For existing MU sites, you can just upgrade. For single-site installations, you have to add a constant to the wp-config.php file: define( 'WP_ALLOW_MULTISITE', true ); Save the file, and when you log back in to the Dashboard, you'll see that there is now a Network menu under Tools. Go to it, and you'll see the initial page of the network setup process, as shown in Figure 13-1.
First, choose subdomains or subdirectories for your setup, keeping in mind the requirements we've discussed. Then, enter a title for your network. (The one WordPress suggests might not make sense, as shown in Figure 13-1.) You'll be asked to enter an administrator email address again; this one will be used for notifications related to new sites, and it does not have to be the same as the address you chose for your initial WordPress installation (which will be used for comment notifications and so on related to that site only). Press Install!
On this second screen (Figure 13-2), you'll be given a set of constants to copy into your wp-config.php file. This is not the entire file, so don't overwrite the whole thing! Just add these few constants. I recommend creating a network section.
You'll also be given a new set of rewrite rules for your .htaccess file. Paste these in; they can replace the WordPress section that's already there.
If you get a warning as shown in Figure 13-2, your wildcard DNS setup is not working. If you plan to add each subdomain by hand, as I do on this test installation, you can ignore this warning. Otherwise, contact your server administrator or hosting provider to resolve the problem.
You also need to create a directory called blogs.dir in your wp-content directory. Since your network sites won't have their own uploads directory, blogs.dir is where their uploaded media will be stored. This directory needs to be writeable by the server, just as your uploads directory is.
Once you've saved both files and created blogs.dir, return to the Dashboard. You'll see some new things there, as shown in Figure 13-3.
The warning you'll probably see at the top of the screen appears because once you've activated the network, you have to explicitly allow file uploads. If you're using a theme that supports post thumbnails (like Twenty Ten), you'll be warned that file uploads are not yet enabled.
Other than the warning, there are several new items here. On the left, you'll see that there's a whole new section of the navigation menu, Super Admin. The options pages under this section of the menu will allow you to configure your network, and I'll walk you through each of them in a moment.
Under the Dashboard section of the menu, you'll see a new My Sites option. This leads to a page where you'll see all the sites on which you are a user. Of course, you are the Super Administrator for all the other sites, even if they are not listed here.
In the Right Now box, you'll see new statistics related to storage space. Each site, including this main one, is limited to a certain amount of storage space. You'll be able to change this under Super Admin ^ Options. If you allow network users to upload files, you'll need to set these limits to something your server can handle.
For now, everything else on the Dashboard is just the same as it was before you enabled the Network, so let's start configuring.
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