Internet DRAFT - draft-daboo-srv-caldav
draft-daboo-srv-caldav
Network Working Group C. Daboo
Internet-Draft Apple Inc.
Updates: 4791,CardDAV-RFC-to-be September 16, 2010
(if approved)
Intended status: Standards Track
Expires: March 20, 2011
Locating CalDAV and CardDAV services
draft-daboo-srv-caldav-10
Abstract
This specification describes how DNS SRV records, DNS TXT records and
well-known URIs can be used together or separately to locate
Calendaring Extensions to WebDAV (CalDAV) or vCard Extensions to
WebDAV (CardDAV) services.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on March 20, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 4
3. CalDAV SRV Service Labels . . . . . . . . . . . . . . . . . . 4
4. CalDAV and CardDAV Service TXT Records . . . . . . . . . . . . 4
5. CalDAV and CardDAV Service Well-Known URI . . . . . . . . . . 5
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. caldav Well-Known URI Registration . . . . . . . . . . . . 10
9.2. carddav Well-Known URI Registration . . . . . . . . . . . 10
9.3. SRV Service Label Registration . . . . . . . . . . . . . . 10
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Change History (to be removed prior to
publication as an RFC) . . . . . . . . . . . . . . . 13
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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.
[I-D.ietf-vcarddav-carddav] 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 Resource Records as per
[I-D.cheshire-dnsext-dns-sd].
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 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 a
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 make use of. 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.
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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 ([RFC2818]).
_caldavs: Identifies a CalDAV server that uses HTTP with transport
layer security ([RFC2818]).
Clients MUST honor "Priority" and "Weight" values in the SRV RRs, 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 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
[I-D.cheshire-dnsext-dns-sd] 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 transport layer security
_caldavs._tcp TXT path=/caldav
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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, 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
location of the redirect might change over time, then a "no-cache"
value would be used.
To facilitate "context path's" 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 which the client can parse suitable information from. 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].
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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 "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.
+ If the calendar user address is an "http:" [RFC2616] or
"https:" [RFC2818] URI, the "userinfo" and "host" portion
of the URI [RFC3986] is extracted.
* 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 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-SSL
connections are appropriate.
* If an SRV record is returned, the client extracts the target
FQDN and port number. In the case of multiple SRV records
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 transport layer security.
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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 bootstrap procedure using the appropriate
".well-known" URI instead.
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 transport layer security negotiation is done
immediately upon connection. The client MUST do certificate
verification using the procedure outlined in Section 4 of
[I-D.saintandre-tls-server-id-check] 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
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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).
* 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 re-
use those when connecting again at a later time.
If a subsequent connection attempt fails, or authentication fails
persistently, clients SHOULD re-try 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 "user identifiers" extracted from them. In the
former case, the addresses MUST NOT conflict with other forms of
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
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corresponds to the principal-URL of the user making the request.
o If the service provider uses transport layer security, the service
provider MUST ensure a certificate is installed that can be
verified by clients using the procedure outlined in Section 4 of
[I-D.saintandre-tls-server-id-check] in regard to verification
with an SRV RR as the starting point.
o Install the appropriate SRV records for the offered services.
Optionally include TXT records.
8. 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, or 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 transport layer security is being used for the
service, clients MUST use the procedure outlined in Section 4 of
[I-D.saintandre-tls-server-id-check] to verify the service.
Implementations of TLS [RFC5246], used as the basis for transport
layer security ([RFC2818]), typically support multiple versions of
the protocol as well as the older Secure Sockets Layer (SSL)
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
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. caldav Well-Known URI Registration
URI suffix: caldav
Change controller: IETF.
Specification document(s): This RFC.
Related information: See also [RFC4791].
9.2. 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].
9.3. SRV Service Label Registration
Service labels have been registered according to
<http://www.dns-sd.org/ServiceTypes.html> [1] and will be
incorporated into IANA once a new registry is available there.
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
[I-D.cheshire-dnsext-dns-sd] Cheshire, S. and M. Krochmal,
"DNS-Based Service Discovery",
draft-cheshire-dnsext-dns-sd-06
(work in progress), March 2010.
[I-D.ietf-vcarddav-carddav] Daboo, C., "vCard Extensions to
WebDAV (CardDAV)",
draft-ietf-vcarddav-carddav-10
(work in progress),
November 2009.
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[I-D.saintandre-tls-server-id-check] Saint-Andre, P. and J. Hodges,
"Representation and
Verification of Domain-Based
Application Service Identity in
Certificates Used with
Transport Layer Security", draf
t-saintandre-tls-server-id-
check-09 (work in progress),
August 2010.
[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.
[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.
[RFC3744] Clemm, G., Reschke, J., Sedlar,
E., and J. Whitehead, "Web
Distributed Authoring and
Versioning (WebDAV)
Access Control Protocol",
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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.
11.2. Informative References
[RFC5545] Desruisseaux, B., "Internet
Calendaring and Scheduling Core
Object Specification
(iCalendar)", RFC 5545,
September 2009.
URIs
[1] <http://www.dns-sd.org/ServiceTypes.html>
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Appendix A. Change History (to be removed prior to publication as an
RFC)
Changes in -09:
1. IESG Review: minor editorial changes.
2. GenART Review: minor editorial changes.
3. GenART Review: "guideline" -> "procedure".
4. GenART Review: "port" -> "port number".
5. GenART Review: added definition of "context path".
6. GenART Review: clarified OPTIONAL nature of suggested client
procedure.
7. GenART Review: clarified that TXT lookup is an additional query.
8. IESG Review: now allow any HTTP redirect response, not just 301.
9. IESG Review: added text on cache interaction with redirect.
Changes in -10:
1. AD Review: make client procedure a SHOULD.
Changes in -08:
1. Clarify that email address is a valid input in Section 7 for
CardDAV.
2. Clarified aspects of DAV:current-user-principal handling for
servers.
3. Added additional text to indicate TXT being used in abstract and
introduction.
Changes in -07:
1. Add password to required minimal user input
2. Section 3 -> Section 4 of server-id check draft.
Changes in -06:
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1. Last call comments: Revised title, abstract and text to indicate
that SRV and .well-known can be done separately.
2. Revised IANA section to use dns-sd registry for now.
3. Added optional TXT RR with path key for service context path in
the DNS
4. Re-organized client bootstrap to take account of TXT and to call-
out the different "phases" involved via a numbered list.
Changes in -05:
1. AD Review: Added "Updates" for 4791 and CardDAV.
2. AD Review: Changed SHOULD to MUST for honoring priority and
weight.
3. AD Review: Added additional reference to 3986 when talking about
userinfo/host portions of the URI.
4. AD Review: Changed section reference for tls-server-id-check
draft.
5. AD Review: Changed should to SHOULD when describing PROPFIND
request and made 5397 normative.
6. AD Review: Made 3744 and 5322 normative references.
7. AD Review: Added IANA SRV registration request.
Changes in -04:
1. Added addition text to client guidelines indicating that clients
cache the discovery details and can re-do discovery if
connections later fail.
2. Changed principal-URI to principal-URL.
Changes in -03:
1. Updated to RFC 5785 reference.
2. Added SSL v2 restriction from srv-email document added after IESG
review.
3. Tweaked client/server guidelines to better match HTTP challenge/
response authentication mechanism.
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Changes in -02:
1. Re-organized introduction.
2. Brought terminology into line with srv-email document which has
been through last call.
3. Brought security section into line with srv-email document which
has been through last call.
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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