Locating CalDAV and CardDAV services
Author(s): Cyrus Daboo
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....
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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." 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. 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 Daboo Expires March 20, 2011 [Page 1] Internet-Draft SRV for CalDAV & CardDAV September 2010 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 Daboo Expires March 20, 2011 [Page 2] Internet-Draft SRV for CalDAV & CardDAV September 2010 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. Daboo Expires March 20, 2011 [Page 3] Internet-Draft SRV for CalDAV & CardDAV September 2010 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 Daboo Expires March 20, 2011 [Page 4] Internet-Draft SRV for CalDAV & CardDAV September 2010 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]. Daboo Expires March 20, 2011 [Page 5] Internet-Draft SRV for CalDAV & CardDAV September 2010 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. Daboo Expires March 20, 2011 [Page 6] Internet-Draft SRV for CalDAV & CardDAV September 2010 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 Daboo Expires March 20, 2011 [Page 7] Internet-Draft SRV for CalDAV & CardDAV September 2010 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 Daboo Expires March 20, 2011 [Page 8] Internet-Draft SRV for CalDAV & CardDAV September 2010 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]. Daboo Expires March 20, 2011 [Page 9] Internet-Draft SRV for CalDAV & CardDAV September 2010 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>  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. Daboo Expires March 20, 2011 [Page 10] Internet-Draft SRV for CalDAV & CardDAV September 2010 [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, RFC2119, March 1997. [RFC2368] Hoffman, P., Masinter, L., and J. Zawinski, "The mailto URL scheme", RFC2368, 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", RFC2616, 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", RFC2617, June 1999. [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC2782, February 2000. [RFC2818] Rescorla, E., "HTTP Over TLS", RFC2818, May 2000. [RFC3744] Clemm, G., Reschke, J., Sedlar, E., and J. Whitehead, "Web Distributed Authoring and Versioning (WebDAV) Access Control Protocol", Daboo Expires March 20, 2011 [Page 11] Internet-Draft SRV for CalDAV & CardDAV September 2010 RFC3744, May 2004. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC3986, January 2005. [RFC4791] Daboo, C., Desruisseaux, B., and L. Dusseault, "Calendaring Extensions to WebDAV (CalDAV)", RFC4791, March 2007. [RFC4918] Dusseault, L., "HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)", RFC4918, June 2007. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC5246, August 2008. [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC5322, October 2008. [RFC5397] Sanchez, W. and C. Daboo, "WebDAV Current Principal Extension", RFC5397, December 2008. [RFC5785] Nottingham, M. and E. Hammer- Lahav, "Defining Well-Known Uniform Resource Identifiers (URIs)", RFC5785, April 2010. 11.2. Informative References [RFC5545] Desruisseaux, B., "Internet Calendaring and Scheduling Core Object Specification (iCalendar)", RFC5545, September 2009. URIs  <http://www.dns-sd.org/ServiceTypes.html> Daboo Expires March 20, 2011 [Page 12] Internet-Draft SRV for CalDAV & CardDAV September 2010 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: Daboo Expires March 20, 2011 [Page 13] Internet-Draft SRV for CalDAV & CardDAV September 2010 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 RFC5785 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. Daboo Expires March 20, 2011 [Page 14] Internet-Draft SRV for CalDAV & CardDAV September 2010 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 RFC5545 reference. 4. Added reference to certificate verification spec. Author's Address Cyrus Daboo Apple Inc. 1 Infinite Loop Cupertino, CA 95014 USA EMail: firstname.lastname@example.org URI: http://www.apple.com/ Daboo Expires March 20, 2011 [Page 15]