audience:administrators

Use the Tango controlled access system

User rights definition

Within the Tango control system, you give rights to a user. The ‘user’ is the name used to log in to the computer where the application trying to access a device is running. There are two kinds of users within the Tango control system:

1. Users with defined rights

2. Users without any rights defined in the control system. These users will have the rights associated with the pseudo-user called All Users

The control system manages two kinds of rights:

• Write access: all types of requests are allowed on the device

• Read access: only read access is allowed meaning that write_attribute, write_read_attribute and set_attribute_config network calls are forbidden. Executing a command is also forbidden except for commands defined as Allowed commands. Getting a device state or status using the command_inout call is always allowed. The definition of the allowed commands is done at the device class level. Therefore, all devices belonging to the same class will have the same allowed commands set.

The rights given to a user are determined from checks at two levels:

1. At the host level: You define from which hosts the user may have write access to the control system by specifying the host name. If the request comes from a host which is not defined, the right will be read access only. If nothing is defined at this level for the user, the rights of the All Users user will be used. It is also possible to specify the host by its IP address. You can define a host family using wildcards in the IP address (eg. 160.103.11.* meaning any host with an IP address starting with 160.103.11). Only IP V4 is supported.

2. At the device level: You define on which device(s) request are allowed using the device name. Device families can be specified using wildcards in the device name like domain/family/*

The control system is doing the following checks when a client try to access a device:

• Get the user’s name

• Get the host IP address

• If rights are defined at the host level for this specific user and this IP address then give the user temporary write access to the control system

• If nothing is specified for this specific user on this host, give the user temporary access rights equal to the host access rights of the All User user.

• If the temporary right given to the user is write access to the control system then check what rights are defined at the device level:

  • If there is a right defined for the device to be accessed (or for the device family), give user that defined right

  • Otherwise if no right is defined at the device level for this user then check:

    • If rights are defined for the All Users user for this device, give this right to the user

    • Otherwise, give the user read Access only for this device

• Otherwise the default access right will be read Access

Then, when the client tries to access the device, the following algorithm is used:

• If right is read access

  • If the call is a write type call, refuse the call

  • If the call is a command execution

    • If the command is one of the command defined in the allowed commands for the device class, send the call

    • Else, refuse the call

All these checks are done during the DeviceProxy instance constructor except those related to the device class allowed commands which are checked during the command_inout call.

To simplify the rights management, give the All Users user host access right to all hosts (.*.*.*) and read access to all devices (/*/*). With such a set-up for this user, each new user without any rights defined in the control access will have only read access to all devices on the control system from any hosts. Then, on request, gives write access to specific user on specific host (or family) and on specific device (or family).

The access rights are managed using the Tango Astor tool which provides a graphical interface to grant/revoke user rights and to define device class allowed commands set. The following screenshot shows an example of the Astor window:

In this example, the user taurel has write access to the device sr/d-ct/1 and to all devices belonging to the domain fe but only from the host pcantares. They have read access to all other devices but always only from the host pcantares. The user verdier has write access to the device sys/dev/01 from any host on the network 160.103.5 and Read Access to all the remaining devices from the same network. All the other users have read access only from any host.

Running a Tango control system with the controlled access

All the users rights are stored in two tables of the Tango database. A dedicated device server called TangoAccessControl accesses these tables without using the classical Tango database server. This TangoAccessControl device server must be configured with only one device. The property Services belonging to the free object CtrlSystem is used to run a Tango control system with controlled access. This property is an array of strings with each string describing the services running in the control system. For controlled access, the service name is AccessControl. The service instance name has to be defined as tango. The device name associated with this service must be the name of the TangoAccessControl server device. For instance, if the TangoAccessControl device server device is named sys/access_control/1, one element of the Services property of the CtrlSystem object has to be set to: AccessControl/tango:sys/access_control/1.

If the service is defined but without a valid device name corresponding to the TangoAccessControl device server, all users from any host will have write access (simulating a Tango control system without controlled access). Note that this device server connects to the MariaDB database and therefore may need the MariaDB connection related environment variables MYSQL_USER and MYSQL_PASSWORD - see database environment variables.

Even if a controlled access system is running, it is possible to by-pass it if, in the environment of the client application, the environment variable SUPER_TANGO is defined to true. If for one reason or another, the controlled access server is defined but not accessible, the device right checked at that time will be read access.