multiple versions of the protocol as
well as the older SSL (Secure Sockets Layer) protocol. Because of
known security vulnerabilities, clients and servers MUST NOT request,
offer, or use SSL 2.0. See Appendix E.2 of [RFC5246] for further
details.

9. IANA Considerations

9.1. Well-Known URI Registrations

This document defines two ".well-known" URIs using the registration
procedure and template from Section 5.1 of [RFC5785].

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9.1.1. caldav Well-Known URI Registration

URI suffix: caldav

Change controller: IETF

Specification document(s): This RFC

Related information: See also [RFC4791].

9.1.2. carddav Well-Known URI Registration

URI suffix: carddav

Change controller: IETF

Specification document(s): This RFC

Related information: See also [RFC6352].

9.2. Service Name Registrations

This document registers four new service names as per [RFC6335]. Two
are defined in this document, and two are defined in [RFC6352],
Section 11.

9.2.1. caldav Service Name Registration

Service Name: caldav

Transport Protocol(s): TCP

Assignee: IESG <[email protected]>

Contact: IETF Chair <[email protected]>

Description: Calendaring Extensions to WebDAV (CalDAV) - non-TLS

Reference: [RFC6764]

Assignment Note: This is an extension of the http service. Defined
TXT keys: path=<context path>

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9.2.2. caldavs Service Name Registration

Service Name: caldavs

Transport Protocol(s): TCP

Assignee: IESG <[email protected]>

Contact: IETF Chair <[email protected]>

Description: Calendaring Extensions to WebDAV (CalDAV) - over TLS

Reference: [RFC6764]

Assignment Note: This is an extension of the https service. Defined
TXT keys: path=<context path>

9.2.3. carddav Service Name Registration

Service Name: carddav

Transport Protocol(s): TCP

Assignee: IESG <[email protected]>

Contact: IETF Chair <[email protected]>

Description: vCard Extensions to WebDAV (CardDAV) - non-TLS

Reference: [RFC6352]

Assignment Note: This is an extension of the http service. Defined
TXT keys: path=<context path>

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RFC 6764 SRV for CalDAV & CardDAV February 2013

9.2.4. carddavs Service Name Registration

Service Name: carddavs

Transport Protocol(s): TCP

Assignee: IESG <[email protected]>

Contact: IETF Chair <[email protected]>

Description: vCard Extensions to WebDAV (CardDAV) - over TLS

Reference: [RFC6352]

Assignment Note: This is an extension of the https service. Defined
TXT keys: path=<context path>

10. Acknowledgments

This specification was suggested by discussion that took place within
the Calendaring and Scheduling Consortium's CalDAV Technical
Committee. The author thanks the following for their contributions:
Stuart Cheshire, Bernard Desruisseaux, Eran Hammer-Lahav, Helge Hess,
Arnaud Quillaud, Wilfredo Sanchez, and Joe Touch.

11. References

11.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[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.

[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, 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.

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[RFC3744] Clemm, G., Reschke, J., Sedlar, E., and J. Whitehead, "Web
Distributed Authoring and Versioning (WebDAV)
Access Control Protocol", RFC 3744, May 2004.

[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.

[RFC4918] Dusseault, L., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918, June 2007.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.

[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.

[RFC5397] Sanchez, W. and C. Daboo, "WebDAV Current Principal
Extension", RFC 5397, December 2008.

[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785,
April 2010.

[RFC6068] Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto'
URI Scheme", RFC 6068, October 2010.

[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011.

[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, August 2011.

[RFC6352] Daboo, C., "CardDAV: vCard Extensions to Web Distributed
Authoring and Versioning (WebDAV)", RFC 6352, August 2011.

[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, February 2013.

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11.2. Informative References

[RFC5545] Desruisseaux, B., "Internet Calendaring and Scheduling
Core Object Specification (iCalendar)", RFC 5545,
September 2009.

Author's Address

Cyrus Daboo
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
USA

EMail: [email protected]
URI: http://www.apple.com/

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