Add sudo rules to Active Directory and access them with SSSD

Centralizing sudo rules in a centralized identity store such as FreeIPA is usually a good choice for your environment as opposed to copying the sudoers files around – the administrator has one place to edit the sudo rules and the rule set is always up to date. Replication mitigates most of the single-point-of-failure woes and by using modern clients like the SSSD, the rules can also be cached on the client side, making the client resilient against network outages.

What if your identity store is Active Directory though? In this post, I'll show you how to load sudo rules to an AD server and how to configure SSSD to retrieve and cache the rules. A prerequisite is a running AD instance and a Linux client enrolled to the AD instance using tools like realmd or adcli. In this post, I'll use dc=DOMAINNAME,dc=LOCAL as the Windows domain name.

The first step is to load the sudo schema into the AD server. The schema describes the objects sudo uses and their attributes and is not part of standard AD installations. In Fedora, the file describing the schema is part of the SUDO RPM and is located at /usr/share/doc/sudo/schema.ActiveDirectory. You can copy the file to your AD server or download it from the Internet directly.

Next, lauch the Windows command line and load the schema to AD's LDAP server using the ldifde utility:

ldifde -i -f schema.ActiveDirectory -c dc=X dc=DOMAINNAME,dc=LOCAL

Before creating the rule, let's also crate an LDAP container that would store the rules. It's not a good idea to mix sudo rules into the same OU that already stores other objects, like users – a separate OU makes management easier and allows to set more fine-grained permissions. You can create the sudoers OU in "ADSI Edit" quite easily by right-clicking the top-level container (dc=DOMAINNAME,dc=LOCAL), selecting "New->Object". In the dialog that opens, select "organizationalUnit", click "Next" and finally name the new OU "sudoers". If you select a different name or a different OU altogether, you'll have to set a custom ldap_sudo_search_base in sssd.conf, the default is ou=sudoers,$BASE_DN".

Now, let's add the rule itself. For illustration purposes, we'll allow the user called 'jdoe' to execute less on all Linux clients in the enterprise.

In my test, I used "ADSI Edit" again. Just right-click the SUDO container, select "New->Object" and then you should see sudoRole in the list of objectClasses. Create the rule based on the syntax described in the sudoers.ldap man page; as an example, I created a rule that allows the user called "jdoe" to run less, for instance to be able to inspect system log files.

dn: CN=lessrule,OU=sudoers,DC=DOMAINNAME,DC=LOCAL
objectClass: top
objectClass: sudoRole
cn: lessrule
distinguishedName: CN=lessrule,OU=sudoers,DC=DOMAINNAME,DC=LOCAL
name: lessrule
sudoHost: ALL
sudoCommand: /usr/bin/less
sudoUser: jdoe

The username of the user who is allowed to execute the rule is stored in the sudoUser attribute. Please note that the username must be stored non-qualified, which is different from the usual username@DOMAIN (or DOM\username) syntax used in Windows.For a more detailed description of how the sudo rules in LDAP work, refer to the sudoers.ldap manual page.

The client configuration involves minor modifications to two configuration files. First, edit /etc/nsswitch.conf and append 'sss' to the 'sudoers:' database configuration:

sudoers: files sss

If the sudoers database was not present in nsswitch.conf at all, just add the line as above. This modification would allow SSSD to communicate with the sssd with the libsss_sudo library.
Finally, open the /etc/sssd/sssd.conf file and edit the [sssd] section to include the sudo service:

services = nss, pam, sudo

Then just restart sssd and the setup is done! For testing, log in as the user in question ("jdoe" here) and run:

sudo -l

You should be able to see something like this in the output:
User jdoe may run the following commands on adclient:
(lcl) /usr/bin/less

That's it! Now you can use your AD server as an centralized sudo rules storage and the rules are cached and available offline with the SSSD.

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