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How to use Kerberos with 389 Directory Server

Read Me First

Please refer to https://access.redhat.com/site/documentation/en-US/Red_Hat_Directory_Server/9.0/html/Administration_Guide/SASL.html and https://access.redhat.com/site/documentation/en-US/Red_Hat_Directory_Server/9.0/html/Administration_Guide/Configuring_Kerberos.html before continuing.

How do I configure 389 to use SASL and GSSAPI to authenticate against a local Kerberos realm?

This describes how to configure 389 to allow users to present their Kerberos credentials (their ticket) to 389 for authentication, using the SASL GSSAPI mechanism. This allows 389 to participate in Single Sign On - a user acquires his/her ticket via kinit or login and can use it to authenticate to various services, including 389.

This requires a previously configured Kerberos realm. We've tested this with MIT Kerberos 5, but other implementations such as Heimdal should also work. You need a key for the LDAP service, an appropriate SASL mapping for GSSAPI, and the cyrus-sasl-gssapi package. 389 uses the cyrus-sasl package to interface to Kerberos.

DNS/hostnames

Kerberos relies heavily on the fully qualified host and domain name (FQDN) for service hosts and service principal names. Make sure all of your clients can resolve the Kerberos server host and LDAP server host by their FQDN. This usually means getting DNS working correctly on all of your server and client machines, but for testing or evaluation purposes you can usually hack /etc/hosts and /etc/nsswitch.conf to make it work correctly. When configuring aliases for hosts, the FQDN MUST COME FIRST BEFORE ANY ALIASES. That is, when IP to hostname or hostname to canonical hostname resolution is done, the very first result must be the FQDN, before any aliases. So if you have "ldap.example.com" and an alias for it called "ldap", you must make sure any IP address or hostname resolution for "ldap" or "ldap.example.com" always returns "ldap.example.com" first.

Time Sync

Kerberos works best if all of your servers and clients are in time sync. This doesn't mean you must use something like NTP, but you need to ensure that your machines are not wildly out of time sync.

krb5.conf

Consult your system documentation for Kerberos configuration, usually the file /etc/krb5.conf. There is usually a default one, and you should be able to use man krb5.conf to get additional information.

Keys

First, make sure that you have created a kerberos principal ldap/FQDN or ldap/FQDN@REALM. If you do not specify the @REALM part, it will use the default value from your krb5.conf (which must be the same on all clients and servers).

kadmin -q "add_principal -randkey  ldap/FQDN@REALM"

or

kadmin -q "add_principal -randkey  ldap/FQDN"

Then, export that key to a keytab file. If you've deployed other services which also authenticate users using Kerberos on the same system, it's recommended that you give each one its own keytab file.

kadmin -q "ktadd -k  /etc/dirsrv/ds.keytab ldap/FQDN" # or ldap/FQDN@REALM

The keytab file needs to be readable by the account under which the directory server runs (i.e dirsrv):

chown dirsrv:dirsrv /etc/dirsrv/ds.keytab
chmod 600 /etc/dirsrv/ds.keytab

Next, set the KRB5_KTNAME environment variable, so your Directory Server can find the keytab file. For FDS 1.1 and later, edit the file

/etc/sysconfig/dirsrv

and add the line below, replacing INSTANCE appropriately:

KRB5_KTNAME=/etc/dirsrv/ds.keytab ; export KRB5_KTNAME

For a systemd system, omit the "; export KRB5_KTNAME" part.

For FDS 1.0 and earlier, you can edit the start-slapd shell script in /etc/init.d/dirsrv and set KRB5_KTNAME in there:

export KRB5_KTNAME=path_to_service_keytab

Using Windows to generate a service keytab

  • Step 1 - Create a user account on AD to use for the principal - this should be a "pseudo" account, not a real user account - make sure the password never expires, and use a really long, random password (you will never actually use this password)
  • Step 2 - use ktpass to map the user account you created to the ldap service principal and export the keytab
ktpass -princ ldap/<fqdn of the 389 server>@DOMAIN.COM
  -mapuser <user on AD> -crypto rc4-hmac-nt -ptype KRB5_NT_SRV_HST -pass <password> -out
  ldap.keytab

Copy ldap.keytab to the directory server machine and change mode and ownership as above.

Maps

The directory server already has some default SASL/GSSAPI maps as described in https://access.redhat.com/site/documentation/en-US/Red_Hat_Directory_Server/9.0/html/Administration_Guide/SASL.html. So you might not have to do anything to get identity mapping working.

However, if you want/need to do your own mapping, see below.

Let's assume your entry in the DS has the DN "dn: uid=uid,o=realm.edu", and assume your Kerberos realm is REALM.EDU. Then, the map would be something like this (as seen in "Managing SASL" in the Administrator's Guide):

dn: cn=mapname,cn=mapping,cn=sasl,cn=config
objectclass: top
objectclass: nsSaslMapping
cn: mapname
nsSaslMapRegexString: \(.*\)@\(.*\)
nsSaslMapBaseDNTemplate: uid=\1,o=\2
nsSaslMapFilterTemplate: (objectclass=inetOrgPerson)

This assumes the Kerberos principal name being sent to the DS is in the form "username@REALM". If this is not the case, and the realm is not being sent, you may have to use something like the following:

nsSaslMapRegexString: \(.*\)
nsSaslMapBaseDNTemplate: uid=\1,dc=myorg,dc=tld

where myorg and tld correspond to your domain and top level domain.

You can use a regex of the form \([^/]+\)/\(.+\) to map kerberos principals with an instance, like service/fqdn or user/admin. For example if you want to map all services from hostname.domain to the uid=hostname.domain,ou=hosts,dc=domain you can use [^/]+/\(.+\) and a map base of uid=\1,ou=hosts,dc=domain or you might want to map all principals with an admin instance to uid=user,ou=Managers,dc=domain so you'll use \([^/]+\)/admin and a mapbase of uid=\1,ou=Managers,dc=domain.