First, configure this consumer as client.

Create a certificate and sign with CA, valid for 3 years. You probably need to perform this step on the LDAP provider or CA system:

# certutil -S -n 'OpenLDAP Consumer' -t ",," \
-c LDAP-CA \
-f /etc/openldap/certs/password \
-d /etc/openldap/certs \
-z /tmp/noise.txt \
-s "CN=OpenLDAP Consumer,OU=IT,O=Company,L=City,ST=State,C=NL" \
-8 "ldap.domain.tld,ldap.mgmt.domain.tld-example!" \
-v 36 \
-Z SHA256 \
-g 4096

Export the signed certificate and key:

# pk12util -d /etc/openldap/certs -o /root/consumer.p12 -n "OpenLDAP Consumer" -k /etc/openldap/certs/password 
Enter password for PKCS12 file: ...
Re-enter password: ...
pk12util: PKCS12 EXPORT SUCCESSFUL

Export the CA certificate:

# certutil -L -d /etc/openldap/certs -n "LDAP-CA" -a > /tmp/ca.crt

On the consumer, create the certificate database directory, if it does not already exisist:

# mkdir /etc/openldap/certs

Generate a secure password:

# pwgen -sy 32 1 > /etc/openldap/certs/password

Create a new empty database, if it does not already exists:

# certutil -d /etc/openldap/certs -N -f /etc/openldap/certs/password

Copy the file to the consumer and import it to the NSS database:

# pk12util -d /etc/openldap/certs -i /tmp/consumer.p12 -k /etc/openldap/certs/password 
Enter password for PKCS12 file: 
pk12util: PKCS12 IMPORT SUCCESSFUL

Import the CA certificate:

# certutil -A -n "LDAP-CA" -t "TCu,Cu,Cu" -i /tmp/ca.crt -d /etc/openldap/certs

Modify rights so that LDAP can read the NSS database:

# chmod 440 /etc/openldap/certs/password
# chown ldap. /etc/openldap/certs/*

List all certificates:

# certutil -L -d /etc/openldap/certs/

List all private keys in the database:

# certutil -K -d /etc/openldap/certs/ -f /etc/openldap/certs/password

View certificate:

# certutil -L -d /etc/openldap/certs/ -n "OpenLDAP Consumer"

Verify certificate:

# certutil -V -d /etc/openldap/certs -n "OpenLDAP Consumer" -u C
certutil: certificate is valid

You now have an encrypted private key and signed certificate for the consumer server.

Install required packages:

# yum install openldap-clients openldap-servers openldap pam_ldap nss-pam-ldapd pam_krb5 sssd migrationtools openldap-devel

Activate LDAPS (TLS), change ldap to ldaps, only start LDAPS and LDAPI (IPC socket):

# vi /etc/sysconfig/slapd
SLAPD_URLS="ldapi:/// ldaps:///"

# Any custom options
SLAPD_OPTIONS="-g ldap"

Modify /etc/openldap/ldap.conf:

# vi /etc/openldap/ldap.conf

BASE    dc=<domain>,dc=<tld>
URI     ldaps://<FQDN>
TLS_CACERTDIR  /etc/openldap/certs
TLS_REQCERT demand

Use the DB config example. You can configure the DB_LOG_AUTOREMOVE directive in DB_CONFIG, but If the log files are removed automatically, recovery after a catastrophic failure is likely to be impossible.:

# install -m 644 -o ldap -g ldap /usr/share/openldap-servers/DB_CONFIG.example /var/lib/ldap/DB_CONFIG

Start OpenLDAP:

slaptest -u
systemctl start slapd
systemctl enable slapd

Add required schema's:

core.schema		OpenLDAP core (required)
cosine.schema		Cosine and Internet X.500 (useful)
inetorgperson.schema	InetOrgPerson (useful)
nis.schema		Network Information Services (FYI)

# ldapadd -Q -Y EXTERNAL -H ldapi:/// -f /etc/openldap/schema/cosine.ldif
# ldapadd -Q -Y EXTERNAL -H ldapi:/// -f /etc/openldap/schema/inetorgperson.ldif
# ldapadd -Q -Y EXTERNAL -H ldapi:/// -f /etc/openldap/schema/nis.ldif

Generate an LDAP root password, you need this hash in the next steps:

# slappasswd
New password: <password>
Re-enter new password: <password>
{SSHA}<hash>

Export variables used in next steps:

# export MYHASH="{SSHA}your-hash"
# export MYDOMAIN=your-domain
# export MYTLD=your-tld

olcDatabase={0}config: Add a root password, and modify the olcAccess in order to (at least) require a password using simple authentication

# ldapmodify -Q -Y EXTERNAL -H ldapi:/// <<EOF
dn: olcDatabase={0}config,cn=config
changetype: modify
add: olcRootPW
olcRootPW: ${MYHASH}
-
replace: olcAccess
olcAccess: {0}to * 
       by dn.base="cn=Manager,dc=${MYDOMAIN},dc=${MYTLD}" manage
       by * none
EOF

olcDatabase={1}monitor: Change the monitor ACL:

# ldapmodify -H ldapi:/// -x -D "cn=config" -W <<EOF
dn: olcDatabase={1}monitor,cn=config
changetype: modify
replace: olcAccess
olcAccess: {0}to *
       by dn.base="gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth" read
       by dn.base="cn=Manager,dc=${MYDOMAIN},dc=${MYTLD}" read
       by * none
EOF

olcDatabase={2}hdb: Change the standard DN suffix and root password

# ldapmodify -H ldapi:/// -x -D "cn=config" -W <<EOF
dn: olcDatabase={2}hdb,cn=config
changetype: modify
replace: olcSuffix
olcSuffix: dc=${MYDOMAIN},dc=${MYTLD}
-
replace: olcRootDN
olcRootDN: cn=Manager,dc=${MYDOMAIN},dc=${MYTLD}
-
add: olcRootPW
olcRootPW: ${MYHASH}
EOF

olcDatabase={2}hdb: configure indexing:

# ldapmodify -H ldapi:/// -x -D "cn=config" -W <<EOF
dn: olcDatabase={2}hdb,cn=config
changetype: modify
replace: olcDbIndex
olcDbIndex: objectClass eq,pres
olcDbIndex: ou,cn,mail,surname,givenname eq,pres,sub
olcDbIndex: uid,memberUid,gidNumber eq
-
EOF

Modify the TLS configuration. …the current implementation doesn't support encrypted keys so the key must not be encrypted and the file itself must be protected carefully.

# ldapmodify -H ldapi:/// -x -D "cn=config" -W <<EOF
dn: cn=config
changetype: modify
replace: olcTLSCertificateFile
olcTLSCertificateFile: "OpenLDAP Consumer"
-
replace: olcTLSCipherSuite
olcTLSCipherSuite: HIGH
-
replace: olcTLSProtocolMin
olcTLSProtocolMin: 3.1
-
replace: olcDisallows
olcDisallows: bind_anon
-
replace: olcIdleTimeout
olcIdleTimeout: 120
EOF

olcDatabase={-1}frontend: Due to a bug i was not able to perform this step… I had to manually edit the olcDatabase={-1}frontend.ldif file and set olcDatabase to “olcDatabase: {-1}frontend”. Then restart slapd!

Set global options: Some backend types, such as frontend and monitor use a hard-coded suffix which may not be overridden in the configuration. Access controls defined in the frontend are appended to all other databases' controls. Configure password and require LDAPv3:

# ldapmodify -H ldapi:/// -x -D "cn=config" -W <<EOF
dn: olcDatabase={-1}frontend,cn=config
changetype: modify
add: olcPasswordHash
olcPasswordHash: ${MYHASH}
-
add: olcRequires
olcRequires: LDAPv3 authc
EOF

Accept only TLS:

# ldapmodify -H ldaps://<FQDN> -x -D "cn=config" -W <<EOF
dn: cn=config
changetype: modify
add: olcSecurity
olcSecurity: tls=1
EOF

Open port 636:

# firewall-cmd --permanent --zone public --add-service=ldaps
success
# firewall-cmd --reload
success

Test server config, restart and test connectivity:

# slaptest -u
# systemctl restart slapd

# openssl s_client -connect localhost:636 -showcerts -CAfile /etc/openldap/cacerts/ca.crt
...
    Verify return code: 0 (ok)

# ldapwhoami -H ldaps://<FQDN> -x -D "cn=Manager,dc=<domain>,dc=<tld>" -W

# ldapsearch -H ldaps://<FQDN> -x -D "cn=Manager,dc=<domain>,dc=<tld>" -W

# ldapsearch -H ldap://<FQDN> -x -D "cn=Manager,dc=<domain>,dc=<tld>" -W
ldap_bind: Confidentiality required (13)
	additional info: TLS confidentiality required

or:
ldap_sasl_bind(SIMPLE): Can't contact LDAP server (-1)

On the provider (master): create a replicator user:

# ldapadd -H ldaps://<FQDN> -x -D "cn=Manager,dc=${MYDOMAIN},dc=${MYTLD}" -W <<EOF
dn: cn=replicator,dc=${MYDOMAIN},dc=${MYTLD}
objectClass: simpleSecurityObject
objectClass: organizationalRole
cn: replicator
userPassword: <your replicator password SSHA hash>
EOF

On the provider (master): create an ACL for the replicator user:

# ldapadd -H ldaps://<FQDN> -x -D "cn=Manager,dc=${MYDOMAIN},dc=${MYTLD}" -W <<EOF
dn: olcDatabase={2}bdb,cn=config
changetype: modify
replace: olcAccess
olcAccess: {0}to attrs=userPassword,shadowLastChange
       by dn.exact="cn=Manager,dc=${MYDOMAIN},dc=${MYTLD}" write
       by dn.exact="cn=replicator,dc=polaire,dc=nl" read
       by self =xw
       by anonymous auth
       by * none
olcAccess: {1}to *
       by dn.exact="cn=Manager,dc=${MYDOMAIN},dc=${MYTLD}" write
       by dn.exact="cn=replicator,dc=polaire,dc=nl" read
       by self read
       by users read
       by * none
EOF

On all servers (provider and consumers) install the syncprov module:

# ldapadd -H ldaps://<FQDN> -x -D "cn=Manager,dc=${MYDOMAIN},dc=${MYTLD}" -W <<EOF
dn: cn=module,cn=config
objectClass: olcModuleList
cn: module
olcModulePath: /usr/lib64/openldap
olcModuleLoad: syncprov.la
EOF

On the provider configure the sync overlay:

# ldapadd -H ldaps://<FQDN> -x -D "cn=Manager,dc=${MYDOMAIN},dc=${MYTLD}" -W <<EOF
dn: olcOverlay=syncprov,olcDatabase={2}hdb,cn=config
changetype: add
objectClass: olcOverlayConfig
objectClass: olcSyncProvConfig
olcOverlay: syncprov
EOF

Configure slave:

ldapadd -H ldaps://ldapconsumer.yourdomain.tld -x -D "cn=Manager,dc=${MYDOMAIN},dc=${MYTLD}" -W <<EOF
dn: olcDatabase={2}hdb,cn=config
changetype: modify
add: olcSyncrepl
olcsyncrepl: {0}rid=014
  provider=ldaps://ldap.yourdomain.tld
  type=refreshAndPersist
  retry="5 5 300 +" 
  searchbase="dc=domain,dc=tld"
  attrs="*,+"
  bindmethod=simple
  binddn="cn=replicator,dc=domain,dc=tld"
  credentials=yourpassword
EOF

contextCSN needs to be the same on provider and consumer:

# ldapsearch -H ldaps://provider -x -D "cn=Manager,dc=<basedn>" -w <passwd> -s base contextcsn | grep contextCSN
# ldapsearch -H ldaps://consumer -x -D "cn=Manager,dc=<basedn>" -w <passwd> -s base contextcsn | grep contextCSN