I use WordPress Multisite... and BuddyPress installed on the main site on mysite.com
Practically I want users to be automatically redirected to the main site login page every time they try to login from any of the blogs/sites/subdomains.
*.mysite.com/login.php => mysite.com/login.php
Let me give some exemples:
site1.mysite.com/wp-login.php redirected to mysite.com/wp-login.php
site2.mysite.com/wp-login.php redirected to mysite.com/wp-login.php
xyzkw.mysite.com/wp-login.php redirected to mysite.com/wp-login.php
.......................
any future subdomain LOGIN PAGE - with no exceptions - will have to be redirected to mysite.com/wp-login.php