KerberosAndLDAP
Debian
The guide I followed was https://wiki.debian.org/LDAP/OpenLDAPSetup#Kerberos which while it worked required some minor tweaks. I obtained edit privileges for the Debian Wiki and updated the guide with the fixes that I found. I however have a Synology NAS and that can run an LDAP Server. So this guide differs from the upstream Debian Guide.
Assumption is that you have installed the LDAP Server package on your NAS and gone through initial configuration steps, so it has a domain, there is a DN you can bind as and so forth. It is also assumed you have a Debian system (12.5 or later, though this guide should work with 11.x and likely 10.x as well) that will become your KDC and KAdmin server.
Recommendation is that you create actual .ldif files rather than use here-documents as used in this guide. It is far easier to make adjustments to things if you have a file to edit rather than having to type it all out again or paste it and then have to try and make edits to it without making mistakes.
The guide is for illustration. Expectation is that you do not follow it verbatim but adapt it to your needs.
Install packages:
The packages you need are krb5-kdc-ldap, krb5-admin-server for the actual KDC and schema2ldif plus slapd for adding the schema and slappasswd. They are to be installed on your designated Debian host.
$ sudo apt install krb5-kdc-ldap krb5-admin-server schema2ldif
Load kerberos LDAP schema:
Now you need to load the kerberos schema into the LDAP server on the Synology. And here you need to use the cn=config DN.
$ zcat /usr/share/doc/krb5-kdc-ldap/kerberos.openldap.ldif.gz | ldapadd -H ldaps://nas.example.com/ -D cn=config -W
Enter LDAP Password:
adding new entry "cn=kerberos,cn=schema,cn=config"
$
Create Index on krbPrincipalName:
Having an index on the krbPrincipalName improves performance and also suppresses some log messages if slapd is configured to log more than default for the database(s) where you intend to store Kerberos data. As this is OpenLDAP on the Synology, it does not use mdb format, it uses bdb. If you install slapd on Debian, it uses mdb format. It is different database format, but the principle is the same. Again, as you are modifying config, the DN is cn=config. Interestingly, it is still the main password you set for the Synology LDAP server.
$ ldapmodify -H ldaps://nas.example.com/ -D cn=config -W <<EOF
dn: olcDatabase={1}bdb,cn=config
add: olcDbIndex
olcDbIndex: krbPrincipalName eq,pres,sub
EOF
Enter LDAP Password:
modifying entry "olcDatabase={1}bdb,cn=config"
$
Create principals kadmin and kdc:
Next, you create and configure two entries which will be used by the Kerberos servers to connect to OpenLDAP. As you will not run the Kerberos KDC and Admin Server on the same host as OpenLDAP, these steps are required. In order to keep things nicely separated, everything will be created under a separate organizationalUnit. I diverge from the official Debian guide here as I do not agree with the DN they use. I also had to make changes to the DNs of kdc and kadmin due to pwdPolicy applied by Synology to their LDAP server. The official guide will have you use placeholder passwords and that does not work with the Synology LDAP server. You need to generate them upfront with slappasswd -h {SSHA}.
$ ldapadd -H ldap://nas.example.com/ -D uid=root,cn=users,dc=example,dc=com -W <<EOF
dn: ou=kerberos,dc=example,dc=com
objectClass: organizationalUnit
objectClass: top
ou: kerberos
dn: cn=kdc,ou=kerberos,dc=example,dc=com
cn: kdc
sn: kdc
objectClass: person
objectClass: pwdPolicy
pwdAttribute: userPassword
pwdMinLength: 8
pwdCheckQuality: 2
pwdPolicySubentry: cn=kdc,ou=kerberos,dc=example,dc=com
userPassword: {SSHA}<password-hash>
description: Kerberos KDC Account
dn: cn=kadmin,ou=kerberos,dc=example,dc=com
cn: kadmin
sn: kadmin
objectClass: person
objectClass: pwdPolicy
pwdAttribute: userPassword
pwdMinLength: 8
pwdCheckQuality: 2
pwdPolicySubentry: cn=kadmin,ou=kerberos,dc=example,dc=com
userPassword: {SSHA}<password-hash>
description: Kerberos KDC Account
EOF
Enter LDAP Password:
adding new entry "ou=kerberos,dc=example,dc=com"
adding new entry "cn=kdc,ou=kerberos,dc=example,dc=com"
adding new entry "cn=kadmin,ou=kerberos,dc=example,dc=com"
Synology LDAP server did not like adding users with placeholder passwords, because Synology puts in place password policies. Hence changing this to a cn instead of a uid. It seems to have worked out fine as my KDC is fully functioning.
Grant kdc and kadmin permissions:
This switches back to the cn=config DN as you are changing the permissions.
$ ldapmodify -H ldaps://ds723.trudheim.com -W -D cn=config <<EOF
dn: olcDatabase={1}bdb,cn=config
add: olcAccess
olcAccess: {0}to attrs=krbPrincipalKey
by anonymous auth
by dn.exact="cn=kdc,ou=kerberos,dc=trudheim,dc=com" write
by dn.exact="cn=kadmin,ou=kerberos,dc=trudheim,dc=com" write
by self write
by * none
-
add: olcAccess
olcAccess: {1}to dn.subtree="cn=krbContainer,ou=kerberos,dc=example,dc=com"
by dn.exact="cn=kdc,ou=kerberos,dc=trudheim,dc=com" write
by dn.exact="cn=kadmin,ou=kerberos,dc=trudheim,dc=com" write
by * none
EOF
Enter LDAP Password:
modifying entry "olcDatabase={1}bdb,cn=config"
$
Note that we now reference our kdc and kadmin accounts and we grant them permission to the krbContainer which will house all our kerberos principals. Give both of them write access, because we do want to have the ability to track last login and lock accounts if there are login failures. We like security.
Create krb5.conf:
Over to adjusting /etc/krb5.conf so that it will point to the right thing later. It should look something like this:
[libdefaults]
default_realm = EXAMPLE.COM
dns_lookup_realm = false
dns_lookup_kdc = false
ticket_lifetime = 24h
forwardable = true
proxiable = true
rdns = false
[realms]
EXAMPLE.COM = {
kdc = debian.example.com
admin_server = debian.example.com
default_domain = example.com
}
[domain_realm]
.example.com = EXAMPLE.COM
example.com = EXAMPLE.COMMake sure your designated debian server have ports 88, 464 and 749 open, both for TCP and UDP, in its firewall. 88 is for the kdc, 464 and 749 is for kadmin.
Create kdc.conf:
Next, we need to write up /etc/krb5kdc/kdc.conf. Something like this should work
[libdefaults]
[realms]
TRUDHEIM.COM = {
database_module = openldap_ldapconf
max_life = 7d
max_renewable_life = 6d
}
[dbdefaults]
ldap_kerberos_container_dn = cn=krbContainer,ou=kerberos,dc=trudheim,dc=com
[dbmodules]
openldap_ldapconf = {
db_library = kldap
disable_last_success = false
disable_lockout = false
ldap_conns_per_server = 5
ldap_servers = ldaps://ds723.trudheim.com
ldap_kdc_dn = "cn=kdc,ou=kerberos,dc=trudheim,dc=com"
ldap_kadmind_dn = "cn=kadmin,ou=kerberos,dc=trudheim,dc=com"
ldap_service_password_file = /etc/krb5kdc/service.keyfile
}
Create kadm5.acl:
Then you need to create /etc/krb5kdc/kadm5.acl and put in it
*/admin@EXAMPLE.COM *
so that administrator principals can run kadmin. Now we are ready to create the domain. And that we do with
Create the kerberos domain:
#
kdb5_ldap_util -D uid=root,cn=users,dc=trudheim,dc=com -H ldaps://ds723.trudheim.com -r TRUDHEIM.COM create -subtrees dc=trudheim,dc=com -maxtktlife '7 Days' -maxrenewlife '6 Days' -s
Password for "uid=root,cn=users,dc=trudheim,dc=com":
Initializing database for realm 'TRUDHEIM.COM'
You will be prompted for the database Master Password.
It is important that you NOT FORGET this password.
Enter KDC database master key:
Re-enter KDC database master key to verify:
kdb5_ldap_util -D uid=root,cn=users,dc=trudheim,dc=com -H ldaps://ds723.trudheim.com stashsrvpw -f /etc/krb5kdc/service.keyfile cn=kdc,ou=kerberos,dc=trudheim,dc=com
Password for "uid=root,cn=users,dc=trudheim,dc=com":
Password for "cn=kdc,ou=kerberos,dc=trudheim,dc=com":
Re-enter password for "cn=kdc,ou=kerberos,dc=trudheim,dc=com":
kdb5_ldap_util -D uid=root,cn=users,dc=trudheim,dc=com -H ldaps://ds723.trudheim.com stashsrvpw -f /etc/krb5kdc/service.keyfile cn=kadmin,ou=kerberos,dc=trudheim,dc=com
Password for "uid=root,cn=users,dc=trudheim,dc=com":
Password for "cn=kadmin,ou=kerberos,dc=trudheim,dc=com":
Re-enter password for "cn=kadmin,ou=kerberos,dc=trudheim,dc=com":
Create the first principals: