I cannot fully answer the SEO parts of this question. But as far as I know, it is no good idea to list a page with different domains at Google. Beside this, subdomains are not made for this purpose. At least I like addresses like http://pastry.com/cake much more than your examples.
The technical answer:
- You need an wildcard DNS record to map all subdomains to pastry.com
- You can rewrite with mod_rewrite
I’ve not tested this, but this is more an example to show how this might work:
RewriteEngine on
RewriteCond %{HTTP_HOST} !^www\.pastry\.com$
RewriteCond %{HTTP_HOST} !^pastry\.com$
RewriteCond %{HTTP_HOST} ([^\.]+)\.pastry\.com [NC]
RewriteRule ^(.*)$ http://pastry.com/#%1 [NE,R=301,L]
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solved From subdomain to root: best approach? [closed]