Ben Stienstra

RHEL 7 Kerberos 5 with OpenLDAP backend

Prerequisites

Working DNS environment.
Working NTP environment, ntpd or chronyd.

Modifications on LDAP server

Install prerequisite packages:
```
yum install krb5-server-ldap
```

Copy schema files:

cp /usr/share/doc/krb5-server-ldap-1.13.2/kerberos.* /etc/openldap/schema/

Workaround problem with importing the kerberos LDIF:

mkdir /tmp/ldap-kerberos/
echo "include /etc/openldap/schema/kerberos.schema" > /tmp/ldap-kerberos/schema_convert.conf

mkdir /tmp/ldap-kerberos/krb5_ldif

slaptest -f /tmp/ldap-kerberos/schema_convert.conf -F /tmp/ldap-kerberos/krb5_ldif

# Edit /tmp/ldap-kerberos/krb5_ldif/cn=config/cn=schema/cn={0}kerberos.ldif and replace

dn: cn={0}kerberos
cn: {0}kerberos

with

dn: cn=kerberos,cn=schema,cn=config
cn: kerberos

# Remove (at the end of the file)

structuralObjectClass: olcSchemaConfig
entryUUID: ...
creatorsName: cn=config
createTimestamp: ...
entryCSN: ...
modifiersName: cn=config
modifyTimestamp: ...

You can now add the schema:

ldapadd -H ldaps://<FQDN> -x -D "cn=Manager,dc=domain,dc=tld" -W -f  /tmp/ldap-kerberos/krb5_ldif/cn=config/cn=schema/cn={0}kerberos.ldif

Create a LDAP user than can modify LDAP data, for example krbadmin.

Modify LDAP ACLs, for example:

olcAccess: {0}to attrs=userPassword,shadowLastChange,krbPrincipalKey
      by dn.exact="cn=Manager,dc=domain,dc=tld" write
      by dn.exact="cn=krbadmin,dc=domain,dc=tld" write
      by dn.exact="cn=replicator,dc=domain,dc=tld" read
      by self =xw
      by anonymous auth
      by * none
olcAccess: {1}to *
      by dn.exact="cn=Manager,dc=domain,dc=tld" write
      by dn.exact="cn=krbadmin,dc=domain,dc=tld" write
      by dn.exact="cn=replicator,dc=domain,dc=tld" read
      by self read
      by users read
      by * none

Add index to speed up the access:

# ldapmodify -H ldapi:/// -x -D "cn=config" -W <<EOF
dn: olcDatabase={2}hdb,cn=config
add: olcDbIndex
olcDbIndex: krbPrincipalName eq,pres,sub
-
EOF

Install and configure KDC server

Install required packages:

yum install -y krb5-server krb5-server-ldap krb5-workstation

Edit /var/kerberos/krb5kdc/kdc.conf and replace EXAMPLE.COM with your domain. Convention is to make it the same as your domain name, in upper-case letters.
Edit /var/kerberos/krb5kdc/kdc.conf add below [realms]:
```
default_principal_flags = +preauth
```

Edit /etc/krb5.conf uncomment all lines and replace EXAMPLE.COM (and the lower-case ones) with your domain.

Edit /etc/krb5.conf and add below [realms].

  default_domain = example.com
  database_module = openldap_ldapconf

Edit /etc/krb5.conf add LDAP config:

[dbdefaults]
        ldap_kerberos_container_dn = cn=krbContainer,dc=example,dc=com

[dbmodules]
        openldap_ldapconf = {
                db_library = kldap
                ldap_kdc_dn = "cn=krbadmin,dc=example,dc=com"

                # this object needs to have read rights on
                # the realm container, principal container and realm sub-trees
                ldap_kadmind_dn = "cn=krbadmin,dc=example,dc=com"

                # this object needs to have read and write rights on
                # the realm container, principal container and realm sub-trees
                ldap_service_password_file = /etc/kerberos/service.keyfile
                ldap_servers = ldaps://ldap01.example.com ldaps://ldap02.example.com
                ldap_conns_per_server = 5
        }

Edit /var/kerberos/krb5kdc/kadm5.acl, replace EXAMPLE.COM with your own realm.

Create the realm, choose a bind user that has rights to create the LDAP/Kerberos container:

 kdb5_ldap_util -D  cn=krbadmin,dc=example,dc=com create -subtrees dc=example,dc=com -r EXAMPLE.COM -s -H ldap://ldap01.example.com

Create directory /etc/kerberos

Stash the admin password:

kdb5_ldap_util -D cn=krbadmin,dc=example,dc=com stashsrvpw -f /etc/kerberos/service.keyfile cn=krbadmin,dc=example,dc=com

Start and activate Kerberos:

systemctl start krb5kdc kadmin
systemctl enable krb5kdc kadmin

Add principal:

kadmin.local:  addprinc -randkey host/kdc.dc.polaire.nl

kadmin.local:  ktadd host/kdc.dc.polaire.nl

kadmin.local:  addprinc root/admin

kadmin.local:  addprinc -x dn="uid=example,ou=people,dc=example,dc=com" example

kadmin.local:  quit

Firewall

Open firewall ports:

firewall-cmd --zone public --add-service kerberos --permanent
firewall-cmd --reload

SSH clients

Install the required packages:

yum -y install krb5-workstation pam_krb5

Edit the /etc/krb5.conf file
- Uncomment all lines.
- Replace al example domain names and realms.
- Change example kdc and admin_server.

Add principals on KDC:

kadmin -p root/admin
kadmin:  addprinc -randkey host/test1.example.com

kadmin:  addprinc -randkey host/test2.example.com

kadmin:  addprinc -randkey host/test3.example.com

kadmin:  ktadd host/test1.example.com

kadmin:  ktadd host/test2.example.com

kadmin:  ktadd host/test3.example.com

Configure SSH Server

Configure server as LDAP client: centos7client

Edit /etc/ssh/sshd_config file to include the following lines:

KerberosAuthentication yes
GSSAPIAuthentication yes
GSSAPICleanupCredentials yes
#UsePAM no   # set to no if you don't want to allow logins with local accounts.

Configure SSH client

Edit /etc/ssh/ssh_config to include following lines:

Host *.domain.com
  GSSAPIAuthentication yes
  GSSAPIDelegateCredentials yes

Retrieve ticket:
```
kinit benst
```
Login to other host:
```
ssh user@host
```

Ben Stienstra

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