apache2.4 mod ldap unable to connect to 2012R2 LDAP after upgrade from Stretch to Buster

Question

after upgrading Stretch to Buster authnz-ldap/mod-ldap is unable to connect to 2012R2 AD with ldaps.

This is my configuration:

#Must be set to a low value to permit failover from a server to the other.
LDAPConnectionTimeout 1
LDAPRetries 1
# SSL
LDAPVerifyServerCert on
LDAPTrustedMode SSL
LDAPTrustedGlobalCert CA_BASE64  /etc/apache2/ssl/ca_der.cer

#LDAP SERVERS DECLARATION START

<AuthnProviderAlias ldap MyLDAPS>
  AuthLDAPBindDN "CN=svc-otp,CN=Users,DC=otp,DC=local"
  AuthLDAPBindPassword REDACTED #checked and working
  # Search user
  AuthLDAPURL "ldaps://ad03.otp.local/CN=Users,DC=otp,DC=local?userPrincipalName?sub?(&(memberOf:1.2.840.113556.1.4.1941:=CN=Admins,OU=2FA,OU=PCI,DC=otp,DC=local)(objectClass=user))" 
</AuthnProviderAlias>

<LocationMatch /manage >
   # Using this to bind
   AuthType Basic
   AuthName "USE YOUR user@domain ACCOUNT"
   AuthBasicProvider MyLDAPS
   require valid-user
</LocationMatch>

I tried openssl s_client -connect ad03.otp.local:636 -state -nbio which resulted in

CONNECTED(00000003)
Turned on non blocking io
SSL_connect:before SSL initialization
SSL_connect:SSLv3/TLS write client hello
SSL_connect:error in SSLv3/TLS write client hello
write R BLOCK
SSL_connect:error in SSLv3/TLS write client hello
read:errno=104
---
no peer certificate available
---
No client certificate CA names sent
---
SSL handshake has read 0 bytes and written 306 bytes
Verification: OK
---
New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
Early data was not sent
Verify return code: 0 (ok)
---

then I discovered that /etc/ssl/openssl.conf added

[system_default_sect]
MinProtocol = TLSv1.2
CipherString = DEFAULT@SECLEVEL=2

I commented both lines and the openssl s_client worked as it worked on Stretch.

But Apache always complain that it can't connect...

I googled a lot around similar problems and Apache docs but did not found a hint or solution.

Answer 1

I don't understand yet why but it seems that LDAPTrustedGlobalCert CA_BASE64 /etc/apache2/ssl/ca_der.cer is not read.

I added the CA cert from Active Directory in /etc/ssl/certs/ and rerun update-ca-certificates and ldapsearch worked. apache mod_ldap worked also.

Usefull commands:

• to verify certs: openssl s_client -showcerts -connect ad03.otp.local:636
• to check connection: openssl s_client -connect ad03.otp.local:636 -state -nbio