Provided by: dacs_1.4.40-2_amd64 
NAME
dacs_notices - DACS notice presentation and acknowledgement handler
SYNOPSIS
dacs_notices [dacsoptions[1]]
DESCRIPTION
This web service is part of the DACS suite.
Some web service providers have a requirement that users must acknowledge a notice of some
sort before access can be granted to an associated resource. A user attempting to access
such a resource is shown a web page containing the notice and asked to acknowledge it or
accept its conditions, typically by pressing an "I Accept" button on the web page. These
notices are commonly legal notices, such as copyright notices, licensing notices,
restricted access notices, and terms-of-use notices. This can also be applied to alert
users to news (a warning about upcoming system maintenance, for example).
dacs_acs(8)[2] (the DACS access control service, or ACS) can be configured to invoke
dacs_notices when access control processing determines that one or more notices have not
been acknowledged. dacs_notices acts as both a generic notice presentation handler (it
retrieves notices, presents them to the user, and requests the user to acknowledge the
notices) and a matching notice acknowledgement handler (executed to process the user's
response and take appropriate action). The presentation aspect of the program can be
customized. It takes steps to defeat attempts to bypass notice presentation.
In the DACS implementation, a notice (N) is usually some text, identified by a URL. A
notice is associated with one or more resources (R) such that an attempt to access any of
these resources requires the user to first explicitly acknowledge the textual material. A
DACS event handler (H) is responsible for presenting the notice to the user. The basic
flow of control is roughly as follows:
USER | Apache/DACS
a) --- Request for R --->|---> (DACS Access Control Service)
b) <--- Redirect to H ---|<--
c) --- Request for H --->|---> (a notice presentation handler)
d) <--- Return N ----|<---
e) --- Submit Ack N --->|--> (a notice acknowledgement handler)
f) <--- Redirect to R ---|<--
g) --- Request for R --->|---> (DACS access control service)
h) <--- R ---|<--
More than one notice can be associated with a resource, in which case they are effectively
concatenated for presentation purposes and collectively acknowledged (so N may be N1, N2,
..., Nn and Ack N acknowledges all of them).
Having already acknowledged N in a session, a user should not need to do so again.
dacs_notices uses a purely client-side approach; cookies are issued to remember that
resources have been acknowledged. These cookies are called notice acknowledgement tokens
or NATs. The implementation handles both authenticated and unauthenticated (anonymous)
users. The cookie name consists of a prefix, the federation name, the jurisdiction name,
and a unique component; see dacs.conf(5)[3] for a description of configuration directives
used by this program, including the NOTICES_NAT_NAME_PREFIX[4] directive.
The program accepts the standard FORMAT argument (see dacs(1)[5]) to select between
generation of an HTML presentation page intended for display by a browser (the default) or
XML output intended for middleware. Output customization applies only to the former. When
generating HTML output, dacs_notices emits instructions, the content of the notices, and
an HTML form by which the user submits a response.
Note
Of course, apart from answering a skill-testing question or the like, there's no way
of knowing that a user has actually read and understood the notices. It is unclear to
what extent it is necessary to go in this regard with respect to providing support.
DACS cannot guarantee that a human user has actually read these notices and indicated
acceptance of them, but it can guarantee (optionally, using secure mode[6]) that a NAT
cannot be obtained by a client without the client having received a copy of the
notices. If the client wants to "trick the system" by not actually presenting the
notices to the user or soliciting a response it can, and in this event the service
provider might consider legal recourse.
In future versions of DACS, the idea of notice/acknowledgement processing may be
generalized so that an arbitrary condition can be tested for by access control rules,
remembered through client-side state (for example), and made available via a seamless
workflow.
Operation
Configuring notice acknowledgement processing involves:
• Configuring an ACS_ERROR_HANDLER[7] directive to catch ACK_NEEDED (error code 905)
events and invoke dacs_notices;
• Calling the ack[8] predicate from an access control rule to specify the notices that
must be acknowledged; and
• Configuring dacs_notices.
The following configuration directives are examined by dacs_notices. All of these
directives are optional. Refer to the individual directives for additional details.
• NOTICES_ACK_HANDLER[9]: the URL of the notice acknowledgement handler, if not
dacs_notices.
• NOTICES_ACCEPT_HANDLER[10]: the URL (absolute or relative) to which a user agent will
be redirected after a positive acknowledgement to a notice has been received (i.e.,
the notice or notices were "accepted"), if it is not possible to redirect the user
agent to the request that initiated notice acknowledgement processing (e.g., if that
request used the POST method).
• NOTICES_DECLINE_HANDLER[11]: the URL (absolute or relative) to which a user agent will
be redirected after a negative acknowledgement to a notice has been received (i.e.,
the notice or notices were "declined").
Note
When determining whether a resource has already been acknowledged, the query component
of a resource URI is not considered.
When emitting HTML, the web page generated by dacs_notices can be customized through the
notices VFS item type. The following items are emitted if they exist:
• header: Initial HTML to emit.
• prologue: HTML to emit immediately after the header.
• instructions: HTML to emit immediately after the prologue and before the notices.
• epilogue: HTML to emit immediately after the form that follows the notices.
• trailer: Final HTML to emit.
For example, given the configuration directives:
VFS "[notices]dacs-fs:${Conf::DACS_HOME}/notices"
files named header and trailer in the directory ${Conf::DACS_HOME}/notices would be
expected to contain the initial and final HTML content, respectively. Note that these
consist of text and HTML markup but are not complete HTML documents.
Customization of the form is possible using configuration variables:
• notices_prompt_text: This label appears immediately before the HTML form. The default
is something like "Your response to the preceding notices is requested:<p>".
• notices_accept_label: This label appears next to the radio button corresponding to the
"accepted" response. The default is something like "I Accept".
• notices_decline_label: This label appears next to the radio button corresponding to
the "declined" response. The default is something like "I Decline".
• notices_submit_label: This label appears in the submit button. The default is
something like "Send".
Here are some examples of how these variables might be set to customize the form:
EVAL ${Conf::notices_prompt_text} = "My <b>custom</b> prompt:"
EVAL ${Conf::notices_accept_label} = "I really do accept<br>"
EVAL ${Conf::notices_submit_label} = "Submit me!"
Web Service Arguments
In addition to the standard CGI arguments[12], dacs_notices understands the following CGI
arguments:
ACCEPT_LABEL
The value of this optional parameter is used by the notice presentation handler and
overrides the value of the ${Conf::notices_accept_label} variable, if any.
DECLINE_LABEL
The value of this optional parameter is used by the notice presentation handler and
overrides the value of the ${Conf::notices_decline_label} variable, if any.
HMAC
This value is a secure keyed hash. It is computed by dacs_acs for use by the notice
presentation handler. A hash is also computed (over different material) by the
presentation handler and passed to the notice acknowledgement handler with the user's
response and other information. Its purpose is to make it difficult to obtain a notice
acknowledgement token directly by side-stepping notice acknowledgement processing. The
federation-wide HMAC key is used. Please refer to the description of Secure Mode[6].
Neither this argument nor the TIME[13] argument are used or required if the
NOTICES_SECURE_HANDLER[14] configuration directive has the value "no".
NOTICE_URIS
The value of this argument is a space-separated list of URIs, each of which is invoked
using the GET method and is expected to return a notice document.
Note
The notices are expected to be fragments of HTML text, not complete HTML
documents; each notice is "pasted" into the presentation page exactly as obtained
from its URI.
TIME
This is the Unix time at which dacs_acs invoked the notice presentation handler for
this workflow. It is used to limit the lifetime of the workflow so that it cannot
easily be rerun to obtain notice acknowledgement tokens at will.
RESOURCE_URIS
The value of this argument is a space-separated list of URIs, each of which is
associated with the notice(s).
RESPONSE
Passed to the notice acknowledgement handler, this argument is the user's response and
must either be accepted or declined.
Middleware Support
dacs_notices can be asked to emit various flavours of XML in support of middleware or
thick clients. This is useful when middleware would prefer to prompt the user itself
(acting as a notice presentation handler) and then invoke a acknowledgement handler (such
as dacs_notices) to obtain a NAT. Any customizations specified for HTML output are ignored
when XML is being produced and are not passed to middleware.
The XML emitted by dacs_notices conforms to the DTD dacs_notices.dtd[15]. When acting as a
notice presentation handler, it returns a presentation_reply element and when acting as a
notice acknowledgement handler, it returns a ack_reply element; in either mode of
operation an error reply is possible (via the common_status element).
In conjunction with dacs_acs(8)[2], dacs_notices can optionally operate in a "secure"
mode, where a particular control flow is enforced.
The simple (non-secure) mode will be described first.
Simple Mode
The presentation_reply element lists one or more notices that must be acknowledged by
the user. It includes a space-separated list of the URIs of the notices and a
space-separated list of the URIs of resources that require these notices to be
acknowledged. The text of the notices are base-64 encoded within the notice element,
as specified by RFC 2045[16] (Section 6.8). The notice's URI is included as an
attribute.
The ack_reply element returns the user's response and, optionally, a URI to which the
user can be redirected (depending on the context, this may be the URI of the request
that required notices to be acknowledged, the value of the NOTICES_ACCEPT_HANDLER[10]
directive, or the value of the NOTICES_DECLINE_HANDLER[11] directive). If a NAT is
issued, it is returned (as an HTTP cookie) by the notice presentation handler.
Secure Mode
The secure mode of operation, which may not be necessary in some environments, serves
two main purposes:
1. a NAT can be cryptographically protected against forgery and alteration. Refer to
dacs.nat(5)[17].
2. DACS can enforce a flow of control so that a client cannot obtain a NAT without
having received a copy of the notice(s); this is the purpose of the hmac and time
attributes. That is, DACS can make it necessary for the client to first call
dacs_acs(8)[2] with -check_only or -check_fail, then call the notice presentation
handler to get the text of the notices, and then call the notice acknowledgement
handler to request the NAT. No other control flow is possible (ignoring errors).
When combined, these protections make it difficult for an attacker or unfriendly user
to bypass having to acknowledge notices by manufacturing NATs or having DACS simply
issue arbitrary NATs.
The value of the NOTICES_SECURE_HANDLER[14] configuration directive determines whether
the secure mode is disabled; it is enabled by default (see dacs.conf(5)[3]).
In secure mode, the total duration of the work flow is limited to 120 seconds by
default. This limit can be set using the NOTICES_WORKFLOW_LIFETIME_SECS[18] directive.
Regardless of the selected output format, the required flow of control is:
1. dacs_acs receives a service request
Access to the requested resource will not be granted by dacs_acs(8)[2] until one
or more notices have been acknowledged by the user. dacs_acs either redirects the
client to the notice presentation handler or returns an XML document
(dacs_acs.dtd[19]) that describes which notices must be displayed and
acknowledged; the behaviour depends on the service request. The notice
presentation handler must be invoked and will expect to be passed the HMAC[20] and
TIME[13] arguments.
Provided they are in the same federation, the notice presentation handler may be
in a different jurisdiction from dacs_acs.
2. Notice presentation handler is invoked
The user is expected to be presented with the notices and asked to accept or
decline them. The handler either returns a web page containing an HTML form or an
XML document (dacs_notices.dtd[15]). In either case, the handler will verify that
dacs_acs had been called "recently" (the security-related arguments expire after a
set amount of time and cannot be reused). Its output will include HMAC[20] and
TIME[13] arguments, either of which may differ from the values passed into the
program; the notice acknowledgement handler expects to be passed these arguments.
3. Notice acknowledgement handler is invoked
The user's response is directed to the notice acknowledgement handler, which
verifies that the notice presentation handler has been called. The handler either
redirects the user appropriately or returns an XML document
(dacs_notices.dtd[15]). If no error has occurred and the user has accepted the
notices, a NAT will also be returned.
Note
In secure mode, presentation handling and acknowledgement handling are "matched".
If dacs_notices provides the latter functionality but not the former, presentation
handling must behave as dacs_notices expects (in its acknowledgement handling
mode) with respect to security. When secure mode is used with middleware that
performs its own presentation handling, for example, middleware will likely need
to invoke dacs_notices in its presentation handling mode solely to obtain security
parameters to pass to dacs_notices in its acknowledgement handling mode.
When dacs_notices acts as a notice presentation handler, it will validate its
arguments (which originate from dacs_acs) and emit values that may be validated by
the notice acknowledgement handler. When dacs_notices acts as a notice
acknowledgement handler, it will validate its arguments. Therefore, if the notice
acknowledgement handler runs in secure mode, the notice presentation handler must
also run in secure mode.
The presentation handling mode and the acknowledgement handling mode of
dacs_notices must agree on the URL of the acknowledgement handler. This means that
either the identical URL must be used for both modes or both modes must find the
NOTICES_ACK_HANDLER directive configured to the same value (as when two different
jurisdictions are involved).
DIAGNOSTICS
The program exits 0 if everything was fine, 1 if an error occurred.
BUGS
A client-side approach is used to note that resources have been acknowledged. While this
is probably the simplest approach that works with both authenticated and unauthenticated
users, it does not offer much support if one wants acknowledgements by authenticated users
to be remembered across sessions (i.e., permanently). One possible solution is to allow
persistent notice acknowledgements to be enabled for authenticated users, suppressing NAT
cookies and causing a record to be written to a configured VFS item type when an
authenticated user has accepted a notice. The ack() predicate would be extended so that
the existence of persistent acknowledgement records could be checked, and some means of
maintaining the persistent records might be added.
The method used for generation of custom web pages is clunky and should be reconsidered.
SEE ALSO
dacs.nat(5)[17], dacs_acs(8)[2]
AUTHOR
Distributed Systems Software (www.dss.ca[21])
COPYING
Copyright © 2003-2012 Distributed Systems Software. See the LICENSE[22] file that
accompanies the distribution for licensing information.
NOTES
1. dacsoptions
http://dacs.dss.ca/man/dacs.1.html#dacsoptions
2. dacs_acs(8)
http://dacs.dss.ca/man/dacs_acs.8.html
3. dacs.conf(5)
http://dacs.dss.ca/man/dacs.conf.5.html
4. NOTICES_NAT_NAME_PREFIX
http://dacs.dss.ca/man/dacs.conf.5.html#NOTICES_NAT_NAME_PREFIX
5. dacs(1)
http://dacs.dss.ca/man/dacs.1.html
6. secure mode
http://dacs.dss.ca/man/#secure_mode
7. ACS_ERROR_HANDLER
http://dacs.dss.ca/man/dacs.conf.5.html#ACS_ERROR_HANDLER
8. ack
http://dacs.dss.ca/man/dacs.exprs.5.html#ack
9. NOTICES_ACK_HANDLER
http://dacs.dss.ca/man/dacs.conf.5.html#NOTICES_ACK_HANDLER
10. NOTICES_ACCEPT_HANDLER
http://dacs.dss.ca/man/dacs.conf.5.html#NOTICES_ACCEPT_HANDLER
11. NOTICES_DECLINE_HANDLER
http://dacs.dss.ca/man/dacs.conf.5.html#NOTICES_DECLINE_HANDLER
12. standard CGI arguments
http://dacs.dss.ca/man/dacs.services.8.html#standard_cgi_args
13. TIME
http://dacs.dss.ca/man/#TIME
14. NOTICES_SECURE_HANDLER
http://dacs.dss.ca/man/dacs.conf.5.html#NOTICES_SECURE_HANDLER
15. dacs_notices.dtd
http://dacs.dss.ca/man/../dtd-xsd/dacs_notices.dtd
16. RFC 2045
http://www.rfc-editor.org/rfc/rfc2045.txt
17. dacs.nat(5)
http://dacs.dss.ca/man/dacs.nat.5.html
18. NOTICES_WORKFLOW_LIFETIME_SECS
http://dacs.dss.ca/man/dacs.conf.5.html#NOTICES_WORKFLOW_LIFETIME_SECS
19. dacs_acs.dtd
http://dacs.dss.ca/man/../dtd-xsd/dacs_acs.dtd
20. HMAC
http://dacs.dss.ca/man/#HMAC
21. www.dss.ca
http://www.dss.ca
22. LICENSE
http://dacs.dss.ca/man/../misc/LICENSE