Imagine I have wordpress multisite running on domain1.com in subdomain mode.
I want to redirect any subdomain that DOESN'T exist on domain1.com to another subdomain on domain2.com
For example, if mysubdomain.domain1.com doesn't exist, I want to redirect to mysubdomain.domain2.com
first.domain1.com -> first.domain2.com
But if second.domain1.com exists, I want the regular WP multisite blog to load up.
How can I accomplish these? How can I hook into "subdomain not found"?