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Applications 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.

defines a DNS-based service discovery protocol that has been widely adopted as a means of locating particular services within a local area network and beyond, using SRV RR records. defines the CalDAV Calendar Access protocol, based on HTTP , for accessing calendar data stored on a server. CalDAV clients need to be able to discover appropriate CalDAV servers within their local area network and at other domains, e.g., to minimize the need for end users to know specific details such as hostname and port for their servers. defines the CardDAV vCard Access protocol based on HTTP , for accessing contact data stored on a server. As with CalDAV, clients also need to be able to discover CardDAV servers. This specification defines new SRV service types for the CalDAV protocol, and gives an example of how clients can use this together with other protocol features to enable simple client configuration. SRV service types for CardDAV are already defined in Section 11 of . Another issue with CalDAV or CardDAV service discovery is that the service may not be located at the "root" URI of the HTTP server hosting it. For example, if CalDAV is implemented as a "servlet" in a web server "container", the servlet "context path" might be "/caldav/". So the URI for the CalDAV service would be, e.g., "http://caldav.example.com/caldav/" rather than "http://caldav.example.com/". SRV records by themselves only provide a hostname and port for the service, not a path. Since the client "bootstrapping" process requires initial access to the "context path" of the service, there needs to be a simple way for clients to also discover what that path is. This specification makes use of the "well known URI" feature 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".

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 .

This specification adds two service types for use with SRV records: Identifies a CalDAV server that uses HTTP without transport layer security (). Identifies a CalDAV server that uses HTTP with transport layer security (). Clients SHOULD honor "TTL", "Priority" and "Weight" values in the SRV records, as described by .

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.

Two "well-known" URIs are registered by this specification for CalDAV and CardDAV services, "caldav" and "carddav" respectively (see ). These URIs point to a resource that the client can use as the "context path" for the service they are trying to use. The actual service could be located at that specific path. Alternatively the server MAY redirect HTTP requests for that resource (using the "301 Moved Permanently" status response) to the actual "context path". Clients MUST handle HTTP redirects on the well-known URI.

A CalDAV server has a "context path" that is the same as the well-known URI, so the client will use "/.well-known/caldav" as the path for its "bootstrapping" process after it has first found the hostname and port via an SRV lookup.

A CalDAV server has a "context path" that is "/servlet/caldav". The client will use "/.well-known/caldav" as the path for its "bootstrapping" process after it has first found the hostname and port via an SRV lookup. When the client makes its initial HTTP request against "/.well-known/caldav", the server would issue an HTTP 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.

This section describes a procedure that CalDAV or CardDAV clients MAY use to do their initial configuration based on minimal user input. For a CalDAV server, minimal input from a user would consist of a calendar user address. A calendar user address is defined by iCalendar to be a URI. 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:" URI, the "mailbox" portion of the URI is examined and the "local-part" and "domain" portions extracted. The "local-part" is used for the user identifier and the "domain" is used as the service domain. If the calendar user address is an "http:" or "https:" URI, the "userinfo" and "host" portion of the URI is extracted. The "userinfo" is used for the user identifier and the "host" is used as the service domain. For a CardDAV server, minimal input from a user would consist of their email address for the domain where the CardDAV service is hosted. The "mailbox" portion of the email address is examined and the "local-part" and "domain" portions extracted. The "local-part" is used for the user identifier and the "domain" is used as the service domain. Once the user identifier and service domain have been extracted, the following is done: An SRV lookup for _caldavs._tcp (for CalDAV) or _carddavs._tcp (for CardDAV) is done against the extracted service domain. 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. If an SRV record is returned the client extracts the server host name and port number. The client does an authenticated "PROPFIND" request using the user identifier, host name and port number and 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 property as one of the properties to return. Note that clients MUST properly handle HTTP redirect responses for the request. 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 "/". If the DAV:current-user-principal property is returned on the initial request, the client uses that value for the principal-URI of the authenticated user. With that, it can do a "PROPFIND" on the principal-URI and discover additional properties for configuration. If the DAV:current-user-principal property is not returned, then the client will need to request the principal-URI path from the user in order to continue with configuration.

Clients that support transport layer security as defined by 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 . A malicious attacker with access to the DNS server data can potentially cause clients to connect to any server chosen by the attacker. In the absence of a secure DNS option, clients SHOULD check that the host name returned in the SRV record matches the original service domain that was queried. If the host is not in the queried domain, clients SHOULD verify with the user that the SRV host name is suitable for use before executing any CalDAV or CardDAV requests against the host.

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

caldav IETF. This RFC. See also .

carddav IETF. This RFC. See also .

This specification was suggested by discussion that took place within the Calendaring and Scheduling Consortium's CalDAV Technical Committee. The authors thank the participants of that group for their input. The "bootstrapping" process is based on diagrams developed by Wilfredo Sanchez.

&rfc2119; &rfc2368; &rfc2616; &rfc2782; &rfc2818; &rfc3986; &rfc4791; &idSiteMeta; &idCardDAV; &idCertCheck; &rfc5322; &rfc5397; &rfc5545;

Changes in -01: Added discovery of CardDAV service. Now makes use of well-known URIs for the service "context path". Updated to RFC 5545 reference. Added reference to certificate verification spec.