slap_global_control: unrecognized control: 1.3.6.1.4.1.4203.666.5.16

Hi,

Since I started adding clients to authenticate users through LDAP I'm getting those messages in the servers log:

slap_global_control: unrecognized control: 1.3.6.1.4.1.4203.666.5.16

Clients use nss-ldapd 0.9.9 with nslcd and nscd for caching. Servers are running OpenLDAP 2.4.47 on Debian/Stretch 9.11.

A quick search shows those are related to deref control. I don't have that overlay loaded on the servers, only slapo-memberof, slapo-syncprov, slapo-refint and slapo-ppolicy

So I wonder if, first, are they harmfull? Or will they get eventually as the number of clients will start to increase in the very near future?

thanks, jf