GLExec Argus Quick Installation Guide

Introduction

gLExec is a program to make the required mapping between the grid world and the Unix notion of users and groups, and has the capacity to enforce that mapping by modifying the uid and gids of running processes. Based on LCAS and LCMAPS, it can both act as a light-weight 'gatekeeper' replacement, and even be used on the worker node in late-binding (pilot job) scenarios.

The gLExec uses the LCMAPS framework to abstract different implementations and techniques that can be implemented through the use of specialized plug-ins. The plug-ins do the actual work (e.g. extensive credentials, procuring an account locally or through a central authorization service, like Argus).

gLExec with Argus

The gLExec will interact with Argus using an LCMAPS plug-in which uses the PEP-C library to communicate to it. The plug-in calls out to the Argus PEP Daemon. The response from the Argus PEP Daemon is extracted and used to deny or fulfill the mapping request to the resource.

Authorization and Mapping

Based on the Grid credentials, LCMAPS C PEP plug-in will send an XACML request to the Argus PEP Daemon, and then parse the XACML response decision to authorize the user and the obligations to map him to a local account which will effectively switch the active Unix account of the running process.

XACML Request

The gLExec LCMAPS PEP plug-in sends an XACML request to the PEP Daemon with the following elements by default (The previously used XACML attribute profile can still be selected, but this needs to be explicitly configured).

• XACML subject with the attribute element:
  • AttributeId: urn:oasis:names:tc:xacml:1.0:subject:key-info
  • Value: The PEM encoded Grid credentials provided by the calling process, fetched from the file content located by the environment variable $GLEXEC_USER_CERT. The location to the proxy file of the pilot job payload owner (e.g. the end user) must be set by this variable in order for gLExec to pass it on. The mapping will be based on this information.
• XACML resource with the attribute element:
  • AttributeId: urn:oasis:names:tc:xacml:1.0:resource:resource-id
  • Value: The type of resource of the node and service from where the request originated (e.g. SE, CE, WN or RB/WMS).
• XACML action with the attribute element:
  • AttributeId: urn:oasis:names:tc:xacml:1.0:action:action-id
  • Value: The type of action to be taken on the node from where the request originated (e.g. access-file, queue, execute-now).
• XACML environment with the attribute element:
  • AttributeId: urn:oasis:names:tc:xacml:1.0:subject:key-info
  • Value: The PEM encoded Grid credentials provided by the calling process, fetched from the file content located by the environment variable $X509_USER_PROXY. The location to the proxy file of the pilot job executor or pilot job framework manager must be set by this variable in case of a pilot job framework scenario.

XACML Response

The PEP Daemon sends back a XACML response to the gLExec LCMAPS PEP plug-in. The following XACML response elements are parsed to authorize and map the user:

• XACML decision element: Contains the authorization decision Permit, Deny, Indeterminate or NotApplicable
• XACML obligation, ObligationId: http://glite.org/xacml/obligation/local-environment-map/posix, with the three possible attribute assignment elements:
  • AttributeId: http://glite.org/xacml/attribute/user-id
  • Value: Contains the Unix User ID of the mapped user account in a string format.
  • AttributeId: http://glite.org/xacml/attribute/group-id/primary
  • Value: Contains the Unix Group ID of the mapped primary group in a string format.
  • AttributeId: http://glite.org/xacml/attribute/group-id
  • Value: Contains the Unix Secondary Group ID of the mapped group in a string format.

The local identity mapping will only succeed if the authorization decision is Permit.