Oracle considers another database to be "trusted" if it meets the following criteria:

1. It is in the same enterprise domain in the directory service as the local database.

2. The enterprise domain is marked as trusted in the directory service.

3. It is not listed as untrusted in the local Trusted Servers List. Current user database links will only be accepted from another database if both databases involved trust each other.

You can list a database server locally in the Trusted Servers List regardless of what is listed in the directory service. However, if you list a database that is not in the same domain as the local database, or if that domain is untrusted, the entry will have no effect.

This functionality is part of the Enterprise User Security feature of the Oracle Advanced Security Option.

It is important to know whether all servers are trusted or not trusted. Trusting a particular server with the ALLOW_SERVER procedure does not have any effect if the database already trusts all databases, or if that database is already trusted. Similarly, denying a particular server with the DENY_SERVER procedure does not have any effect if the database already does not trust any database or if that database is already untrusted.

The procedures DENY_ALL and ALLOW_ALL delete all entries (in other words, server names) that are explicitly allowed or denied using the ALLOW_SERVER procedure or DENY_SERVER procedure respectively.

SELECT * FROM TRUSTED_SERVERS;
TRUST      NAME                                                                            
--------- ---------------------
Trusted   All                                                                             

Because all servers are currently trusted, you can execute the DENY_SERVER Procedure and specify that a particular server is not trusted:

EXECUTE DBMS_DISTRIBUTED_TRUST_ADMIN.DENY_SERVER
                           ('SALES.US.AMERICAS.ACME_AUTO.COM');
PL/SQL procedure successfully completed.

SELECT * FROM TRUSTED_SERVERS;
TRUST       NAME                                                                            
--------- -----------------------------------------------
Untrusted SALES.US.AMERICAS.ACME_AUTO.COM 

By executing the DENY_ALL Procedure, you can choose to not trust any database server:

EXECUTE DBMS_DISTRIBUTED_TRUST_ADMIN.DENY_ALL; 

PL/SQL procedure successfully completed.

SELECT * FROM TRUSTED_SERVERS;

TRUST       NAME                                                                            
--------- -----------------------------------------------
Untrusted All                                                                             

The ALLOW_SERVER Procedure can be used to specify that one particular database is to be trusted:

EXECUTE DBMS_DISTRIBUTED_TRUST_ADMIN.ALLOW_SERVER 
                            ('SALES.US.AMERICAS.ACME_AUTO.COM');
PL/SQL procedure successfully completed.

SELECT * FROM TRUSTED_SERVERS;
TRUST       NAME                                                                            
--------- ------------------------------------------------
Trusted   SALES.US.AMERICAS.ACME_AUTO.COM