Symptoms
The following error message is returned when you attempt to configure publishing using a remote distributor if the password required for the publisher to establish an administrative link with the distributor isNULL:

SQL Server Enterprise Manager could not configure ‘<Distributor_name>’ as the Distributor for ‘<Publisher_name>’.
Error 18456: Login failed for user ‘distributor_admin’.
Resolution
Configure the publisher as a trusted publisher to the distributor so that a password is not required to establish the administrative link between the publisher and the distributor. To change the security settings for the publisher, follow these steps:On the Tools menu, select Replication.On the Publisher tab, highlight your publisher in the list, and then click the ellipsis (…) next to that publisher.In the Replication Agents on the Distribution Log into the Publisher section, select the By impersonating the SQL Server Agent account on ‘{ServerName}’ (trusted connection) check box.
-or-Do not use a login that has a NULL password for establishing the administrative link between the publisher and the distributor. The administrative link password can be set in Enterprise Manager. To do this, follow these steps: On the Tools menu, select Replication.On the Distributor tab, under the Administrative link password section, enter your password in the password and confirm password fields.
Note We recommend that you use a strong password in this step.