Se servisse questa e' la configurazione da aggiungere/modificare:
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auth_param basic program /usr/lib/squid/ldap_auth -v 3 -b "ou=People,dc=isi,dc=lan" -f "uid=%s" nome_server
external_acl_type ldap_group %LOGIN /usr/lib/squid/squid_ldap_group -v 3 -b ou=Groups,dc=isi,dc=lan -B ou=People,dc=isi,dc=lan -f "(&(memberUid=%v)(objectClass=posixGroup)(cn=%g))" nome_server
acl ldapauth proxy_auth REQUIRED
acl ldapgroup-nowww external ldap_group nowww
http_access deny ldapgroup-nowww
http_access allow ldapauth
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FYI