LDAP

Clients on the network use LDAP to authenticate user logins. We are running an LDAP server on Einstein. This server serves up the user information (passwd and shadow) and also lists of servers and workstations which tie into various permission schemes.

Organization

LDAP runs on einstein and tomato. The service program slapd handles requests from clients. For passwords and such it is protected with TSL encryption. (See Certificates.) The certificate "unh_physics_ca.crt" is used.

Good instructions for setting up a single LDAP server.

Setting up a master/slave server pair

From "Migrating a Mail Server to Postfix/Cyrus/OpenLDAP":

The OpenLDAP slave servers have a similar configuration to the master server. You have to keep some important things in mind.

• The slave has neither replica nor replogfile directives. (the master does, though)
• The slave has an additional updatedn option that must be the same as the binddn used in the master config file. ("binddn" doesn't seem to exist and the man page says "updatedn" should be different than "rootdn")
• The DN mentioned in the updatedn option must have write access to the tree.
• If you wish to write to your slave server, you have to include an updateref directive that points to your master server. Clients issuing write operations will then be redirected to the master.

Introducing a new slave server needs synchronisation with the master database. OpenLDAP cannot copy the whole master data. The master only transfers changes to the slaves. The initial distribution of the whole LDAP tree must be done manually.

1. Shut down the master server.
2. Extract a copy of the master's data in LDIF notation by executing slapcat -f /etc/ldap/openldap/slapd.conf master_dump.ldif.
3. Copy the resulting master_dump.ldif file to your slave server.
4. Fill the slave's database by executing slapadd -f /etc/ldap/openldap/slapd.conf master_dump.ldif. Make sure the OpenLDAP server there is shut down!
5. Restart the master server.
6. Start the slave server.

After that, your master server should log into the slave and send updates as soon as you modify the master LDAP tree. Master and slaves have a steady TCP connection that you can see by using netstat. Important note: Don't forget to create a new key and certificate for the slave or else the encryption won't work.

Configuration

For clients, configuration for LDAP is in two locations: /etc/ldap.conf and /etc/openldap/ldap.conf. Here you set the host(s) that is serving the information. Specifically, einstein.unh.edu or einstein.farm.physics.unh. Also, ldap must be referenced in /etc/nsswitch.conf like so:

passwd:     files ldap
shadow:     files ldap
group:      files ldap

Those may not be the only entries requiring a reference to ldap, but the GUI tool system-config-authentication is good at taking care of things. If a machine doesn't have that program, it's safe to look at another machine's files.

Once a user is authenticated, the client mounts the user's home directory. See Automount.

Troubleshooting

The best way to check whether LDAP is working is getent passwd, which should show user passwords. If it does not work, then env HOME=/root ldapsearch -ZZ '(uid=someusername)' may give more diagnostics. Try ldapsearch -x '(uid=someusername)' to test LDAP without using the encryption layer.

If your node has a bad system time, the certificate may look like it is from the future and will not be accepted. So check system time if users cannot log in.

Another thing to check is if the client is able to connect to the LDAP server, and that the server is referenced in the appropriate configuration files.

If LDAP isn't configured properly, or the machine cannot reach the LDAP server, then the machine may experience a very long boot time (usually hanging during "Starting system message bus...").

What is needed for a succesful login?

• Entry in ldap tables or /etc/password and /etc/shadow
• Be in the correct group. Which group is correct is controlled by PAM in /etc/security/access.conf

Administration

Administrators can use the programs /usr/sbin/luseradd, /usr/sbin/luserdel, and /usr/sbin/lusermod to add, delete, and modify users from the directory. There is also a more-friendly(??) program for adding users: /usr/local/bin/adduser-npg, however it requires that the above programs and more be accessible via the PATH environmental variable (root has the appropriate path set up, but sudo won't use it). When running any of these programs, it is important that the HOME environment variable is set to "/root". Best way to use them is: "env HOME=/root /usr/local/bin/adduser-npg".

There appears to be a problem with the libuser stuff in that a new user is added with objectClass of "posixUser" and "account", and you cannot (as far as I (Maurik) could figure out) change "account" to "person". The "account" has a mandatory field for "userid" which is redundant with "uid" and actually gets translated to "uid" when you add something there (in JXplorer). The result is that you cannot modify anything in the account, since you must have something for "userid" which must be different from "uid". See below for the difficult way of changing user info, which works.

Standard defining things like dn, cn, ou, and object classes

Servers must have certificates signed by the Certificate Authority on roentgen.

Some Useful LDAP Hints

• Add a user account: Use env HOME=/root /user/local/bin/adduser-npg
• You can look at and manipulate the tables with JXplorer (installed on improv). Login as root with the standard password for ldap.
• slapcat, slappadd can be used to dump and restore the entire ldap database. Be careful: The slapadd command can only be run when slapd is NOT running
• Modify user items (i.e. shell, home, uid, gid) env HOME=/root /usr/sbin/lusermod --shell /bin/false minuti
• Add a user to a group: env HOME=/root /usr/sbin/lgroupmod --member-add uid group

We've got to do something about needing the "env HOME=/root" stuff to get those scripts to work properly. The representation of the programs used in a script shouldn't be dependent on a certain user's PATH variable. It's already annoying enough when we've got a bunch of scripts that are dependent on a bunch of non-default programs, and the only ways to find out what they are is to pour over the entire source code, or try to run the script over and over, seeing what program it fails to find each time, hoping that what did run didn't mess the system up.

Chaning Items the "hard" way: ldapadd & ldapmodify

This is NOT the easy way, but it works when the easy way does not. You need to create a file which contains "ldif" commands for LDAP. That is the hard part. There is information on the LDAP man pages, but this is pretty terse stuff. Here is a hack:

First, dump out the record you want to change (for user "editme"):

 ldapsearch -x -L 'uid=editme' > editme.ldif

Next edit the editme.ldif file. In the case of making someone a person, you want to change the "objectClass: account" to "objectClass:person". The person class has a MUST field "sn" or sirname, so add a line: "sn: LastName". You now need to delete the old entry, and for me "ldapdelete" did not work since I could not get the correct credentials (?). Instead I used ldapmodify with the following input script:

 dn: uid=editme,ou=People,dc=physics,dc=unh,dc=edu  # This is the first line of the editme.ldif
 changetype: delete

Save the file and run:

 env HOME=/root ldapmodify -f editme_delete.ldif
 env HOME=/root ldapadd -f editme.ldif

The second command restores the record, with the changes you wanted.

It is sometimes easier to use ldapmodify, see the man pages. This would not let me change the objectClass though.

External Information

• Setup information
• Documentation for RHEL 5
• OpenLDAP Software 2.3 Administrator's Guide
• OpenLDAP Faq-O-Matic