Integrated Kerberos-OpenLDAP consumer on Debian lenny

This page describes how to set up an OpenLDAP consumer server and an MIT Kerberos V slave server on the same host so that Kerberos uses LDAP as it back-end database. It depends on a previously installed Kerberos-OpenLDAP provider server, kls1.example.com. The system also relies heavily on timestamps, so reasonably accurate time synchronization among all participating hosts is essential.

In this example, OpenLDAP and MIT Kerberos V are installed on a host running Debian 5.0 (lenny). If followed properly, the step-by-step process should produce an OpenLDAP provider server with a new DIT, followed by a Kerberos master server that stores its database in that same DIT.

But, before the interesting parts can begin, it will first be necessary to install the operating system on a new host called kls2.example.com. A DNS server must also be available on the network with a zone file to which forward and reverse mappings can be added for this host. After the initial installation of the operating system, make sure these packages are installed on the system as well:

~# apt-get install ssh ntp ntpdate nmap

Afterwards, edit /etc/ntp.conf so that the machine synchronizes to a common NTP server (preferably a local one) and edit /etc/default/ntpdate to use the same host also. Now the installation of the new server can begin.

First, run the following command to test if the MIT Kerberos V server installed previously is available on the network:

~# nmap kls1.example.com

This should be among the results:

PORT    STATE SERVICE
749/tcp open  kerberos-adm

If there is a problem, fix it first. If not, continue by installing these three packages:

~# apt-get install krb5-{config,user} libpam-krb5

A total of four packages are installed as a result, including one dependency:

krb5-config             1.22                               Configuration files for Kerberos Version 5
krb5-user               1.6.dfsg.4~beta1-5lenny2           Basic programs to authenticate using MIT Kerberos
libkadm55               1.6.dfsg.4~beta1-5lenny2           MIT Kerberos administration runtime libraries
libpam-krb5             3.11-4                             PAM module for MIT Kerberos

During the installation, the krb5-config package will automatically have the default realm set to EXAMPLE.COM, but a few questions have to be answered for it as well:

Kerberos servers for your realm: kls1.example.com
Administrative server for your Kerberos realm: kls.example.com

These settings, along with the default realm, are saved in /etc/krb5.conf.

Use kadmin to create a Kerberos principal for the LDAP service and a matching keytab file by issuing a few commands:

~# kadmin -p admin
Authenticating as principal admin with password.
Password for admin@EXAMPLE.COM: arietans
kadmin:  addprinc -randkey ldap/kls2.example.com
WARNING: no policy specified for ldap/kls2.example.com@EXAMPLE.COM; 
defaulting to no policy
Principal "ldap/kls2.example.com@EXAMPLE.COM" created.
kadmin:  ktadd ldap/kls2.example.com
Entry for principal ldap/kls2.example.com with kvno 3, encryption type
AES-256 CTS mode with 96-bit SHA-1 HMAC added to keytab
WRFILE:/etc/krb5.keytab.
Entry for principal ldap/kls2.example.com with kvno 3, encryption type
ArcFour with HMAC/md5 added to keytab 
WRFILE:/etc/krb5.keytab.
Entry for principal ldap/kls2.example.com with kvno 3, encryption type
Triple DES cbc mode with HMAC/sha1 added to keytab
WRFILE:/etc/krb5.keytab.
Entry for principal ldap/kls2.example.com with kvno 3, encryption type
DES cbc mode with CRC-32 added to keytab 
WRFILE:/etc/krb5.keytab.
kadmin:  q
~# _

The -randkey switch is used because a machine cannot enter a password. To list the keys in /etc/krb5.keytab, use the klist -ke command. A host (or service) principal and a keytab file should be created for and saved on all of the various client machines that are part of a Kerberos realm.

Before starting the install process, run the following command to test if the previously installed OpenLDAP provider server is actually available on the network:

~# nmap kls1.example.com

This should be among the results:

PORT    STATE SERVICE
389/tcp open  ldap

If there is a problem, fix it first. Otherwise, install these two packages:

~# apt-get install slapd ldap-utils

A total of ten packages are installed as a result, including eight dependencies:

ldap-utils                 2.4.11-1                    OpenLDAP utilities
libdb4.2                   4.2.52+dfsg-5               Berkeley v4.2 Database Libraries [runtime]
libltdl3                   1.5.26-4+lenny1             A system independent dlopen wrapper for GNU libtool
libperl5.10                5.10.0-19lenny2             Shared Perl library
libsasl2-modules           2.1.22.dfsg1-23+lenny1      Cyrus SASL - pluggable authentication modules
libslp1                    1.2.1-7.5                   OpenSLP libraries
odbcinst1debian1           2.2.11-16                   Support library and helper program for accessing odbc ini file
psmisc                     22.6-1                      Utilities that use the proc filesystem
slapd                      2.4.11-1                    OpenLDAP server (slapd)
unixodbc                   2.2.11-16                   ODBC tools libraries

During the install process, an administrator password will be requested for slapd. Use somalica:

Administrator password: somalica
Confirm password: somalica

This password is actually inconsequential and will not be needed again.

Run the following command to test if the OpenLDAP server is actually running:

~# nmap kls2.example.com

This should be among the results:

PORT    STATE SERVICE
389/tcp open  ldap

Perform a quick test by generating an LDIF dump of the contents of a the database:

~# slapcat
dn: dc=example,dc=com
objectClass: top
objectClass: dcObject
objectClass: organization
o: example.com
dc: example
structuralObjectClass: organization
entryUUID: e12251be-a923-102e-8428-ff3aa2ce9756
creatorsName:
createTimestamp: 20100208173356Z
entryCSN: 20100208173356.975696Z#000000#000#000000
modifiersName:
modifyTimestamp: 20100208173356Z

dn: cn=admin,dc=example,dc=com
objectClass: simpleSecurityObject
objectClass: organizationalRole
cn: admin
description: LDAP administrator
userPassword:: e2NyeXB0fXpwYldxSC5GbjVzamc=
structuralObjectClass: organizationalRole
entryUUID: e122f4de-a923-102e-8429-ff3aa2ce9756
creatorsName:
createTimestamp: 20100208173356Z
entryCSN: 20100208173356.980385Z#000000#000#000000
modifiersName:
modifyTimestamp: 20100208173356Z

~# _

Edit /etc/ldap/ldap.conf and use these two lines:

BASE    dc=example,dc=com
URI     ldap://kls2.example.com/

This configuration file is used to set system-wide defaults for LDAP clients.

To set up a Kerberos slave server, install these two packages:

~# apt-get install krb5-{kdc,kdc-ldap}

These are also the only two packages that are installed as a result:

krb5-kdc                    1.6.dfsg.4~beta1-5lenny2             MIT Kerberos key server (KDC)
krb5-kdc-ldap               1.6.dfsg.4~beta1-5lenny2             MIT Kerberos key server (KDC) LDAP plugin

Since the idea is for Kerberos to use OpenLDAP as its back-end database, see to it that the Kerberos KDC and administrative services start up after slapd by changing the names of the symbolic links for their respective startup scripts:

~# mv /etc/rc2.d/S18krb5-kdc /etc/rc2.d/S20krb5-kdc
~# _

Following this automated configuration sequence for the package, a message appears regarding krb5kdc: that the realm, EXAMPLE.COM cannot be initialize. This is because the realm, or rather the database for it, has not yet been created, but that is an issue that will be dealt with later.

Edit /etc/krb5kdc/kdc.conf and modify these two lines − the same as the Kerberos master server is configured:

        max_life = 1d 0h 0m 0s
        max_renewable_life = 90d 0h 0m 0s

This version of Kerberos includes the V to IV ticket conversion daemon, krb524d. Unfortunately, it suffers from a type of bug, called a file descriptor leak, that manifests itself when running Kerberos together with an LDAP back-end. The bug causes slapd to slowly run out of file descriptors and eventually become unresponsive. Since it is unlikely that the Kerberos developers will address this problem (krb524d has been removed from current Kerberos V releases), a workaround is necessary. Create a file, called /usr/local/bin/slapd-check, with the following contents:

#!/bin/sh

PATH=/usr/bin:/bin

openfiles=`lsof |grep -c slapd` 

daemonactive=`ps ax |grep -c krb524d`

if [ $daemonactive -gt 0 ] && [ $openfiles -gt 100 ]; then 

	killall krb524d
	krb524d -m
fi

exit 0

This script checks if krb524d is running, and if so, whether the number of files that slapd has open is greater than 100. If both these tests are true, krb524d is killed and restarted.

Make the new script executable:

~# chmod 750 /usr/local/bin/slapd-check

Finally, have cron (crontab -e) run this script every fifteen minutes:

*/15 * * * * /usr/local/bin/slapd-check

Unzip and copy the Kerberos schema file to the OpenLDAP server's schema directory:

~# gunzip -c /usr/share/doc/krb5-kdc-ldap/kerberos.schema.gz > \
   /etc/ldap/schema/kerberos.schema

Edit /etc/ldap/slapd.conf and make a number of changes to it. Starting at the top of the file, add a line to include support for the Kerberos schema in the DIT:

# Schema and objectClass definitions
include         /etc/ldap/schema/core.schema
include         /etc/ldap/schema/cosine.schema
include         /etc/ldap/schema/nis.schema
include         /etc/ldap/schema/inetorgperson.schema
include         /etc/ldap/schema/kerberos.schema

Comment out the logging option, which will return it to its default value (256):

#loglevel "none"

Later on, after the configuration is finished and the whole system is working properly, it will likely be desirable to once again return the loglevel value to "none", because Kerberos tends to cause a lot of slapd log output (at least once every minute) as it accesses its back-end database. This level of logging may also affect performance.

Add an entry for a hard-coded root user, which is necessary for replication:

# rootdn          "cn=admin,dc=example,dc=com"

rootdn          "cn=manager"

It does not matter that the rootdn is not part of the DIT; just as long as it exists and is unique within the same system of replicating servers.

Add two new index directives for the uid and krb5principalname attributes:

index           objectClass eq
index           uid eq
index           krbPrincipalName eq,pres,sub

The first eq index will facilitate searches for uid entries, while the second will do the same for Kerberos principal entries. Both will speed up the login process.

Last, modify the ACLs to look like this:

access to attrs=userPassword,shadowLastChange
	by dn="cn=admin,dc=example,dc=com" read
        by anonymous auth
        by * none

access to dn.subtree="ou=krb5,dc=example,dc=com"
	by dn="cn=admin,dc=example,dc=com" read
	by dn="cn=adm-srv,ou=krb5,dc=example,dc=com" read
	by dn="cn=kdc-srv,ou=krb5,dc=example,dc=com" read
	by * none

access to dn.base=""
	by * read

access to *
        by users read
        by * none

It is pointless to add the same access directives here as on the provider to allow clients to write, since all OpenLDAP database synrepl replicas are strictly read-only. If clients attempt to write to it anyway, they will be given a referral (URL) to the provider before these ACLs are ever encountered.

Edit /etc/default/slapd and allow the LDAP service to listen for IPC connections (Unix domain sockets) by adding this line to the end of the file:

SLAPD_SERVICES="ldap:/// ldapi:///"

This will instruct slapd to listen on IPC, instead of only TCP port 389, which is the default.

Save all of these changes, but do not restart the LDAP service until later.

Edit the Kerberos realm configuration file, /etc/krb5.conf. This file is initially created by the Debian installer and contains information about the realms of a number of famous institutions, but none of that is necessary in this case. Instead, replace its contents with this:

[libdefaults]
	default_realm = EXAMPLE.COM
	forwardable = true
	proxiable = true

[realms]
	EXAMPLE.COM = {
		kdc = kls2.example.com
		admin_server = kls.example.com
		database_module = openldap_ldapconf
	}

[domain_realm]
	.example.com = EXAMPLE.COM
	example.com = EXAMPLE.COM

[dbdefaults]
	ldap_kerberos_container_dn = ou=krb5,dc=example,dc=com

[dbmodules]
	openldap_ldapconf = {
		db_library = kldap
		ldap_kdc_dn = cn=kdc-srv,ou=krb5,dc=example,dc=com
		ldap_kadmind_dn = cn=adm-srv,ou=krb5,dc=example,dc=com
		ldap_service_password_file = /etc/krb5kdc/service.keyfile
		ldap_conns_per_server = 5
	}

[logging]
	kdc = FILE:/var/log/krb5/kdc.log
	default = FILE:/var/log/krb5/kdc.log

See this section for a more detailed explanation of this file.

Note the absence of the ldap_servers statement from the list of openldap_ldapconf options. This is possible because the Kerberos daemons use IPC (Unix domain sockets) to connect to the LDAP back-end by default.

After /etc/krb5.conf has been saved, create the Kerberos log directory:

~# mkdir /var/log/krb5
~# _

To prevent the log file from growing too large, create a logrotate configuration file. Edit /etc/logrotate.d/krb5-kdc and give it the following contents:

/var/log/krb5/kdc.log {
	daily
	missingok
	rotate 7
	compress
	delaycompress
	notifempty
	postrotate
		/etc/init.d/krb5-kdc restart > /dev/null
	endscript
}

The various password stashes that were created previously on the provider − for the KDC database master key, the KDC service (for cn=kdc-srv) and the Kerberos administration server (cn=adm-srv) − are also required on this host. However, instead of creating them from scratch as was done before, simply copy them from the provider:

~# scp kls1.example.com:/etc/krb5kdc/s* /etc/krb5kdc/

Two files will be copied as a result: stash contains the KDC database master key, while service.keyfile contains the two service passwords.

Switch over to the provider server, kls1.example.com, to make some changes there. Start by installing a package that will prevent a replication error from appearing in the syslog:

root@kls1:~# apt-get install sasl2-bin

A total of two packages are installed as a result, including one dependency:

db4.6-util             4.6.21-11                      Berkeley v4.6 Database Utilities
sasl2-bin              2.1.22.dfsg1-23+lenny1         Cyrus SASL - administration programs for SASL users database

The installation also warns that saslauthd requires some tending to before it will start up automatically, but that can be ignored. All that is important here is that the install process creates a small database file, /etc/sasldb2, that slapd needs to have write access to so that certain error messages will not be generated. Next, make sure that the sasl group has access to the file:

root@kls1:~# chgrp sasl /etc/sasldb2
root@kls1:~# chmod 660 /etc/sasldb2
root@kls1:~# _

Then edit /etc/group on kls1 and add openldap to the sasl group:

sasl:x:45:openldap

Create a new LDAP entry that will be used to represent and authorize kls2 to read the entire DIT. Start by creating a file, ~/kls2.ldif, with the following contents:

dn: ou=consumers,dc=example,dc=com
ou: consumers
objectClass: organizationalUnit

dn: cn=kls2,ou=consumers,dc=example,dc=com
cn: kls2
objectClass: simpleSecurityObject
objectClass: organizationalRole
description: LDAP server2 replicator
userPassword: {CRYPT}*

Add these new LDAP entries to the DIT with:

root@kls1:~# kinit admin
Password for admin@EXAMPLE.COM: arietans
root@kls1:~# ldapadd -f ~/kls2.ldif
SASL/GSSAPI authentication started
SASL username: admin@EXAMPLE.COM
SASL SSF: 56
SASL data security layer installed.
adding new entry "ou=consumers,dc=example,dc=com"

adding new entry "cn=kls2,ou=consumers,dc=example,dc=com"

root@kls1:~# _

Next, edit /etc/ldap/slapd.conf on kls1 and make a number of changes to it. First, add this extra authz-regexp stanza:

authz-regexp
     uid=host/([^/\.]+).example.com,cn=example.com,cn=gssapi,cn=auth
     cn=$1,ou=hosts,dc=example,dc=com

authz-regexp
     uid=ldap/([^/\.]+).example.com,cn=example.com,cn=gssapi,cn=auth
     cn=$1,ou=consumers,dc=example,dc=com

authz-regexp
     uid=([^,]+),cn=example.com,cn=gssapi,cn=auth
     uid=$1,ou=people,dc=example,dc=com

This is to map the GSSAPI name for kls2 to that of of the new LDAP entry, cn=kls2.

Add two new eq index directives:

index           objectClass eq
index           uid eq
index           krbPrincipalName eq,pres,sub
index           entryUUID eq
index           entryCSN eq

Then modify the ACLs to ensure that cn=kls2 has read access to everything in the database:

access to attrs=userPassword,shadowLastChange
        by dn="cn=admin,dc=example,dc=com" write
	by dn.one="ou=consumers,dc=example,dc=com" read
        by anonymous auth
        by * none

access to dn.subtree="ou=krb5,dc=example,dc=com"
        by dn="cn=admin,dc=example,dc=com" write
	by dn.one="ou=consumers,dc=example,dc=com" read
        by dn="cn=adm-srv,ou=krb5,dc=example,dc=com" write
        by dn="cn=kdc-srv,ou=krb5,dc=example,dc=com" read
        by * none

access to attrs=loginShell
        by dn="cn=admin,dc=example,dc=com" write
        by self write
        by users read
        by * none

access to dn.base=""
        by * read

access to *
        by dn="cn=admin,dc=example,dc=com" write
        by users read
        by * none

This ensures that all entries directly under ou=consumers will have full read access to the DIT.

Lastly, add these four lines to the end of the first database section to load, activate and configure the Sync Provider overlay:

access to *
        by dn="cn=admin,dc=example,dc=com" write
        by users read
        by * none

moduleload      syncprov
overlay         syncprov

syncprov-checkpoint 100 10
syncprov-sessionlog 100

Having made certain changes to slapd.conf on kls1 that include new index directives, it not only becomes necessary to restart slapd, but also to reindex the database. Since it is best to run the slapindex command as the openldap user, first make this possible by altering the following line (that may not have the same UID and GID numbers) in the /etc/passwd file:

openldap:x:105:107:OpenLDAP Server Account,,,:/var/lib/ldap:/bin/sh

Now stop the LDAP service, regenerate the database indices as the openldap user, and restart the LDAP service:

root@kls1:~# /etc/init.d/krb5-admin-server stop
root@kls1:~# /etc/init.d/krb5-kdc stop
root@kls1:~# /etc/init.d/slapd stop
Stopping OpenLDAP: slapd.
root@kls1:~# su -c /usr/sbin/slapindex openldap
root@kls1:~# /etc/init.d/slapd start
Starting OpenLDAP: slapd.
root@kls1:~# /etc/init.d/krb5-kdc start
root@kls1:~# /etc/init.d/krb5-admin-server start
root@kls1:~# _

Return to kls2. To keep its copy of the LDAP database in sync, the consumer must contact the provider and authenticate itself just like any other client. It has previously been shown how to do this with a simple bind and a password, but in this case Kerberos will be used to take care of both slapd consumer authentication to the provider and encryption of the entire replication process. To make this possible, an initial Kerberos ticket (a TGT) must be obtained automatically on behalf of the openldap user, which owns the slapd process, and it must be renewed regularly. One way to do this is to run kinit -k with a cron job, but a better solution is to use a modified version of kinit, called k5start. Install it with:

~# apt-get install kstart

This is the only package that gets installed as a result:

kstart                   3.14-1                     Kerberos kinit supporting AFS and ticket refreshing

To configure it, just add this line to the end of the /etc/inittab file to start running k5start in the background soon after the system boots up:

KS:2345:respawn:/usr/bin/k5start -U -f /etc/krb5.keytab -K 10 -l 24h -k /tmp/krb5cc_105 -o openldap

This must be written as a single line. A number of options have been used for this command:

After saving this modification to /etc/inittab, start k5start for the first time by forcing init to reload its configuration file:

~# kill -HUP 1

A new ticket cache file, /tmp/krb5cc_105, should be created almost immediately as a consequence.

Install a package, the sole purpose of which is to prevent a certain error from appearing in the syslog:

~# apt-get install sasl2-bin

A total of two packages are installed as a result, including one dependency:

db4.6-util           4.6.21-11                       Berkeley v4.6 Database Utilities
sasl2-bin            2.1.22.dfsg1-23+lenny1          Cyrus SASL - administration programs for SASL users database

The sasldb2 database file is installed here for the same reasons as on kls1. Begin by making sure that the sasl group has access to the file:

~# chgrp sasl /etc/sasldb2
~# chmod 660 /etc/sasldb2
~# _

Then edit /etc/group and add openldap to the sasl group:

sasl:x:45:openldap

This step is about kerberizing OpenLDAP. A number of requirements have already been met, but there is more. First, install this one package:

~# apt-get install libsasl2-modules-gssapi-mit

Only one package is installed as a result with no dependencies:

libsasl2-modules-gssapi-mit       2.1.22.dfsg1-23+lenny1        Cyrus SASL - pluggable authentication modules (GSSAPI)

Change the permissions and ownership of the Kerberos service keytab file to allow slapd to read it:

~# chmod 640 /etc/krb5.keytab
~# chown root.openldap /etc/krb5.keytab
~# _

Next, three different files must be modified. First, edit /etc/default/slapd and and uncomment a line near the end of the file that will export the location of the Kerberos system keytab file as a variable:

export KRB5_KTNAME=/etc/krb5.keytab

Edit /etc/ldap/ldap.conf and add the following line, which specifies the authentication mechanism, to the end of the file:

SASL_MECH GSSAPI

Then, edit /etc/ldap/slapd.conf and add these lines to the end of the Global section (before any database definitions):

tool-threads 1

authz-regexp
     uid=admin,cn=example.com,cn=gssapi,cn=auth
     cn=admin,dc=example,dc=com

authz-regexp
     uid=host/([^/\.]+).example.com,cn=example.com,cn=gssapi,cn=auth
     cn=$1,ou=hosts,dc=example,dc=com

authz-regexp
     uid=([^,]+),cn=example.com,cn=gssapi,cn=auth
     uid=$1,ou=people,dc=example,dc=com

sasl-realm      EXAMPLE.COM

All of these changes were also made on the provider server, kls1.

Continue to edit /etc/ldap/slapd.conf, this time to enable the replication process. Start by adding two more eq index entries:

index           objectClass eq
index           uid eq
index           krbPrincipalName eq,pres,sub
index           entryUUID eq
index           entryCSN eq

These two eq indexes will accelerate the replication process.

Then add this replication stanza to the end of the first database section:

access to *
        by users read
        by * none

syncrepl rid=123
        provider=ldap://kls.example.com:389/
        type=refreshAndPersist
        retry="60 30 300 +"
        searchbase="dc=example,dc=com"
        bindmethod=sasl
        saslmech=gssapi

With the refreshAndPersist replication type, any changes made to the provider database are immediately sent to the consumer. Note that the provider's alias, ldapks, is used here, so if the OpenLDAP provider is ever moved to a different host, only the alias will need to be changed − not the configurations of the various consumers.

Alternatively, in case a more bandwidth-friendly solution is desired, the refreshOnly replication strategy can be used instead:

syncrepl rid=123
        provider=ldap://kls.example.com:389/
        type=refreshOnly
        interval=00:00:05:00
        searchbase="dc=example,dc=com"
        bindmethod=sasl
        saslmech=gssapi

Usually, complete replication of the provider's copy of the OpenLDAP database is only possible if specific access rules are added to the provider's slapd.conf to ensure that each consumer server has read access to everything. However, in this case it is currently unnecessary to change the provider's ACLs, because other than a few unnecessary passwords, all users have complete access to the DIT anyway.

Still editing /etc/ldap/slapd.conf, add a referral to the end of the file, below the syncrepl stanza, to inform clients where to find the provider when they need to make changes to the LDAP database:

        bindmethod=sasl
        saslmech=gssapi

updateref       ldap://kls.example.com:389/

Finally, since some LDAP client applications do not understand referrals, implement the chain overlay to allow slapd to chase referrals on behalf of such clients by adding the following lines to the global section of slapd.conf, before any database definitions:

sasl-realm      EXAMPLE.COM

moduleload              back_ldap
overlay                 chain

chain-uri               ldap://kls.example.com:389/
chain-rebind-as-user    TRUE
chain-idassert-bind     bindmethod=sasl
                        saslmech=gssapi
                        mode=self
chain-return-error      TRUE

Having saved all of the previous changes to /etc/slapd.conf, stop the LDAP service, delete the now-redundant local database, and restart the service. This will cause the consumer to synchronize its database with the provider for the first time:

~# /etc/init.d/slapd stop
Stopping OpenLDAP: slapd.
~# rm /var/lib/ldap/*
~# /etc/init.d/slapd start
Starting OpenLDAP: slapd.
~# _

If the original copy of the local database on kls2 is not deleted, its two existing entries − the domain component and the admin user (with password somalica) − will not be replaced with those of the provider, even though the rest of the DIT will be synchronized.

If everything works as expected, it should now be possible to run the slapcat command on both the consumer and the provider and verify that, in both cases, the entire output is exactly the same.

Start the Kerberos admin and KDC servers for the first time:

~# /etc/init.d/krb5-kdc start
~# _

If there are no errors, run the following command to verify that the new MIT Kerberos V master server is indeed available on the network:

~# nmap -sU kls2.example.com.

Note the trailing dot! This should be among the results:

PORT     STATE SERVICE
88/udp   open|filtered kerberos-sec
750/udp  open|filtered kerberos
4444/udp open|filtered krb524

Run some tests. First try a simple unauthenticated (-x) LDAP query:

~# ldapsearch -x -LLL cn=kls2
No such object (32)
~# _

As on kls1, this no longer works. The authenticated version will also give an error:

~# ldapsearch -LLL cn=kls2
SASL/GSSAPI authentication started
ldap_sasl_interactive_bind_s: Local error (-2)
~# _

Acquire a Kerberos ticket for the admin user (password arietans):

~# kinit admin
Password for admin@EXAMPLE.COM: arietans
~# _

A verification of the ticket should show a success:

~# klist -5
Ticket cache: FILE:/tmp/krb5cc_0
Default principal: admin@EXAMPLE.COM

Valid starting     Expires            Service principal
02/09/10 03:49:04  02/10/10 03:49:04  krbtgt/EXAMPLE.COM@EXAMPLE.COM
~# _

Now the authenticated version of the query should also work:

~# ldapsearch -LLL cn=kls2
SASL/GSSAPI authentication started
SASL username: admin@EXAMPLE.COM
SASL SSF: 56
SASL data security layer installed.
dn: cn=kls2,ou=consumers,dc=example,dc=com
cn: kls2
objectClass: simpleSecurityObject
objectClass: organizationalRole
description: LDAP server2 replicator
userPassword:: e0NSWVBUfSo=

~# _

Before the chaining configuration added in step 14 will work, more changes first have to be made to the OpenLDAP provider server, kls1.example.com. What is required is something called SASL proxy authorization. With this mechanism it is possible to allow an authenticated user, such as the consumer server in this example, to assume the identity of other users and use that to relay their write requests on to the provider server. The ability to assume another user's identity is important, as it would otherwise not be possible to enforce existing access rules.

Two changes are needed to enable proxy authorization. First, the cn=kls2 LDAP object must be modified to include the authzTo attribute, which will allow it to act as an authorization proxy for certain users. Second, an authz-policy directive must be added to the slapd.conf file. Start by creating an LDIF file on the provider server, called ~/kls2-mod.ldif, with the following contents:

dn: cn=kls2,ou=consumers,dc=example,dc=com
changetype: modify
add: authzTo
authzTo: dn.regex:^uid=[^,]+,ou=people,dc=example,dc=com$
authzTo: dn.exact:cn=admin,dc=example,dc=com
-

The authzTo attribute is a source rule that determines which user identities it is allowed to assume; in this case its value is a regular expression that matches all uid objects in the ou=people,dc=example,dc=com container.

Run the following command to apply this modification:

root@kls1:~# ldapmodify -f ~/kls2-mod.ldif
SASL/GSSAPI authentication started
SASL username: admin@EXAMPLE.COM
SASL SSF: 56
SASL data security layer installed.
modifying entry "cn=kls2,ou=consumers,dc=example,dc=com"

root@kls1:~# _

Next, edit /etc/ldap/slapd.conf on kls1 and add this line to the end of the Global directives section:

sasl-realm      EXAMPLE.COM

authz-policy    to

This statement enables proxy authorization using rules associated with the authzTo attribute of the authentication DN (in this case cn=kls2). Without this, the authzTo attribute that was added to the cn=kls2 entry would be ignored.

Having made these changes to /etc/slapd.conf, save them and restart the LDAP service:

root@kls1:~# /etc/init.d/slapd restart
Stopping OpenLDAP: slapd.
Starting OpenLDAP: slapd.
root@kls1:~# _

This last step will test whether the proxy authorization actually works. This is easily done on the consumer server using the ldapadd command. This exercise will also be used as an excuse to create a new user account. Start by using kadmin to create an account for a user ccolumbus with password NewWorld:

~# kadmin -p admin
Authenticating as principal admin with password.
Password for admin@EXAMPLE.COM: arietans
kadmin:  addprinc ccolumbus
WARNING: no policy specified for ccolumbus@EXAMPLE.COM; defaulting 
to no policy
Enter password for principal "ccolumbus@EXAMPLE.COM": NewWorld
Re-enter password for principal "ccolumbus@EXAMPLE.COM": NewWorld
Principal "ccolumbus@EXAMPLE.COM" created.
kadmin: q
~# _

Having completed that, the object is now to test the proxy authentication configuration by creating a matching LDAP user account for ccolumbus. If this works, the changes will be immediately available on the consumer server (or within five minutes if refreshOnly replication is used). First, create a file, called ~/ccolumbus.ldif, with the following contents:

dn: cn=ccolumbus,ou=groups,dc=example,dc=com
cn: ccolumbus
gidNumber: 20001
objectClass: top
objectClass: posixGroup

dn: uid=ccolumbus,ou=people,dc=example,dc=com
uid: ccolumbus
uidNumber: 20001
gidNumber: 20001
cn: Christopher
sn: Columbus
objectClass: top
objectClass: person
objectClass: posixAccount
objectClass: shadowAccount
loginShell: /bin/bash
homeDirectory: /home/ccolumbus
userPassword: {CRYPT}*

Then apply this change with the ldapadd command:

~# ldapadd -f ~/ccolumbus.ldif
SASL/GSSAPI authentication started
SASL username: admin@EXAMPLE.COM
SASL SSF: 56
SASL data security layer installed.
adding new entry "cn=ccolumbus,ou=groups,dc=example,dc=com"

adding new entry "uid=ccolumbus,ou=people,dc=example,dc=com"

~# _

That looked like a success. Over on kls1, this is what appeared in the syslog as a result:

Feb 13 23:20:09 kls1 slapd[3968]: conn=2 op=2 \
	BIND authcid="ldap/kls2.example.com@EXAMPLE.COM" \
	authzid="ldap/kls2.example.com@EXAMPLE.COM"
Feb 13 23:20:09 kls1 slapd[3968]: conn=2 op=2 \
	BIND dn="cn=kls2,ou=consumers,dc=example,dc=com" \
	mech=GSSAPI sasl_ssf=56 ssf=56
Feb 13 23:20:09 kls1 slapd[3968]: \
	conn=2 op=2 RESULT tag=97 err=0 text=
Feb 13 23:20:09 kls1 slapd[3968]: \
	conn=2 op=3 PROXYAUTHZ dn="cn=admin,dc=example,dc=com"
Feb 13 23:20:09 kls1 slapd[3968]: \
	conn=2 op=3 ADD dn="cn=ccolumbus,ou=groups,dc=example,dc=com"

This confirms that kls2 is using its Kerberos ID, ldap/kls2.example.com@EXAMPLE.COM, which is then translated by the authz-regexp statement on kls1 to cn=kls2, which is then authorized to act as a proxy for cn=admin, which is in turn authorized to add the new cn=ccolumbus entry.

For another confirmation of success, a search for uid=ccolumbus should show that this entry has indeed been added to the DIT:

~# ldapsearch -LLL uid=ccolumbus
SASL/GSSAPI authentication started
SASL username: admin@EXAMPLE.COM
SASL SSF: 56
SASL data security layer installed.
dn: uid=ccolumbus,ou=people,dc=example,dc=com
uid: ccolumbus
uidNumber: 20001
gidNumber: 20001
cn: Christopher
sn: Columbus
objectClass: top
objectClass: person
objectClass: posixAccount
objectClass: shadowAccount
loginShell: /bin/bash
homeDirectory: /home/ccolumbus
userPassword:: e0NSWVBUfSo=

~# _

• Integrated Kerberos-OpenLDAP provider on Debian lenny
• Integrated Kerberos-OpenLDAP client on Debian lenny
• Integrated Kerberos-OpenLDAP consumer on Debian squeeze

• Eastlake D, Panitz A. 1999. RFC2606 − Reserved Top Level DNS Names. The Internet Society. HTML at the Internet FAQ Archives.
• Hodges J, Morgan R. 2002. RFC3377 − Lightweight Directory Access Protocol (v3): Technical Specification. The Internet Society. HTML at the Internet FAQ Archives.
• Kohl J, Neuman C. 1993. RFC1510 − The Kerberos Network Authentication Service (V5). HTML at the Internet FAQ Archives.
• Linn J. 2000. RFC2743 − Generic Security Service Application Program Interface Version 2, Update 1. The Internet Society. HTML at the Internet FAQ Archives.
• Myers J. 1997. RFC2222 − Simple Authentication and Security Layer (SASL). The Internet Society. HTML at the Internet FAQ Archives.
• Wahl M, Howes T, Kille S. 1997. RFC2251 − Lightweight Directory Access Protocol (v3). The Internet Society. HTML at the Internet FAQ Archives.
• Wray J. 2000. RFC2744 − Generic Security Service API Version 2 : C-bindings. The Internet Society. HTML at the Internet FAQ Archives.
• Yeong W, Howes T, Kille S. 1993. RFC1487 − X.500 Lightweight Directory Access Protocol. The Internet Society. HTML at the Internet FAQ Archives.

• Carter G. 2003. LDAP System Administration. O'Reilly & Associates, Inc. ISBN 1-56592-491-6. 294 pp.
• Massachusetts Institute of Technology. 1985-2007. Kerberos V5 System Administrator's Guide. HTML at the Massachusetts Institute of Technology (MIT). See section 6: Configuring Kerberos with OpenLDAP back-end.
• Milicchio F, Gehrke WA. 2007. Distributed Services with OpenAFS. Springer-Verlag. ISBN-13 978-3-540-36633-1. 395 pp.
• OpenLDAP Project. 2009. OpenLDAP Software 2.4 Administrator's Guide. HTML at OpenLDAP.
• Ubuntu documentation. Ubuntu 9.04. Ubuntu Server Guide, Network Authentication, Kerberos and LDAP. HTML at the Ubuntu 9.04 Server Guide.