Domain Mapping in WordPress 4.5 appears to have been absorbed into the core and is not working properly with Multisite. I've tried to get help from the WordPress forums and would like to know if it is possible to contact the authors of this book.

The WordPress MU Domain Mapping plugin referenced in the third edition of Professional WordPress: Design and Development, has not been updated in over a year. This plugin performs the domain mapping functions successfully in Multisite 4.5.2; however, it prevents the preview from loading when using Appearance -> Customize from the Dashboard.

The WordPress forum and to my knowledge the plugin's developer, has indicated the new way to map registered domain names to WordPress subdomains is by editing the: Network Admin Dashboard -> Sites -> Edit Site -> Site Address (URL) field. This procedure does indeed map registered domain names to WordPress subdomains but prevents access to the subdomain's dashboard in the following manner:

1. When logged in as the Super Admin and clicking on the subsite's (subdomain's) dashboard link, one is sent to the WordPress Admin login screen, which fails to log one in with the following message: "ERROR: Cookies are blocked or not supported by your browser. You must enable cookies to use WordPress."
   
   NOTE: If one inserts: define(‘COOKIE_DOMAIN’ , ‘the-registered-mapped-domain-name’) into wp-config.php, one can login to the subdomain’s dashboard just fine; however, logging into the Super Admin Dashboard will no longer be possible! (I also tried define(‘COOKIE_DOMAIN’ , ‘*.*’) and multiple define() functions in the same wp-config.php file).
   
   
2. When using https://subdomain.main-domain.tld/wp-login.php, one is redirected to the home page of main domain's website.

The end result of using the "accepted" method of domain mapping in Multisite 4.5.2 is essentially a complete lock out of the subdomain Dashboards. It's hard to believe something this critical wasn't completely tested, so I'm assuming there's a quick fix I haven't yet discovered.

I haven't finished chapters 8 through 10 yet but will continue to look for ideas on how to rectify this situation... in the mean time, any help would be great!

Thank you,
Don