Hi all,
I answer to your replies, good news: I found the problem.

@ Howard
Thank you for told me that this is not a bug, it was a good point to start from.

@ Michael
the back-sock listener is the same for Debian9 and for Debian10, the most important information is that neither "servers/slapd/back-sock/searchexample.pl" worked on Debian10, but only on Debian9. The back-sock listener is a gevent python3 server. Thank you for apparmor hints, I found this information reading openldap archives. On Debian10 we do not have SElinux but only apparmor, I confirm all you wrote.

I just made some mistake in ACL, because I can read results with
"ldapsearch -H ldapi:// -Y EXTERNAL  -b "dc=proxy,dc=myorg,dc=it""

but not with
ldapsearch -H ldap://localhost:389 -D "cn=admin,dc=myorg,dc=it" -w slapdsecret -b "dc=proxy,dc=myorg,dc=it"

So I understood it was a silly ACL problem behind this.
I just added an ACL as follow and everything works fine!

export BASEDC="dc=myorganization,dc=it"

ldapadd -Y EXTERNAL -H ldapi:/// <<EOF
dn: olcDatabase={4}sock,cn=config
changeType: modify
add: olcAccess
olcAccess: to *
 by dn.exact=gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth manage
 by * break
olcAccess: to dn.subtree="ou=people,$BASEDC"
 by dn.children="ou=auth,$BASEDC" read
 by self read
 by * break
olcAccess: to *
 by anonymous auth
 by * break

the question about Debian9 / Debian10 derived from different ldapsearch authentication methods,
Thank you all for your time

Il giorno gio 25 lug 2019 alle ore 17:28 Michael Ströder <michael@stroeder.com> ha scritto:
On 7/25/19 11:31 AM, Giuseppe De Marco wrote:
> I made a configuration to get slapd-sock to work with a python3 server
> (gevent).

Is this an asyncio server?

> [25-07-2019 10:33:57] slapd debug  sock: fgets failed: Success (0)

Are you sure your back-sock listener really responded on the correct
socket? Does it have an own debug log.

FWIW: My back-sock listeners just work fine with 2.4.47+. But on Debian
Stretch/Buster I'm using the LTB builds.

Ciao, Michael.

Dott. Giuseppe De Marco
University of Calabria
87036 Rende (CS) - Italy
Phone: +39 0984 496961
e-mail: giuseppe.demarco@unical.it