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Thursday, June 3, 2010

Mac OS X and OpenLDAP

At work we had some issues trying to join Mac OS X machines into our Samba Windows domain. Turned out Mac OS X was doing a search with scope base and empty base, which is meant to return some information that can be used for compatibility or some global knowledge about the LDAP tree. This object is the RootDSE object. In our case that search would return nothing, instead of the descriptive entry.

After quite a while we noticed closed bug #427842 on Launchpad. The bug describes some missing access control rules that can lead to this problem. Although this bug is closed, it can still show up when migrating data from an older release, which was also the case for us. The bug also has the required ldif, which I'll copy here for future reference:
dn: olcDatabase={-1}frontend,cn=config
changetype: modify
add: olcAccess
olcAccess: to dn.base="" by * read
olcAccess: to dn.base="cn=subschema" by * read

You can feed this to ldapmodify or ldapadd (yes, ldapadd can also do modifies). A quick ldapsearch will reveal if it worked:
$ ldapsearch -x -b '' -s base

This should return an object of the OpenLDAPRootDSE objectClass (and empty distinguished name).

Now we're at it, let's add another useful gem for Mac OS X: altServer attributes. Mac OS X searches for altServer attributes in order to find other servers that should provide the same data, in case the server is down (although I don't know when this data is cached).

It's possible to add attributes to the OpenLDAPRootDSE object by creating an LDIF file and pointing the olcRootDSE attribute on the config object to the created LDIF file. Create the following file, place it at /etc/ldap/rootdse.ldif:
altServer: ldap://server2.domain.tld/dc=domain,dc=tld
altServer: ldap://server3.domain.tld/dc=domain,dc=tld

Now add the following LDIF to OpenLDAP:
dn: cn=config
changetype: modify
add: olcRootDSE
olcRootDSE: /etc/ldap/rootdse.ldif

You can add this one using ldapmodify again.

Another quick ldapsearch will verify the attributes are really there:
$ ldapsearch -x -b '' -s base "+"

This should present quite a list detailing some support, including the just added altServer attributes.

Now there's one last thing that we should add to offer our Mac OS X users (or better, ourselves as sys admins!) a more pleasant experience: an Avahi (bonjour/zeroconf) entry for our OpenLDAP server. This will make the server show up as an option in some dialogs, for instance when adding an LDAPv3 directory server for authentication or contacts. To do this, add the following service file to avahi, for instance as /etc/avahi/services/slapd.service:
<?xml version="1.0" standalone='no'?><!--*-nxml-*-->
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">

<name replace-wildcards="yes">%h</name>

The only additional step to integrating OpenLDAP even more with Mac OS X would be by adding the Apple schemas and providing OpenDirectory support using OpenLDAP on Linux. I'll probably come to that later, but one thing I'll definitely post about is authentication against our existing OpenLDAP user tree.

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