Generating a memberOf attribute for posixGroups

Mon 22 July 2013 by Lars Kellogg-Stedman Tags ldap

This showed up on #openstack earlier today:

2013-07-22T13:56:10  <m0zes> hello, all. I am looking to
setup keystone with an ldap backend. I need to filter
users based on group membership, in this case a
non-rfc2307 posixGroup. This means that memberOf doesn't
show up, and that the memberUid in the group is not a
dn. any thoughts on how to accomplish this?

It turns out that this is a not uncommon question, so I spent some time today working out a solution using the dynlist overlay for OpenLDAP.

The LDIF data presented in this article can be found on github.

Assumptions

I'm assuming that you have a traditional posixGroup that looks something like this:

dn: cn=lars,ou=groups,dc=oddbit,dc=com
objectClass: posixGroup
cn: lars
gidNumber: 1000
memberUid: lars

That is, members are recorded in the memberUid attribute which corresponds to the uidNumber attribute of a user object.

Loading the dynlist module

This solution makes use of the dynlist dynamic overlay, so you'll first need to make sure that module is loaded. Most modern OpenLDAP deployments make use of the new slapd.d configuration directory, which means you'll modify your configuration by loading the following LDIF file:

dn: cn=modules,cn=config
objectClass: olcModuleList
cn: modules
olcModuleLoad: dynlist

You would load this into your running instance with something like the following:

# ldapadd -Y EXTERNAL -H ldapi://%2fvar%2frun%2fldapi -f dynlist.ldif

This makes certain assumptions about how your permissions are configured (in particular, it assumes that your server is configured to permit administrative access to system UID 0 when accessing the ldapi socket).

If you already have a cn=modules{0},cn=config object, you'll need to modify instead using the following:

dn: cn=modules,cn=config
changetype: modify
add: olcModuleLoad
olcModuleLoad: dynlist

And use ldapmodify:

# ldapmodify -Y EXTERNAL -H ldapi://%2fvar%2frun%2fldapi -f dynlist.ldif

Schema modifications

In an ideal world, we would be able to make our solution populate the standard memberOf attribute. Unfortunately, this is an "operational" attribute in OpenLDAP, which means we can't make it available to a user class...so, we're going to define (a) a new attributeType that is largely identical to the memberOf attribute, and (b) a new auxiliary object class that allows the new attribute.

dn: cn=oddbit,cn=schema,cn=config
objectClass: olcSchemaConfig
cn: oddbit
olcAttributeTypes: ( 1.3.6.1.4.1.24441.1.1.1 
 NAME 'obMemberOf' 
 DESC 'Distinguished name of a group of which the object is a member' 
 EQUALITY distinguishedNameMatch 
 SYNTAX 1.3.6.1.4.1.1466.115.121.1.12 )
olcObjectClasses: ( 1.3.6.1.4.1.24441.2.1.1 
 NAME 'obPerson' DESC 'oddbit.com person' 
 AUXILIARY MAY ( obMemberOf ) )

This gives us the obMemberOf attribute and the obPerson object class. NOTE: the OIDs I'm using here are using my own IANA-assigned OID prefix. You should replace 1.3.6.1.4.1.24441 with your own OID prefix. If you don't have one (and you're sure your organization doesn't already have one), you can register for your own.

Defining a dynamic list

We're going to configure the dynlist overlay so that when it sees an obPerson object, it will use the labeledURI attribute of that object to generate a list of obMemberOf attributes containing the distinguished names of the groups of which the user is a member. We'll load the following LDIF file into our server:

dn: olcOverlay=dynlist,olcDatabase={2}hdb,cn=config
objectClass: olcOverlayConfig
objectClass: olcDynamicList
olcOverlay: dynlist
olcDlAttrSet: obPerson labeledURI obMemberOf

Note that the distinguished name for this entry depends on the DN of the database which you are configuring, so you'll need to modify the olcDatabase= component in the DN.

Setting user attributes

With the above configuration in place, we can now add the necessary labeledURI attribute to a user and see what happens. For our purposes, this attribute needs to contain an LDAP URI that returns the groups of which the user is a member. Assuming a user like this:

dn: cn=user1,ou=people,dc=oddbit,dc=com
objectClass: posixAccount
objectClass: inetOrgPerson
cn: user1
sn: testuser
uid: user1
uidNumber: 1001
gidNumber: 1001
homeDirectory: /home/user1

We'll need to add the following:

labeledURI: ldap:///ou=groups,dc=oddbit,dc=com??sub?(&(
 objectclass=posixgroup)(memberuid=user1))

You could do this with the following LDIF file and ldapmodify:

dn: cn=user1,ou=people,dc=oddbit,dc=com
changetype: modify
add: labeledURI
labeledURI: ldap:///ou=groups,dc=oddbit,dc=com??sub?(&(
 objectclass=posixgroup)(memberuid=user1))

Testing things out

Assuming we have the following groups:

dn: cn=user1,ou=groups,dc=oddbit,dc=com
objectClass: posixGroup
cn: user1
gidNumber: 1001
memberUid: lars
memberUid: user1

dn: cn=staff,ou=groups,dc=oddbit,dc=com
objectClass: posixGroup
cn: staff
gidNumber: 2000
memberUid: user1

If we look up the user1 user:

# ldapsearch -Y EXTERNAL -H ldapi://%2fvar%2frun%2fldapi -b \
  ou=people,dc=oddbit,dc=com cn=user1

We should see obMemberOf attributes in the result:

dn: cn=user1,ou=people,dc=oddbit,dc=com
cn: user1
sn: testuser
uid: user1
uidNumber: 1001
gidNumber: 1001
homeDirectory: /home/user1
labeledURI: ldap:///ou=groups,dc=oddbit,dc=com??sub?(&(objectclass=posixgroup)
 (memberuid=user1))
objectClass: inetOrgPerson
objectClass: oddPerson
objectClass: posixAccount
obmemberof: cn=user1,ou=groups,dc=oddbit,dc=com
obmemberof: cn=staff,ou=groups,dc=oddbit,dc=com

Caveats

Note that this solution requires searching through all of your group entries every time you look up a user object. Given a sufficiently large directory this may not be an optimal solution.


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