Network Working Group C. Daboo Internet-Draft Apple Inc. Updates: 4791,XXXX-CardDAV June 18, 2010 (if approved) Intended status: Standards Track Expires: December 20, 2010 Use of SRV records for locating CalDAV and CardDAV services draft-daboo-srv-caldav-05 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. Status of This Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 20, 2010. 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 Daboo Expires December 20, 2010 [Page 1]
Internet-Draft SRV for CalDAV June 2010 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 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 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. Client "Bootstrapping" Guidelines . . . . . . . . . . . . . . 5 6. Guidance for Service Providers . . . . . . . . . . . . . . . . 7 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 8.1. caldav Well-Known URI Registration . . . . . . . . . . . . 8 8.2. carddav Well-Known URI Registration . . . . . . . . . . . 8 8.3. SRV Service Label Registration . . . . . . . . . . . . . . 9 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 10.1. Normative References . . . . . . . . . . . . . . . . . . . 9 10.2. Informative References . . . . . . . . . . . . . . . . . . 11 Appendix A. Change History (to be removed prior to publication as an RFC) . . . . . . . . . . . . . . . 11 Daboo Expires December 20, 2010 [Page 2]
Internet-Draft SRV for CalDAV June 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 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. [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 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 RRs by themselves only provide a FQDN 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 [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 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". Daboo Expires December 20, 2010 [Page 3]
Internet-Draft SRV for CalDAV June 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 Well-Known URI Two "well-known" URIs are registered by this specification for CalDAV and CardDAV services, "caldav" and "carddav" respectively (see Section 8). These URIs point to a resource that the client can use as the "context path" for the service they are trying to connect to. 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 FQDN and port via an SRV lookup. Daboo Expires December 20, 2010 [Page 4]
Internet-Draft SRV for CalDAV June 2010 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 FQDN and port via an SRV lookup. When the client makes its initial HTTP request against "/.well-known/caldav", the server would issue an HTTP 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. The goal is to determine an http: or https: URI that describes the full path to the user's principal-URL [RFC3744]. 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. 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. The "mailbox" portion of the email address is examined and the "local-part" and "domain" portions extracted. Once the user input has been processed, the following is done: o An SRV lookup for _caldavs._tcp (for CalDAV) or _carddavs._tcp (for CardDAV) is done with the user provided "domain" as the service domain. o 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. Daboo Expires December 20, 2010 [Page 5]
Internet-Draft SRV for CalDAV June 2010 o If an SRV record is returned, the client extracts the target FQDN and port number. If an SRV record is not found, the client will need to prompt the user to enter the FQDN and port information directly, or use some other heuristic. 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]). o 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. Certificate verification MUST use the procedure outlined in Section 3 of [I-D.saintandre-tls-server-id-check] in regard to verification with an SRV RR as the starting point. o 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 6. If these user identifiers result in authentication failure, the client SHOULD prompt the user for a valid identifier. o The client does an initial "PROPFIND" [RFC4918] request with a 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. The server will need to authenticate the user for the initial request and MAY follow the HTTP authentication procedure outlined in [RFC2617] or use other appropriate authentication schemes. o 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 "/" or prompt the user for a suitable path. o If the DAV:current-user-principal property is returned on the initial 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" Daboo Expires December 20, 2010 [Page 6]
Internet-Draft SRV for CalDAV June 2010 collections). o 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 have been done, clients SHOULD cache the service details (user identity, principal-URL with full scheme/host/port details) that were successfully used, 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. 6. 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 guidelines to ensure proper operation. o CalDAV or CardDAV servers SHOULD be configured to allow authentication with calendar user addresses (just the "mailbox" portion of a "mailto:" URI) 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 the initial client request ("PROPFIND" with a DAV:current-user-principal property requested) to ensure that the value of DAV:current-user-principal property returned 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 3 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. 7. 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 Daboo Expires December 20, 2010 [Page 7]
Internet-Draft SRV for CalDAV June 2010 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 3 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. 8. IANA Considerations This document defines two "well-known" URIs using the registration procedure and template from Section 5.1 of [RFC5785]. 8.1. caldav Well-Known URI Registration URI suffix: caldav Change controller: IETF. Specification document(s): This RFC. Related information: See also [RFC4791]. 8.2. carddav Well-Known URI Registration URI suffix: carddav Change controller: IETF. Daboo Expires December 20, 2010 [Page 8]
Internet-Draft SRV for CalDAV June 2010 Specification document(s): This RFC. Related information: See also [I-D.ietf-vcarddav-carddav]. 8.3. SRV Service Label Registration IANA is requested to add "caldav" and "caldavs" service labels as aliases for "http" and "https" respectively. 9. 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: Bernard Desruisseaux, Helge Hess, Arnaud Quillaud and Wilfredo Sanchez. 10. References 10.1. Normative References [I-D.ietf-vcarddav-carddav] Daboo, C., "vCard Extensions to WebDAV (CardDAV)", draft-ietf-vcarddav-carddav-10 (work in progress), November 2009. [I-D.saintandre-tls-server-id-check] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Server Identity in Certificates Used with Transport Layer Security", draf t-saintandre-tls-server-id- check-06 (work in progress), June 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., Daboo Expires December 20, 2010 [Page 9]
Internet-Draft SRV for CalDAV June 2010 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", 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", Daboo Expires December 20, 2010 [Page 10]
Internet-Draft SRV for CalDAV June 2010 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. 10.2. Informative References [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 -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: Daboo Expires December 20, 2010 [Page 11]
Internet-Draft SRV for CalDAV June 2010 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. 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/ Daboo Expires December 20, 2010 [Page 12]
