GLExec Argus Quick Installation Guide

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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).

Package installation

Enable the correct UMD or EMI repository.

Install either the emi-glexec_wn metapackage or do a manual yum install of the following list of packages:

  • glexec
  • lcmaps
  • lcmaps-plugins-basic
  • lcmaps-plugins-c-pep
  • lcmaps-plugins-tracking-groupid
  • lcmaps-plugins-verify-proxy
  • lcmaps-plugins-voms

You optionally can also install LCAS:

  • lcas
  • lcas-plugins-basic
  • lcas-plugins-check-executable
  • lcas-plugins-voms

the gLExec wrapper scripts:

  • glexec-wrapper-scripts

the MkGlTempDir utility:

  • mkgltempdir

the gLExec nagios plugin:

  • nagios-plugins-emi.glexec

and the YAIM configuration module for the gLExec workernode:

  • yaim-glexec-wn

Create UNIX user and group glexec.

Configuration

As a minimum, gLExec and LCMAPS configuration files need to be configured. For use with Argus, something along the lines of the following gLExec and LCMAPS configuration files suffice:

/etc/glexec.conf

#  Glexec configuration file
#
[glexec]
user_white_list              = batman, superman, .woman
log_level                    = 3
#log_destination              = file
#log_file                     = /var/log/glexec/glexec.log

lcmaps_db_file               = /etc/lcmaps/lcmaps-glexec.db
lcmaps_debug_level           = 3
#lcmaps_log_file              = /var/log/glexec/lcas_lcmaps.log
lcmaps_get_account_policy    = glexec_get_account

#use_lcas                     = no
#lcas_db_file                 = /etc/lcas/lcas-glexec.db
#lcas_debug_level             = 0

Edit at least the user whitelist. See the Man pages of gLExec for full information on all the configuration options.

Make sure the glexec.conf file is only writable for user root and/or glexec. In the preferred switching mode, the preferred installation of glexec binary and config files are: 

-rws--x--x   1   root   root   12345   2010-02-29  12:34   /usr/sbin/glexec
-r--------   1   glexec root     123   2010-02-29  12:34   /etc/glexec.conf

i.e. run 

chown root.root /usr/sbin/glexec
chmod 4711 /usr/sbin/glexec
chown glexec.root /etc/glexec.conf
chmod 400 /etc/glexec.conf

/etc/lcmaps/lcmaps-glexec.db for use with the Argus Authorization Service 

# LCMAPS config file for glexec
#

# where to look for modules
path = /usr/lib64/lcmaps

# module definitions
verify_proxy = "lcmaps_verify_proxy.mod" 
               " -certdir /etc/grid-security/certificates/"
               " --allow-limited-proxy"

pepc        = "lcmaps_c_pep.mod"
              "--pep-daemon-endpoint-url https://argus.testbed:8154/authz"
              " -resourceid wn"
              " -actionid http://glite.org/xacml/action/execute"
              " -capath /etc/grid-security/certificates/"
              " -pep-certificate-mode implicit"

glexec_get_account:
verify_proxy -> pepc

Edit at least the pep-daemon-endpoint-url, to point to the correct Argus PEPd server.

/etc/lcas/lcas.db

In case LCAS is used for e.g. user banning, one can use the following simple /etc/lcas/lcas.db file: 

# LCAS policy file/plugin definition
# Written by: Oscar Koeroo - okoeroo * at * nikhef * dot * nl
pluginname=lcas_userban.mod,pluginargs=/etc/lcas/ban_users.db

Make sure to create an (empty) ban_users.db file: 

touch /etc/lcas/ban_users.db

gLExec with Argus

GLExec will interact with Argus using an LCMAPS plug-in which uses the Argus 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:
  • XACML action with the attribute element:
  • 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:

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

LCMAPS C PEP Plugin configuration

See LCMAPS_C_PEP_plugin for a list of valid options.

Quick test

Two environment variables MUST be setup before trying gLExec:

  • X509_USER_PROXY : Contains the path to the proxy file of the gLExec calling identity. This is the pilot job executor identity.
  • GLEXEC_CLIENT_CERT : Contains the path to the proxy file of the identity that must get authorized and subsequently mapped by gLExec. This is the pilot job payload identity.

Run gLExec with the following command in order to perform a quick test: 

export X509_USER_PROXY=${X509_USER_PROXY:-"/tmp/x509up_`id -u`"}
export GLEXEC_CLIENT_CERT=${GLEXEC_CLIENT_CERT:-$X509_USER_PROXY}

/usr/sbin/glexec /usr/bin/id -a; echo $?

The expected output is something like 

uid=40205(testuseraccount) gid=2013(testusergroup)
0

Debugging hints

See Debugging_hints and Exit codes of gLExec.

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