Autofs and LDAP

Today I began working on replacing the NIS installation at work with an LDAP database.

As I’ve used the PAM and NSS LDAP modules a lot at customer sites integrating against eDirectory, I was rather comfortable with that part of the integration. What I didn’t know much about was how the automounter integrates with LDAP.

It turns out that this was rather easy, although the documentation is sparse. Also, the fact that I have to cope with the rather old autofs in Red Hat Linux 7.3 complicated the installation a bit (I have one machine on the network that must run RHL 7.3. The rest of the machines are running a variety of modern Linux distributions)

Client Configuration

Instead of having to manually specify which mount points to automount in /etc/auto.master on each client, all configuration is stored in LDAP. To instruct autofs to read LDAP to find automountpoints, add ldap to the automount line in /etc/nsswitch.conf. In my case, the line looks like this:

automount: files ldap

This instructs automount to first check /etc/auto.master for mount points, and then search LDAP.

Which LDAP Server is Used?

Autofs has to know which LDAP server to use. It seems the method of aquiring this information is a bit different on different distributions. The Red Hat Linux machine read /etc/ldap.conf which is the configuration file for nss_ldap and pam_ldap, while my Debian sarge workstation reads /etc/ldap/ldap.conf which is configuration for the OpenLDAP libraries. I have yet to see which file is used by Fedora Core et. al.

It also seems like there’s some support for finding which LDAP server via DNS RR records, but I haven’t investigated this further.

Old Autofs - LDAP v2-style bind, but with LDAP v3?

One problem on the Red Hat Linux 7.3 machine was that it was trying to do a LDAP version 2-style bind over LDAP version 3. It was trying to bind as the DN of the ou for an automount map (more about this later), with a null password. My OpenLDAP didn’t like this, expecting either a regular bind with DN and password, or an anonymous bind with null DN and null password.

I solved this problem by patching autofs. I could probably have upgraded autofs as well, but I’m not sure autofs 4 works with the current kernel version, so patching was easier. I still have to include the allow bind_v2 in the server configuration.

Data in LDAP

To find which mount points to handle, autofs will search LDAP for entries with the objectclas automountMap. It will then search for all entries under this DN with the objectclass automount, each of them representing a mount point for the automounter to handle.

Each of the automount entries under the automountMap entry points to another container in the LDAP tree, under which you store one automount entry per possible subdirectory to the mount point.

Confused? Let’s show some example:

The automountMap and its subtree looks like this:

dn: ou=auto.master,ou=autofs,dc=example,dc=com
ou: auto.master
objectClass: top
objectClass: automountMap

dn: cn=/import,ou=auto.master,ou=autofs,dc=example,dc=com
objectClass: automount
cn: /import

dn: cn=/home,ou=auto.master,ou=autofs,dc=example,dc=com
objectClass: automount
cn: /home

This tells the automounter that it should handle /home, and that information about which directories are available to mount under /home is available on the LDAP server under the DN ou=auto.home,ou=autofs,dc=example,dc=com.

There’s similar information for our second automount point, /import

Now, let’s inspect ou=auto.home,ou=autofs,dc=example,dc=com

dn: ou=auto.home,ou=autofs,dc=cendio,dc=se
ou: auto.home
objectClass: top
objectClass: organizationalUnit

dn: cn=wingel,ou=auto.home,ou=autofs,dc=cendio,dc=se
cn: wingel
objectClass: automount
automountInformation: -rsize=8192,wsize=8192,intr fileserver:/export/home/wingel
dn: cn=thomas,ou=auto.home,ou=autofs,dc=cendio,dc=se
cn: thomas
objectClass: automount
automountInformation: -rsize=8192,wsize=8192,intr fileserver:/export/home/thomas
dn: cn=forsberg,ou=auto.home,ou=autofs,dc=cendio,dc=se
cn: forsberg
objectClass: automount
automountInformation:  -rsize=8192,wsize=8192,intr fileserver:/export/home/thomas

As you can see, under the auto.home ou, there’s one entry for each possible mount under /home. The automountInformation attribute contains the information the automounter uses to do the actual mount.

Written on June 27, 2006