Domain aliasing in different variations

[RipperDoc]

Hello!
I have googled like crazy on this topic but never seem to find any good results. (lots of solutions, but none that exactly does what I want). What I want is the following two scenarios:

sub1.domain.com, sub2.domain.com and sub3.domain.com should be aliases of sub.domain.com. I don't want a redirect, I want people that visit sub1.domain.com to believe they are at sub1, when they in fact is at sub. I cannot touch the apache conf of my host, but I can set DNS records and of course, use mod_rewrite. However, I have not yet managed to find any solution.

My second scenario is that I want:
sub.anotherdomain.com to be an alias of sub.yetanotherdomain.com. So if you visit sub.anotherdomain.com, you see the same as when visiting sub.yetanotherdomain.com. Here I still can only access the DNS records and the mod_rewrite. Worth noting is that sub.yetanotherdomain.com is supposed to point to a WebDAV directory.

I would be very thankful for any ideas what to do. It feels like the solution should be easy enough, but I just can't get it to work.

Thanks,
RipperDoc

[David Harrison]

My host allows me to mirror the content on one domain from another, all it takes is a few clicks and key presses in the control panel.

Perhaps your host offers something similar. Here's a screen-shot of what it looks like in my control panel, though it will almost certainly be different in your control panel (assuming that it's there).

I can mirror domains from a sub-domain or sub-domains from a domain, or any other combination, the only requirement is that the domain I'm mirroring is on the same hosting plan.