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Adding the ad_access_filter option

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Problem Statement

The recommended way of connecting a GNU/Linux client to an Active Directory domain is using the AD provider. However, in the default configuration of the Active Directory provider, only account expiration is checked. Very often, the administrator needs to restrict the access to the client machine further, limiting the access to a certain user, group of users, or using some other custom filtering mechanism. In order to do so, the administrator is required to use an alternative access control provider. However, none of the alternatives provide the full required functionality for all users resolvable by the AD provider, moreover they are hard to configure. This design page proposes extension of the AD access provider to address these concerns.

Current access control options

With the existing SSSD, the administrator has two basic means to restrict access control to the GNU/Linux client - using the simple access control provider or configuring the LDAP access control provider. Each approach has its pros and cons.

Using the simple access provider

The simple access provider grants or denies access based on the contents of allow and deny lists. There are separate lists for user and group names as well as allowed and denied objects.

The following example shows configuration that grants access to user named tux and group called linuxadmins. :

access_provider = simple
simple_allow_users = tux
simple_allow_groups = linuxadmins
  • Pros:
    • Easy to configure
    • Realmd provides an interface to configure the simple access provider using its CLI
  • Cons:
    • Account expiration is not checked
    • Limited expressiveness. No way to combine several clauses
    • Does not align with the LDAP structure the Active Directory uses

Using the LDAP access provider

The LDAP access provider offers a way to configure the access control decision based on whether the user matches a preconfigured filter. Moreover, the LDAP access provider also offers chaining other LDAP based checks. For the vanilla AD environment, only account expiration check applies.

The following example illustrates configuration that allows access to those users, who are members of group named linuxadmins AND have a valid home directory set using the ldap_access_filter directive. The users who match the configured filter are also checked whether they are expired (ldap_access_order contains expire). :

access_provider = ldap
ldap_access_order = filter, expire
ldap_account_expire_policy = ad
ldap_access_filter = (&(memberOf=cn=admins,ou=groups,dc=example,dc=com)(unixHomeDirectory=*))
ldap_sasl_mech = GSSAPI
ldap_sasl_authid = CLIENT_SHORTNAME$@EXAMPLE.COM
ldap_schema = ad
  • Pros:
    • Allows the administrator to base access control on a custom LDAP filter, making it possible to combine several conditions
    • Conditions are not limited to user names or group membership
  • Cons:
    • Nontrivial and clumsy configuration that must include several low level LDAP settings, otherwise set automatically by the AD provider. Defeats the whole purpose of the AD provider
    • The admin needs to combine AD and LDAP providers. Judging by experience from triaging support cases with Red Hat support, this is a problem for many admins.
    • Account expiration check must be configured separately, which is not obvious
    • No support for users from trusted AD domains
    • No realmd integration

Proposed solution

The proposal is to add a new access filter configuration option to the existing AD access provider. Adding the option to the AD provider would greatly simplify the configuration when compared to the LDAP access control, while maintaining the full expressiveness of ldap_access_filter. The new option would be called ad_access_filter. If the new option was set, then the AD access provider would first match the entry against the filter in that option. If the entry matched, then the account would be checked for expiration.

The following example illustrates an example similar to the one above, using the proposed AD options: :

access_provider = ad
ad_access_filter = (&(memberOf=cn=admins,ou=groups,dc=example,dc=com)(unixHomeDirectory=*))

The main advantage is simplified configuration. The admin doesn’t have to know or understand what “SASL ID” is.

In comparison with the two legacy solutions explained above:

  • Pros
    • Easy and intuitive configuration. Only one provider type is configured
    • Sane defaults - always checks for expiration, also checks access filter if configured that way
    • Would support users and groups from trusted domains by leveraging the existing AD provider infrastructure
  • Cons
    • No realmd integration

Realmd integration

After a short discussion with the realmd upstream maintainer, it was decided that these options do not fit the realmd use-cases well. If the user needs to use such advanced techniques as LDAP filters, chances are that he doesn’t need a tool like realmd to set them up in the config file.

Implementation details

  1. The default value of what AD access_provider is set to should be changed
    • Currently, if access_provider is not set explicitly, the default is permit, thus allowing even expired accounts
    • The new default would be ad, checking account expiration even with a minimal configuration
  2. A new option would be added. The new option would be called ad_access_filter
  3. The LDAP access provider must be extended to allow connecting to a GC and support subdomains in general
    • Pass in struct sdap_domain and id_conn instead of using the connection from sdap_id_ctx directly
    • The code must not read the sss_domain_info from be_ctx but only from sdap_domain in order to support subdomain users
  4. The AD access provider must call the improved LDAP access provider internally with the right connection
    • The default should be GC
    • If POSIX attributes are in use and GC lookup wouldn’t match, optionally fall back to LDAP. This fallback could be tried just once to speed up subsequent access control
  5. The default chain of LDAP access filter the AD provider sets internally must be changed.
    • Currently AD provider sets ldap_access_order=expire. If (and only if) ad_access_filter was set, the LDAP chain would become ldap_access_order=filter,expire

Parsing the ad_access_filter option

  1. The ad_access_filter option is a comma-separated list of filters that apply globally, per-domain or per-forest. The most specific match is used
  2. If the ad_access_filter value starts with an opening bracket (, it is used as a filter for all entries from all domains and forests
    • example: (&(memberOf=cn=admins,ou=groups,dc=example,dc=com)(unixHomeDirectory=*))
  3. More advanced format can be used to restrict the filter to a specific domain or a specific forest. This format is KEYWORD:NAME:FILTER
    • KEYWORD can be one of DOM or FOREST
      • KEYWORD can be missing
    • NAME is a label.
      • if KEYWORD equals DOM or missing completely, the filter is applied for users from domain named NAME only
      • if KEYWORD equals FOREST, the filter is applied on users from forest named NAME only
    • examples of valid filters are:
      • apply filter on domain called dom1 only:
        • dom1:(memberOf=cn=admins,ou=groups,dc=dom1,dc=com)
      • apply filter on domain called dom2 only:
        • DOM:dom2:(memberOf=cn=admins,ou=groups,dc=dom2,dc=com)
      • apply filter on forest called EXAMPLE.COM only:
        • FOREST:EXAMPLE.COM:(memberOf=cn=admins,ou=groups,dc=example,dc=com)
  4. If no filter matches the user’s domain, access is denied
    • example ad_access_filter = dom1:(memberOf=cn=admins,ou=groups,dc=dom1,dc=com), dom2:(memberOf=cn=admins,ou=groups,dc=dom2,dc=com), user logs in from dom3

Contingency plan

None needed. The existing options would still exist and function as they do now.

How to test

  1. Check that access_provider=ad without any other options allows non-expired users
  2. Check that access_provider=ad without any other options denies expired users
  3. Test that setting ad_access_filter restricts access to users who match the filter
    • test that an expired user, even though he matches the filter, is denied access
    • this test must include users from the primary domain as well as a sub domain
    • Different filters should be tested to make sure the most specific filter applies
      • example: add a restrictive filter for dom1 and permissive filter without specifying the domain. A user from dom1 must be denied access, while a user from other domain must be allowed access
  4. When access is denied, the SSSD PAM responder must return a reasonable return code (6)

Future and optional enhancements

In the future, we should extend the access_provider option itself and allow chaining access providers. This enhancement would allow even more flexibility and would allow the administrator to combine different access providers, but is outside the scope of the change described by this design page.

Author(s)

  • Jakub Hrozek <jhrozek@redhat.com>
  • Sumit Bose <sbose@redhat.com>