Extdom_plugin_enhancement_grouplist

Extdom_plugin_enhancement_grouplist#

Overview#

Currently on FreeIPA clients the full list of group-memberships for users from trusted domains is only available for authenticated users. Authentication is needed because the group-memberships are extracted from the PAC which is a part of the Kerberos ticket of the user. For un-authenticated users only the primary POSIX group is available.

With the introduction of SSSD’s ipa-server-mode IPA servers can resolve the group memberships for user from trusted domains even for un-authenticated users by reading them from the AD DCs of the trusted domains directly. This data should be made available to the IPA clients as well.

Use Cases#

  • Calling the ‘id’ command on a IPA client or calling getgrouplist() in a process should return the full list of secondary groups for any user from a trusted domain without requiring that the user has to log in first

Design#

The IPA extdom plugin is already called by SSSD running on the IPA clients to translate SID to names or POSIX IDs. It can be enhanced to return the secondary groups as well.

To make this change backward compatible in the sense that newer client can still work with older servers

  • a new OID can be added to the extdom plugin, currently it uses 2.16.840.1.113730.3.8.10.4 , if we agree on this approach it might be possible to just add a “.1” to the OID. With this the client can determine from reading the rootDSE if the server supports this operation or not.

  • the client just sends the new request and assumes in the case of a specific error the request is not supported. A suitable error code might be LDAP_UNWILLING_TO_PERFORM which is currently only returned by the extdom plugin if either there is no request data or if there are issues reading and understanding the first enumeration in the request data (see Implementation section for more details).

If the request was parsed successful the extdom plugin will return the secondary groups for the given user as a list of fully qualified group names. Additionally the POSIX user data (UID, GID, gecos, home-directory and shell) can be send. This would reduce the number of requests to the extdom plugin, because refreshing user and group-membership data often happen together. Please note that UID and GID may change due to user-views. But as long as this feature is implemented after user-views everything what is needed will be available.

Implementation#

Request#

The current request looks like this:

/* We expect the following request:
 * ExtdomRequestValue ::= SEQUENCE {
 *    inputType ENUMERATED {
 *        sid (1),
 *        name (2),
 *        posix uid (3),
 *        posix gid (3)
 *    },
 *    requestType ENUMERATED {
 *        simple (1),
 *        full (2)
 *    },
 *    data InputData
 * }
 *
 * InputData ::= CHOICE {
 *    sid OCTET STRING,
 *    name NameDomainData
 *    uid PosixUid,
 *    gid PosixGid
 * }
 *
 * NameDomainData ::= SEQUENCE {
 *    domain_name OCTET STRING,
 *    object_name OCTET STRING
 * }
 *
 * PosixUid ::= SEQUENCE {
 *    domain_name OCTET STRING,
 *    uid INTEGER
 * }
 *
 * PosixGid ::= SEQUENCE {
 *    domain_name OCTET STRING,
 *    gid INTEGER
 * }
 */

Basically only a new request type must be added, e.g.

*    requestType ENUMERATED {
*        simple (1),
*        full (2)
*        full_with_groups (3)
*    },

But as mentioned before the LDAP_UNWILLING_TO_PERFORM error code is only send if there are issues with the inputType. The requestType is evaluated later in the existing code and the quite generic LDAP_OPERATIONS_ERROR error code is returned if an unexpected value is found. This would make the detection based on error code much more fragile. I would suggest if we do not want to use a new OID to indicate the feature, that the inputType is extended as well, e.g. by adding 128 or 256 to the values. Nevertheless I think it is less error prone to use a new OID.

Processing the request#

The extdom plugin will call getgrouplist() and the resolve the fully qualified names by calling getgrgid(). By default groups from the local IPA domain are returned unqualified and the local domain name should be added in this case so that the response only contains fully qualified group names.

There are two things to note here. First getgrgid() returns the full list of group members which might cause some unneeded overhead e.g. with respect to memory allocation. Second easiest way to determine if a domain name is fully qualified is to look for a ‘@’ character. But this will only work if the full_name_format option is not changed from the default. Both can be fixed by adding a call to libsss_nss_idmap to map a GID to user and domain name. If it turns out the such a call is needed it can be added later and the extdom plugin can be updated accordingly.

Additionally the extdom plugin call getpwnam() with user-view code to get the data of the POSIX user entry.

Response#

Currently the response looks like:

/* We send to follwing response:
 * ExtdomResponseValue ::= SEQUENCE {
 *    responseType ENUMERATED {
 *        sid (1),
 *        name (2),
 *        posix_user (3),
 *        posix_group (4)
 *    },
 *    data OutputData
 * }
 *
 * OutputData ::= CHOICE {
 *    sid OCTET STRING,
 *    name NameDomainData,
 *    user PosixUser,
 *    group PosixGroup
 * }
 *
 * NameDomainData ::= SEQUENCE {
 *    domain_name OCTET STRING,
 *    object_name OCTET STRING
 * }
 *
 * PosixUser ::= SEQUENCE {
 *    domain_name OCTET STRING,
 *    user_name OCTET STRING,
 *    uid INTEGER
 *    gid INTEGER
 * }
 *
 * PosixGroup ::= SEQUENCE {
 *    domain_name OCTET STRING,
 *    group_name OCTET STRING,
 *    gid INTEGER
 * }
 */

Here a new responds type e.g.

posix_user_grouplist (5)

is needed which returns OutputData

user_grouplist PosixUserGrouplist

as

PosixUser ::= SEQUENCE {
   domain_name OCTET STRING,
   user_name OCTET STRING,
   uid INTEGER
   gid INTEGER
   gecos OCTET STRING,
   home_directory OCTET STRING,
   shell OCTET STRING,
   grouplist GroupNameList
}

GroupNameList ::= SEQUENCE OF groupname OCTET STRING

Since the new response type will only be returned if requested by the client there are no compatibility concerns because older clients cannot request it.

Feature Management#

The extdom plugin is automatically configured during ipa-adtrust-install. No additional configuration is needed.

Configuration#

No additional configuration is needed. If chosen a new OID can indicate that the feature is available.

How to Test#

It is possible to test the new feature using an IPA client or directly call into the plugin.

Integration tests with SSSD#

The user-visible effect of this feature is that group members, POSIX attributes and user’s group memberships can all be resolved with the help of the extdom plugin.

Please make sure to run these tests on an IPA client as the IPA server doesn’t use the plugin but connects to the server directly!

  1. Prepare a user who is a member of at least one non-primary group in Active Directory

  2. Make sure the that hasn’t logged in previously. Clearing the cache ensures a clean state.

  3. Run “id user”. The output would show all groups the user is a member of

  4. Run “getent group $groupname” where $groupname is an AD group that contains at least one user. All the member users need to be displayed on the command line.

Manual tests of the plugin#

Besides running integration tests with a separate IPA client the plugin can be exercised manually on the server as well and since the extdom plugin uses standard libc and SSS interfaces IPA users can be requested via the extdom plugin as well. This mean the plugin can be tested on the server without established trust which I think would make it possible to include it in the CI tests as well.

The current version of the extdom plugin can be manually tested in the following way:

$ cat extdom_req_user_admin.asc
Example Example.Sid2NameRequestValue
inputType 2
requestType 1
data name
data.name.domain_name ipa20.devel
data.name.object_name admin
$ asn1Coding extdom_req.asn extdom_req_user_admin.asc
Parse: done.
var=Example, value=Example.Sid2NameRequestValue
var=inputType, value=2
var=requestType, value=1
var=data, value=name
var=data.name.domain_name, value=ipa20.devel
var=data.name.object_name, value=admin
name:NULL  type:SEQUENCE
  name:inputType  type:ENUMERATED  value:0x02
  name:requestType  type:ENUMERATED  value:0x01
  name:data  type:CHOICE
    name:name  type:SEQUENCE
      name:domain_name  type:OCT_STR  value:69706132302e646576656c
      name:object_name  type:OCT_STR  value:61646d696e
Coding: SUCCESS
-----------------
Number of bytes=30
30 1c 0a 01 02 0a 01 01 30 14 04 0b 69 70 61 32 30 2e 64 65 76 65 6c 04 05 61 64 6d 69 6e
-----------------
OutputFile=extdom_req_user_admin.out
Writing: done.
$ cat extdom_req_user_admin.out | base64
MBwKAQIKAQEwFAQLaXBhMjAuZGV2ZWwEBWFkbWlu
$ ldapexop -Y GSSAPI 2.16.840.1.113730.3.8.10.4::MBwKAQIKAQEwFAQLaXBhMjAuZGV2ZWwEBWFkbWlu
SASL/GSSAPI authentication started
SASL username: admin@IPA20.DEVEL
SASL SSF: 56
SASL data security layer installed.
# extended operation response
oid: 2.16.840.1.113730.3.8.10.4
data:: MDIKAQEELVMtMS01LTIxLTEyMjMyODkxODgtMzE5ODQ0MDM1My0zMzAwMjExMDMyLTUwMA=
 =
$ echo -n MDIKAQEELVMtMS01LTIxLTEyMjMyODkxODgtMzE5ODQ0MDM1My0zMzAwMjExMDMyLTUwMA== |base64 -d > extdom_resp_user_admin.bin
$ asn1Decoding extdom_resp.asn extdom_resp_user_admin.bin Example.Sid2NameResponseValue
Parse: done.
Decoding: SUCCESS
DECODING RESULT:
name:NULL  type:SEQUENCE
  name:responseType  type:ENUMERATED  value:0x01
  name:data  type:CHOICE
    name:sid  type:OCT_STR  value:532d312d352d32312d313232333238393138382d333139383434303335332d333330303231313033322d353030
$ echo  532d312d352d32312d313232333238393138382d333139383434303335332d333330303231313033322d353030 | xxd -r -p
S-1-5-21-1223289188-3198440353-3300211032-500

RFE Author#

Sumit Bose