Virtuoso Authentication Layer
VAL provides the means to easily add authentication and ACL support to existing or new vsp-based applications. This tutorial shows the three main steps to add authentication and ACL protection to an application. We are using
curi - the Compressed URI Service as an example.
curi we want to add login information, a means for the user to logout, and ACLs to protect the service.
The first and simplest step is to check if the user already provided authentication information as supported by VAL. This can simply be achieved by calling VAL.DBA.get_authentication_details_for_connection() at the top of the
realmparameter and only return authentication data for the given realm. Additionally it will honor the
app_realmsetting in the virtual dir serving the page in question. Thus, there are basically two ways to define the realm for an application: 1. Set it in the virtual dir, and 2. Force it manually via the
After the call to VAL.DBA.get_authentication_details_for_connection() the application can use the information. The most important one is the value of
val_serviceId which defines who is authenticated. If it is
null then the user has not authenticated yet.
VAL provides an authentication and a logout page to support the most simple login and logout links possible. Given that the application page is stored in
pageUrl the following links can be used:
However, in our case a dedicated login page is more desirable since it allows to configure certain aspects of
authenticate.vsp. Thus, we create a new page
login.vsp with the following content (or at least parts of it):
The settings should be obvious:
login.vspinstead of its own URL for all login links.
__val_req_res_label__A custom label for the login dialog to tell the user which service they log into.
__val_oauth_scope__The optional OAuth scope to use (
dav). This is only of interest for applications that reuse the created OAuth sessions for additional API calls to the 3rd-party service.
So we end up with code for creating a login/logout box like the following:
Once the user authenticates, they will be redirected to the
pageUrl with a newly created
sid cookie. the logout page will remove that cookie.
A typical situation for authentication-enabled applications is forcing the user to authenticate. Ideally this is done via
40x page options in the virtual directory in combination with VAL's
authenticate.vsp page (which is also used for login links). One simply creates a new file
40x.vsp which has the following content:
authenticate.vsp can be configured via a set of connection settings:
__val_req_res__The resource which is protected, ie which requires the login. This is only used to retrieve ownership information for the "request access" dialog that
authenticate.vspwill show if access was denied. This will default to the
returntoURL if not provided, and should that also be
null(as is the case if
authenticate.vspis used as
40x_page) then the requested URL will be used.
__val_req_acl_scope__The ACL scope in which the above resource is protected. This is only used to retrieve ownership information for the "request access" dialog that
authenticate.vspwill show if access was denied. If not given, then no "request access" dialog is shown.
__val_req_res_label__An optional label for the login dialog showing the user for which service they are authenticating.
authenticate.vspto use our custom page
login.vspinstead of its own URL for all login links.
val_err_msgAn error message indicating any kind of error. This should be set to
http_param ('error.msg')for the simple reason that Virtuoso does clear the http params before processing the 40x page.
This page will be used as
40x page in the virtual directory configuration:
Then the application can raise a permission denied error as shown in the following example:
null then the user has not logged in and the application simply requests that they do. Otherwise
403 indicates that permission was denied to the authenticate user. The authenticated has to be communicated to
authenticate.vsp via the
__val_denied_service_id__ connection setting.
In 40x Pages we saw how to use
authenticate.vsp as a
40x_page. Now we will add ACL protection to the
curi service and put the new 40x_page to use.
We want to be able to grant people the right to create new compressed URIs and others the right to read these. To that end we define a new scope
urn:virtuoso:val:scopes:curi which is only used for
curi and a virtual resource URI which is used to grant permissions:
urn:virtuoso:access:curi These URIs are arbitrary, they simply follow a random scheme to be easily recognizable. In theory they could be any URI one wanted to use.
VAL makes use of scope definitions to get default access modes for disabled scopes (the default). Thus we start by defining our new scope in the corresponding VAL acl schema graph (Hint: standard scopes for DAV, etc. are defined in the OpenLink ACL ontology, example: oplacl:Dav):
The most important part is
oplacl:hasDefaultAccess which defines the access modes used in case ACL evaluation has not been enabled for the
curi scope. In this case everyone is allowed to create and read compressed URIs.
Now at the top of the
create.vsp page which allows to create new compressed URIs we add the following ACL check (after the code from 40x Pages):
Some of this code we already know from before. But the big first part is new. First we check if we are logged in as an admin user. VAL provides us with the convinience procedure VAL.DBA.is_admin_user() for that. Of course only "real" users, ie. SQL users, can be administrators of the V instance. In case no admin credentials were provided we continue with the ACL check using VAL.DBA.check_access_mode_for_resource() which allows to check for exactly one mode on one resource for one service id. Here we use all the details that were provided by VAL.DBA.get_authentication_details_for_connection() and combine them with the resource and scope URIs we defined above.
Since we want to create a compressed URI we use the
oplacl:Write access mode. Should no ACL exist which grants access we continue to raise a
40x error. But before we do that we set two more variables:
__val_denied_service_id__This is important as it allows
authenticate.vspto know that access has been denied to a certain person and the user should be asked to login again. Without this setting,
authenticate.vspwould simply return to the
returntoURL if authentication information could be found. This would result in an endless loop. Should no authentication information exist yet then
authenticate.vspwill simply ask for it.
__val_req_acl_mode__Like the resource and the scope settings above the mode is only used for the "request access" dialog. It allows
authenticate.vspto create a more detailed access request message to the resource owner.
Finally we add the same code to the
get.vsp page which handles the conversion of comressed to uncompressed URIs. The only difference is the access mode:
authenticate.vsp provides a simple dialog through which users can request access to a certain resource, should it have been denied. This dialog is shown by
authenticate.vsp if the following conditions hold true:
non-null(See 40x Pages for details).
__val_req_res__has to be
non-nulland an owner has to be set (DAV resources are handled as special cases, for every other resource see VAL.DBA.set_resource_ownership(), VAL.DBA.add_ownership_graph() and friends.)
Once these conditions are fulfilled then the user has the option to write a message to the owner of the resource, requesting to grant them access.
In the case of
curi the vsp pages are not executed as
dba but using the dedicated account
CURI which improves security and is generally recommended. However, since most of the internal VAL API procedures require special permissions this user needs to be granted the
VAL_ACL roles to be able to execute:
(The API documentation contains hints about which role grants the right to execute a specific procedure in said procedure's documentation.)
As explained in SPARQL ACL Rules - Defining access to private graphs VAL handles SPARQL ACL rules as a special case, using the fixed rule scope oplacl:PrivateGraphsScope (VAL.DBA.get_sparql_scope()) and a graph security callback. The following example shows how all the different pieces of VAL can be used to perform ACL-protected SPARQL queries (as done by
Imagine the actual SPARQL query is stored in variable
First we get the authentication information for the current connection. It is important to set
null because it is an
inout variable which has to be tested against
null in the procedure.
sid as name for the cookie which stored the session id. This is the default but can be set to any value required. (However, be aware that
authenticate.vsp currently does not allow to change this.)
After the call to VAL.DBA.get_authentication_details_for_connection() we know if a user is logged in or not and can act accordingly. One might either want to show a login link or simply proceed to execute the query as user
nobody (which means that only public ACL rules apply).
As recommended we execute the query as user
VAL_SPARQL_ADMIN to get full access to the entire triple store. We then use the callback to restrict that access according to the ACL rules setup in the current realm. We do not use
dba for security reasons.
VAL_SPARQL_ADMIN is a special user which has full access to all public and private graphs. The permissions reported by the callback include not only the implied permissions set by VAL ACL rules, but also the physical graph permissions set by
Finally we can execute the query and clear the temporary WebID graph: