Alias domain to Multi-site installation in Apache

This is regarding to set an Alias domain to Multi-site installation.

We want to create a short domain as alias to my main website(Multi-Site enabled).I have tried it using a most common options like…

1)Creating cname records,

2)pointing to main installation location as addon domain

3)Domain parking

When we try the first two options it is redirecting to sign up page of Web-Site(I think Wild card redirect for Multi-site installation is the reason).

The third option is making is showing an error :”Debugging a WordPress Network”

I think that I should configure it in Apache to solve the problem(I have root access).But I am not aware of dealing with Apache

When we contact our host,they are not trying to look in to the issue and simply said that they would re-install the whole web-site again. I don’t want to do that.

Could anybody help me with this

Update: I needed this domain Alias for a plugin which enable us to shorten the URL with out any third party service . I have Doncha’s Domain mapping plugin installed and I don’t know how to use it in this situation

Thanks!

Update 2 : : I mean to say that the domain mapping plugin doesn’t work with main site.It is only useful to map the sub-domains under a multi-site installation. I read some where over internet that the main site cannot be aliased with this plugin as it causes some problems . Could any body help to alias the main site in an alternative way ?

Thanks !