Understanding UNC authentication
When you add preferred servers (Tools > Distribution > Content replication / Preferred servers ), you also provide credentials that devices should use when accessing the preferred server. For security reasons, make sure these credentials are divided between read only and write access (for more information, see About the Preferred server properties dialog box). Managed devices obtain read only credentials from the core and use them to authenticate with that preferred server when downloading a file.
When using preferred servers added to the Server Credentials dialog box, you no longer have to configure your package shares to be null-session shares, as was necessary with previous versions. As long as the credentials you provide for the preferred server work with the package share (Click Test credentials in the Configuration dialog box), managed devices should be able to access the share.