[Fedora-directory-users] Server-Side ACLs for pam_ldap logins.

[Dan Cox]

Since I've been meaning to write this document for quite a while now it 
got a bit long so feel free to trim it as needed. I concentrated mainly 
on how to implement login access control, but left some references to 
other possibilities. It may need to have some URL references added when 
it makes its way to the WIKI. I've attached it in OpenDocument  form as 
well, which may be easier to read.


*System Access Control using LDAP backed NIS Netgroups*


There are many ways to control both login and service level 
authentication with Fedora Directory Server. Here, I will discuss a 
specific implementation using LDAP backed NIS Netgroups and detail what 
exactly makes them so powerful.

/Prerequisites/

    *

      Some knowledge of NIS and the netgroup triple syntax is in order.
      For those that do not have a netgroup man page available, you may
      see the Sun NIS FAQ http://www.sunhelp.org/faq/nis.html, Section
      3.15 specifically.

    *

      An understanding of PAM and the PAM module stack.

    *

      A working implementation of nss_ldap, which acts as the
      NSS->NIS->LDAP gateway is required.

/What are NIS netgroups good for?/

First, it's important to understand what a NIS netgroup gains the 
average system administrator. NIS Netgroups provide the ability to 
perform such tasks as:

    *

      Control both user and group login access to individual or groups
      of machines.

    *

      Manage NFS access control lists.

    *

      Control user and group sudo command access.

    *

      Execute remote commands or interactive logins on groups of
      machines with dsh (distributed shell).

    *

      Manage the configuration of your entire network on a role basis
      with CFEngine.

These are just a few of the excellent uses for NIS netgroups. If we take 
this functionality and implement an LDAP based backend, we can not only 
take advantage of these tools but gain the security, manageability and 
fault tolerance of Fedora Directory Server.

/How does it work?/

NIS netgroup entries are stored as an objectClass of type nisNetgroup in 
the directory server. The relative distinguished name attribute is 
typically cn (common name). There are two important attributes in 
creating the netgroup. Note that they are not mutually exclusive. Also, 
neither are required (sometimes having an empty netgroup is as valuable 
as one populated with values).

    *

      nisNetgroupTriple : This can be used to describe a user
      (,bobby,example.com) or a machine name
      (shellserver1,,example.com). This attribute can have multiple values.

    *

      memberNisNetgroup : This is a very powerful attribute. It is used
      to merge the attribute values of another netgroup into the current
      one by simply listing the name (cn) of the merging netgroup. This
      attribute can have multiple values as well.

You also want to attach a description attribute and value to your 
object. You were planning on describing that netgroup, weren't you?

Let's look at an example LDIF:

dn: cn=QAUsers,ou=Netgroup,dc=example,dc=com

objectClass: nisNetgroup

objectClass: top

cn: QAUsers

nisNetgroupTriple: (,bobby,example.com)

nisNetgroupTriple: (,joey,example.com)

description: All QA users in my organization

We can see here that the users 'bobby' and 'joey' belong to the QAUsers 
netgroup. Now, any tool that will query for the QAUsers netgroup will 
get back these values and can act upon them.

With nss_ldap appropriately configured and /etc/nsswitch.conf 
conveniently pointing netgroup queries to ldap, we can test this entry 
on the command line like so:

# getent netgroup QAUsers

QAUsers (,bobby,example.com) (,joey,example.com)

The getent command is part of the glibc-common package on Fedora. It can 
be used to query any of the available NSS databases.

Now, let's look at an LDIF defining which machines are QA systems on our 
network:

dn: cn=QASystems,ou=Netgroup,dc=example,dc=com

objectClass: nisNetgroup

objectClass: top

cn: QASystems

nisNetgroupTriple: (qa01,,example.com)

nisNetgroupTriple: (qa02,,example.com)

description: All QA systems on our network

OK, so we have our users and systems in place, now how do we give 
QAUsers login access to QASystems? Enter PAM's access.conf.

PAM has an often overlooked access control feature, the configuration of 
which is typically located in /etc/security/access.conf. It has the 
ability to use UNIX users and groups as well as NIS netgroups to control 
remote and local console access to the system. The documentation of the 
syntax should be contained within the configuration file itself.

We can give our users remote login access from our 10.x.x.x network with 
this line:

    *

      : @QAUsers@@QASystems : 10.

*NOTE*: PAM operates on a first match basis for granting access. This 
means you want to end your ACL list by denying all unmatched entries, 
but before you do that make sure root and/or your admin users have been 
matched! For example, adding root for console only, users in the Admins 
netgroup remote access and denying all other unmatched entries:

+ : root : LOCAL

+ : @Admins : 10.

- : ALL : ALL

An advantage to using machine groups in the access.conf is the ability 
to push out this access.conf configuration file to all systems in your 
network, regardless if they are related to QA. This gives an admin the 
ability to maintain a central access control list of general user and 
group pairs, which can be deployed via tools like CFEngine. If a QA user 
attempts to login to a non-QA system, PAM will first check for the 
user's name in the users portion of the ACL. If a match is found, it 
will then check if the current machine's hostname exists in the netgroup 
or machine name section. If the current machine does not belong to the 
netgroup, the ACL fails and the next one will be tried.

Since we have created our own framework of system and user group ACLs 
inside the LDAP server, we have decoupled access control from the actual 
posixAccount and posixGroup entries. This means that the user no longer 
requires an account in the LDAP server itself. A simple entry in 
/etc/passwd is good enough to apply access control in this manor.

With this infrastructure in place, we can now start up Fedora's Admin 
Console or our favorite LDAP editor and quickly add or remove login 
access to users and machines!

/Advanced Usage & Tips/

Use sub scope for your netgroup queries as configured in /etc/ldap.conf. 
This will give you the ability to create new netgroups inside 
organizationalUnit and other containers, which will help categorize your 
ACLs. nss_ldap is smart enough to only match objects of type nisNetgroup 
when performing its searches.

With the memberNisNetgroup attribute, we can join together our netgroups 
to achieve cascading access control and system groupings. What if the 
QAUsers bobby and joey were also members of a larger team called 
LinuxTeam, which contains individuals who aren't in QA? An example LDIF 
defining the LinuxTeam:

dn: cn=LinuxTeam,ou=Netgroup,dc=example,dc=com

objectClass: nisNetgroup

objectClass: top

cn: LinuxTeam

nisNetgroupTriple: (,frank,example.com)

nisNetgroupTriple: (,jill,example.com)

memberNisNetgroup: QA

memberNisNetgroup: Development

memberNisNetgroup: Operations

description: The Linux Team

Here we have defined some new users frank and jill as being part of the 
LinuxTeam. We have also automatically imported bobby and joey from the 
QA team as well as any additional users defined in our hypothetical 
Development and Operations groups. Any ACL for the LinuxTeam deployed on 
our network will not only apply to frank and jill, but to all imported 
users!

You may have noticed the nisNetgroupTriple's example.com entry. This is 
an indicator to NIS netgroup clients that the result of the netgroup 
query should only apply to servers in the example.com domain. If you 
have multiple domains, this can be a useful feature to further separate 
your ACLs. It's also completely optional. Leaving this portion of the 
triple empty will remove the domain restriction.

It's worth noting that the LDAP backend implementation discussed here 
can be implemented in other directory servers include Active Directory. 
Also, client functionality can be applied to most modern, PAM enabled 
UNIX systems such as Linux and Solaris.

I hope this information will be useful for systems administrators out 
there trying to implement centralized and maintainable access control in 
their Linux/UNIX network. It can be done!

Dan Cox

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