dacs_notices — DACS notice presentation and acknowledgement handler


dacs_notices [dacsoptions]


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) (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) for a description of configuration directives used by this program, including the NOTICES_NAT_NAME_PREFIX directive.

The program accepts the standard FORMAT argument (see dacs(1)) 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.


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) 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.


Configuring notice acknowledgement processing involves:

  • Configuring an ACS_ERROR_HANDLER directive to catch ACK_NEEDED (error code 905) events and invoke dacs_notices;

  • Calling the ack 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: the URL of the notice acknowledgement handler, if not dacs_notices.

  • NOTICES_ACCEPT_HANDLER: 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: 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").


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 &lt;b&gt;custom&lt;/b&gt; prompt:"
EVAL ${Conf::notices_accept_label} = "I really do accept&lt;br&gt;"
EVAL ${Conf::notices_submit_label} = "Submit me!"

Web Service Arguments

In addition to the standard CGI arguments, dacs_notices understands the following CGI arguments:


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.


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.


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.

Neither this argument nor the TIME argument are used or required if the NOTICES_SECURE_HANDLER configuration directive has the value "no".


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.


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.


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.


The value of this argument is a space-separated list of URIs, each of which is associated with the notice(s).


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. 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), 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 (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 directive, or the value of the NOTICES_DECLINE_HANDLER 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).

  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) 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 configuration directive determines whether the secure mode is disabled; it is enabled by default (see dacs.conf(5)).

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 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) 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) 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 and TIME 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). 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 and TIME 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). If no error has occurred and the user has accepted the notices, a NAT will also be returned.


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).


The program exits 0 if everything was fine, 1 if an error occurred.


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.


dacs.nat(5), dacs_acs(8)


Distributed Systems Software (www.dss.ca)


Copyright 2003-2012 Distributed Systems Software. See the LICENSE file that accompanies the distribution for licensing information.

DACS Version 1.4.38a 23-Nov-2016 DACS_NOTICES(8)

Table of Contents
−− Set ++

$Id: dacs_notices.8.xml 2563 2012-02-07 22:40:41Z brachman $