26 Oct Enabling wildcard subdomains
When you install WPMU by default the subdomain option is checked. This is for blogs in the format of username.yourdomain.com.
There are two steps that need to be done to your server in order for this to work.
In Apache, there needs to be a line in the domain's virtual hosts section as follows:
ServerAlias yourdomain.com *.yourdomain.com
The Server Alias line with the *.yourdomain.com is what controls this. If you do not have access to change this, ask your webhost for support. More and more hosts are enabling this by default.
The second thing that needs to be done is adding the DNS record. Where you add it depends on your domain hosting. Basically, you need to add a line like this:
*. in A YOURIPADDRESS
These wildcards mean that any subdomain request will fall through to the WPMU install, where MU will look up the name in the database. If it finds it, it serves up the blog.
Note: if you choose the subfolder option on installation, you do not have to do either one of these steps.