A user may concurrently possess more than one set of DACS credentials during a session, with each representing a different identity. Zero or more credentials may be submitted with a request for a DACS-wrapped web service. It is sometimes desirable or necessary for a user to switch between identities, or to be considered unauthenticated. Middleware (software situated between a user agent and a DACS-capable web server) and more sophisticated user agents might provide this functionality simply by sending some DACS HTTP cookies and not sending others, under user control. With standard browsers or in other situations where this functionality is not available, achieving this by repeatedly authenticating and signing off (or by manually deleting cookies) would be inconvenient at best.

The dacs_select_credentials web service can be used to temporarily disable credentials, leaving the remaining credentials selected for access control purposes. The user agent continues to send all DACS HTTP cookies as usual, but m[blue]dacs_acs(8)m[][2] will ignore disabled identities before deciding to grant or deny access. This feature can be used to work around the maximum number of identities that DACS allows to be associated with a request - determined by the m[blue]ACS_CREDENTIALS_LIMITm[][3] directive - or for administrative, testing, or other reasons. There are similarities between dacs_select_credentials and m[blue]su(1)m[][4].

A selected identity is handled normally, but a disabled identity is "hidden"; it is not considered for access control purposes and is not reported by m[blue]dacs_current_credentials(8)m[][5]. A disabled identity may be re-enabled by dacs_select_credentials, however, and m[blue]dacs_signout(8)m[][6] will work with disabled identities. All identities are considered for the purposes of revoking access, however, and in other situations described below.

The selected credentials are identified by a cryptographically protected cookie that is issued by dacs_select_credentials. The HTTP cookie name has the following format:

DACS:Federation-Name:::SELECTED

where Federation-Name is the official name assigned to the federation for which the cookie is valid. This cookie confers no identity or access control rights to its possessor. If this cookie is deleted, or just not sent with a request, all credentials accompanying the request are used for access control. If m[blue]dacs_signout(8)m[][6] asks the browser to delete all credentials (i.e., no more credentials exist that dacs_signout is aware of), it will also ask the browser to delete the selected credentials cookie.

The FORMAT argument (see m[blue]dacs(1)m[][7]) determines the type of output, with the default being HTML, using the style sheet m[blue]dacs_select_credentials.cssm[][8]. If XML output is selected, a document conforming to m[blue]dacs_select_credentials.dtdm[][9] is returned.