Enable LDAP over SSL (LDAPS) for Microsoft Active Directory servers

2026-07-26T13:22:00Z

Microsoft active directory servers will default to offer LDAP connections over unencrypted connections (boo!).

The steps below will create a new self signed certificate appropriate for use with and thus enabling LDAPS for an AD server. Of course the "self-signed" portion of this guide can be swapped out with a real vendor purchased certificate if required.

Steps have been tested successfully with Windows Server 2025. Requires a working OpenSSL install (ideally Linux/OSX) and (obviously) a Windows Active Directory server.

Create root certificate

Using OpenSSL, create new private key and root certificate. Answer country/state/org questions as suitable:

$ openssl genrsa -aes256 -out ca.key 4096
$ openssl req -new -x509 -days 3650 -key ca.key -out ca.crt

Hold onto the resulting ca.key and ca.crt.

Import root certificate into trusted store of domain controller

From the active directory server, open Manage computer certificates.
Add the generated ca.crt to the certificate path Trusted Root Certification Authorities\Certificates.
Done.

Create client certificate

We will now create a client certificate to be used for LDAPS, signed against our generated root certificate.

From the active directory server:

Create a new request.inf definition with the following contents - replacing ACTIVE_DIRECTORY_FQDN with the qualified domain name of your active directory server:

 [Version]
 Signature="$Windows NT$"

 [NewRequest]
 Subject = "CN=ACTIVE_DIRECTORY_FQDN"
 KeySpec = 1
 KeyLength = 2048
 Exportable = TRUE
 MachineKeySet = TRUE
 SMIME = FALSE
 PrivateKeyArchive = FALSE
 UserProtected = FALSE
 UseExistingKeySet = FALSE
 ProviderName = "Microsoft RSA SChannel Cryptographic Provider"
 ProviderType = 12
 RequestType = PKCS10
 KeyUsage = 0xa0

 [EnhancedKeyUsageExtension]
 OID = 1.3.6.1.5.5.7.3.1 ; Server Authentication

Run the following to create a client certificate request of client.csr (note: it's critical this is run from the active directory server itself to ensure correct private key -> certificate association):

C:\> certreq -new request.inf client.csr

Back to our OpenSSL system:

Create v3ext.txt containing the following:

keyUsage=digitalSignature,keyEncipherment
extendedKeyUsage=serverAuth
subjectKeyIdentifier=hash

Create a certificate client.crt from certificate request client.csr and root certificate (with private key):

$ openssl x509 -req -days 3650 -in client.csr -CA ca.crt -CAkey ca.key -extfile v3ext.txt -set_serial 01 -out client.crt

Verify generated certificate:

$ openssl x509 -in client.crt -text

Ensure the following X509v3 extensions are all present:
- X509v3 Key Usage: Digital Signature, Key Encipherment
- X509v3 Extended Key Usage: TLS Web Server Authentication
- X509v3 Subject Key Identifier

Accept and import certificate

From the active directory server with client.crt present, run the following:
```
 C:\> certreq -accept client.crt
```
Open Manage computer certificates, the new certificate should now be present under Personal\Certificates. Ensure that:
- Certificate has a private key association.
- The "Intended Purposes" is defined as "Server Authentication".
- Certificate name is the FQDN of the active directory server.

Reload active directory SSL certificate

Alternatively you can just reboot the server, but this method will instruct the active directory server to simply reload a suitable SSL certificate and if found, enable LDAPS:

Create ldap-renewservercert.txt containing the following:

dn:
changetype: modify
add: renewServerCertificate
renewServerCertificate: 1

Run the following command:

 C:\> ldifde -i -f ldap-renewservercert.txt

Test LDAPS using `ldp.exe` utility

From another domain controller, firstly install our generated root certificate ca.crt to the certificate path Trusted Root Certification Authorities\Certificates.
Open utility:
```
 C:\> ldp.exe
```
From Connection, select Connect.
Enter name of target domain controller.
Enter 636 as port number (this is the LDAPS port).
Click OK to confirm the connection works.
You're all done!

Reference

Enable LDAP over SSL with a third-party certification authority: https://support.microsoft.com/en-us/kb/321051

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