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. 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. Synology OpenLDAP does not use mdb format, it uses bdb. Debian slapd 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. Use the main password you set for the Synology LDAP server. Create a file step1.ldif with the following content:
dn: olcDatabase={1}bdb,cn=config add: olcDbIndex olcDbIndex: krbPrincipalName eq,pres,sub
and apply it with
$ ldapmodify -H ldaps://nas.example.com/ -D cn=config -W -f step1.ldif 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. Not running the Kerberos KDC and Admin Server on the same host as OpenLDAP, these steps are required. Keeping things confined, everything will be created under a separate organizationalUnit. My guide differs from the official Debian guide here. Due to Synology OpenLDAP having a strict password policy, it was necessary to adjust the DNs of kdc and kadmin. The official guide use placeholder passwords which does not work with the Synology LDAP server.
Generate the passwords upfront with slappasswd -h {SSHA}. Then create a file step2.ldif with the following content:
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
Apply it with
$ ldapadd -H ldaps://nas.example.com/ -D uid=root,cn=users,dc=example,dc=com -W -f step2.ldif 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"
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 regular principals
Here, you will run kadmin.local to create first a regular user, and then an admin version of that user.
# kadmin.local
Authenticating as principal root/admin@TRUDHEIM.COM with password.
kadmin.local: addprinc user
No policy specified for user@TRUDHEIM.COM; defaulting to no policy
Enter password for principal "user@TRUDHEIM.COM":
Re-enter password for principal "user@TRUDHEIM.COM":
Principal "user@TRUDHEIM.COM" created.
kadmin.local: addprinc user/admin
No policy specified for user/admin@TRUDHEIM.COM; defaulting to no policy
Enter password for principal "user/admin@TRUDHEIM.COM":
Re-enter password for principal "user/admin@TRUDHEIM.COM":
Principal "user/admin@TRUDHEIM.COM" created.
kadmin.local: q
#
Worth to note here is that user@EXAMPLE.COM and user/admin@EXAMPLE.COM can have (and should have) different passwords as the admin variant is allowed to do things to the kerberos database. And this is why you want to have the registering of failures to login enabled. Should you have the system exposed to the internet, you can and should expect intrusion attempts. Having Kerberos deployed makes it harder for perpetrators to gain access, but not impossible.
If you later kerberise your storage and leverage it for NFS4 mounts from your NAS, you can have NFS exposed to the internet as well. Unless someone has a valid kerberos ticket, even if they somehow could mount the share, they see nothing on it without the krbtgt.
Test your new principal
$ kinit user@EXAMPLE.COM
Password for user@EXAMPLE.COM:
$ klist
Ticket cache: FILE:/tmp/krb5cc_1000
Default principal: user@EXAMPLE.COM
Valid starting Expires Service principal
09/05/24 08:07:35 10/05/24 08:07:35 krbtgt/EXAMPLE.COM@EXAMPLE.COM
$
Set up pam and sssd