Enhanced LDAP Authentication using PAM+SSSD+LDAP in Apache Knox

(This article is work in progress)

Apache Knox has always had LDAP based authentication through the Apache Shiro authentication provider which makes the configuration a bit easier and flexible. However there are a number of limitations with the KnoxLdapRealm (KNOX-536), for instance only a single Organizational Unit (OU) is currently supported. Group lookup will not return the groups that are defined within the tree structure below that single OU. Also, group memberships that are indirectly defined through membership in a group that is itself a member of another group are not resolved. In Apache Knox 0.10.0 Knox introduced the ability to leverage the Linux PAM authentication mechanism. KNOX-537 added a KnoxPAMRealm to the Shiro provider for PAM support. This blog post discusses how to set up LDAP using the new PAM support provided by Knox with Linux SSSD daemon  and some of the advantages and key features of SSSD.

Some of the advantages of using this are:

• Supported for nested OUs and nested groups

• Faster lookups

• Support more complex LDAP queries

• Reduce load on the LDAP/AD server (caching by SSSD)

Scenarios

There are two scenarios that were tested

• Nested groups
• Nested OUs
• Using Multiple Search Bases

Nested Groups

Following diagram represents a nested groups structure used for testing

In the above diagram we have OU=data which has multiple nested groups (2 levels) and we have a user 'jerry' who belongs to the final group datascience-b explicitly, but implicitly belongs to all the other groups that nest it (i.e. datascience-a and datascience)

When SSSD is properly configured (as explained later in the post) we get the following results

# id -a jerry
uid=4001(jerry) gid=4000(engineer) groups=4000(engineer),5000(datascientist),6000(datascientist-a),7000(datascientist-b)

When we try to access a resource secured by Knox using the user jerry we can see all the groups that user jerry belongs to are logged in gateway-audit.log (part of Knox logging)

Groups: [datascientist-a, datascientist-b, engineer, datascientist]

Nested OUs

Following diagram shows the nested OU structure used for testing

In this example we can see that the user kim is part of group 'processors' which is part of OU processing which is part of OU data which in turn is part of OU groups.

Following is the output of 'id' command, here we can see that our user kim and group that user belongs to are retrieved correctly.

# id -a kim
uid=8001(kim) gid=8000(processors) groups=8000(processors)

Similarly, when we try to access a resource secured by Knox using the user kim we get the following entry in gateway-audit.log (part of Knox logging)

Groups: [processors]

This demonstrates that Knox can authenticate and retrieve groups against nested OUs.

Using Multiple Search Bases

Following diagram shows nested parallel OUs (processing and processing-2)

In this test we will configure two different search bases 

• ou=processing,ou=data,ou=groups,dc=hadoop,dc=apache,dc=org
• ou=processing-2,ou=data,ou=groups,dc=hadoop,dc=apache,dc=org

sssd.conf settings (relevant) for this test are as follows:

[sssd]
....
domains = default, processing2
....

[domain/default]
....
ldap_search_base = ou=processing,ou=data,ou=groups,dc=hadoop,dc=apache,dc=org
....

[domain/processing2]
....
ldap_search_base = ou=processing-2,ou=data,ou=groups,dc=hadoop,dc=apache,dc=org
....

To check whether SSSD correctly picks up our users we use the id command

# id kim
uid=8001(kim) gid=8000(processors) groups=8000(processors)

# id jon
uid=9001(jon) gid=9000(processors-2) groups=9000(processors-2)

Similarly, when we try to access a resource secured by Knox using the user kim and jon we get the following entry in gateway-audit.log (part of Knox logging)

for kim
success|Groups: [processors]

for jon
success|Groups: [processors-2]

Thanks to Eric Yang for pointing out this scenario.

Setup Overview

Following diagram shows a high level set-up of the components involved.

Following are the component versions for this test

• OpenLDAP - 2.4.40
• SSSD - 1.14.1
• Apache Knox - 0.10.0

LDAP

In order to support nesting of groups LDAP needs to support RFC 2307bis schema. For SSSD to talk to LDAP it has to be secure. Acquire a copy of the public CA certificate for the certificate authority used to sign the LDAP server certificate, you can test the certificate using the following openssl test command

openssl s_client -connect <ldap_host>:<ldap_port> -showcerts -state -CAfile <path_to_ca_directory>/cacert.pem

SSSD

SSSD is stricter than pam_ldap. In order to perform an authentication, SSSD requires that the communication channel be encrypted. This means that if sssd.conf has ldap_uri = ldap://<server>, it will attempt to encrypt the communication channel with TLS (transport layer security). If sssd.conf has ldap_uri = ldaps://<server>, then SSL will be used instead of TLS. This requires that the LDAP server

1. Supports TLS or SSL
2. Has TLS access enabled on the standard LDAP port (636) (or alternate port, if specified in the ldap_uri or has SSL access enabled on the standard LDAPS port (or alternate port).
3. Has a valid certificate trust (can be relaxed by using ldap_tls_reqcert = never,  but it is a security risk and should ONLY be done for development and demos)

Copy the public CA certs needed to talk to LDAP at  /etc/openldap/certs

To configure sssd you can use the following 'authconfig' command

authconfig --enablesssd --enablesssdauth --enablelocauthorize --enableldap --enableldapauth --ldapserver=<ldap_host> --enableldaptls --ldapbasedn=dc=my-company,dc=my-org --enableshadow --enablerfc2307bis --enablemkhomedir --enablecachecreds --update

After the command executes you can see that sssd.conf file has been updated.

An example of sssd.conf file

[sssd]
config_file_version = 2
reconnection_retries = 3
sbus_timeout = 30
services = nss, pam, autofs
domains = default

[nss]
reconnection_retries = 3
homedir_substring = /home

[pam]
reconnection_retries = 3

[domain/default]
access_provider = ldap
autofs_provider = ldap
chpass_provider = ldap
cache_credentials = True
ldap_schema = rfc2307bis

id_provider = ldap
auth_provider = ldap
ldap_uri = ldap://<ldap_host>/

ldap_tls_cacertdir = /etc/openldap/certs
ldap_id_use_start_tls = True

# default bind dn
ldap_default_bind_dn = cn=admin,dc=apache,dc=org
ldap_default_authtok_type = password
ldap_default_authtok = my_pasword
ldap_search_base = dc=apache,dc=org

# For group lookup
ldap_group_member = member

# Enable nesting 
ldap_group_nesting_level = 5

[sudo]

[autofs]

[ssh]

[pac]

[ifp]

The important settings to note are:

• ldap_schema = rfc2307bis - Needed if all groups are to be returned when using nested groups or primary/secondary groups.
• ldap_tls_cacertdir = /etc/openldap/certs - certs to talk to LDAP server
• ldap_id_use_start_tls = True - Secure communication with LDAP
• ldap_group_nesting_level = 5 - Enable group nesting up-to 5 levels
  

NOTE: You might need to add / change some options in sssd.conf file to suite your needs. like debug level etc. After updating just restart the service and changes should be reflected.

Some additional settings that can be used to control caching of credentials by SSSD are

Test SSSD is configuration

To check whether SSSD is configured correctly you can use the standard 'getent' or 'id' commands

$ getent passwd <ldap_user>
$ id -a <ldap_user>

Using the above commands you should be able to see all the groups that <ldap_user> belongs to. If you do not see the secondary groups check the 'ldap_group_nesting_level = 5' option and adjust it accordingly.

Knox

Setting up Knox is relatively easy, install Knox on the same machine as SSSD and update the topology to use PAM based auth

			<param>
                <name>main.pamRealm</name> 
                <value>org.apache.hadoop.gateway.shirorealm.KnoxPamRealm</value>
            </param>
            <param>
                <name>main.pamRealm.service</name> 
                <value>login</value>
            </param>

For more information and explanation on setting up Knox see the PAM Based Authentication section in Knox user guide.

Caveats

• For nested group membership SSSD and LDAP should use rfc2307bis schema

• SSSD requires SSL/TLS to talk to LDAP

Troubleshooting