WP Multisite

Before you begin

Run with care: Converting to multisite edits core
configuration files and adds database tables. It is easy to enable and
awkward to undo, so take a VM snapshot and a backup first.

Connect to the VM as root and save a copy of the site files and database.
This assumes the database is named wordpress; use your
own name if it differs.

ssh root@YOUR_SERVER_IP

mkdir -p /root/wp-backup
mariadb-dump wordpress > /root/wp-backup/wordpress-$(date +%F).sql
tar -czf /root/wp-backup/wp-files-$(date +%F).tar.gz -C /var/www/html .
ls -lh /root/wp-backup

Download these files to another machine so a VM failure cannot take
your only copy with it.

In the WordPress dashboard, open Plugins, select every
plugin, and choose Deactivate. This avoids conflicts
during conversion. You will reactivate them afterward.

Also check that your permalinks work and that
Settings → General shows the correct site address.

Open wp-config.php and add the line below on its own line
above the comment that says That’s all, stop editing!

nano /var/www/html/wp-config.php

define( 'WP_ALLOW_MULTISITE', true );

In nano, press Ctrl+O then Enter to save, and Ctrl+X to exit.

Reload the dashboard and open Tools → Network Setup.
Choose Sub-domains or Sub-directories,
set the network title and administrator email, and select
Install.

WordPress then shows two code blocks made for your site, one for
wp-config.php and one for .htaccess. Keep that
page open and copy from it in the next steps.

Use your own screen’s code: The examples below show the
shape of what WordPress generates, but the Network Setup page contains
your real domain and your chosen type. Prefer its values if they differ.

Open wp-config.php again and paste WordPress’s first code
block below the WP_ALLOW_MULTISITE line, still above the
stop editing comment. Example for a subdirectory network, where
example.com is your domain:

define( 'MULTISITE', true );
define( 'SUBDOMAIN_INSTALL', false );
define( 'DOMAIN_CURRENT_SITE', 'example.com' );
define( 'PATH_CURRENT_SITE', '/' );
define( 'SITE_ID_CURRENT_SITE', 1 );
define( 'BLOG_ID_CURRENT_SITE', 1 );

For a subdomain network, SUBDOMAIN_INSTALL is
true instead.

Open .htaccess and replace the existing block between
# BEGIN WordPress and # END WordPress with
WordPress’s second code block. Copy the version that matches your
network type.

nano /var/www/html/.htaccess

Sub-directory network

RewriteEngine On
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]

Sub-domain network

RewriteEngine On
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^wp-admin$ wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^(.*\.php)$ $1 [L]
RewriteRule . index.php [L]

Return to the WordPress login page and sign in again. You should now
see My Sites → Network Admin in the admin bar. Open
this address to reach the network dashboard:

https://example.com/wp-admin/network/

Replace example.com with your domain, and use
http if you have not enabled HTTPS.

apache2ctl configtest && systemctl reload apache2

In Network Admin → Sites → Add New, create your first
extra site by entering its address, title, and administrator email.
Then go to Network Admin → Plugins and use
Network Activate for plugins every site should have.
Plugins that should run on only some sites can be activated from each
site’s own dashboard.

Some plugins and themes are not multisite compatible. Test each one
after activating it, and enable themes for the network under
Network Admin → Themes.

Add a wildcard DNS record, *.example.com, pointing to the
VM’s IP address, and make sure Apache’s default site answers all
hostnames. A standard single-site certificate does not cover new
subdomains. Wildcard certificates from Let’s Encrypt require DNS
validation, so follow
Certbot’s instructions
for your DNS provider.

If conversion fails, restore the snapshot or the backup from Step 01.
To restore the backup, replace the date in each file name, then run
the commands. This overwrites the current site and database.

mariadb wordpress < /root/wp-backup/wordpress-YYYY-MM-DD.sql
tar -xzf /root/wp-backup/wp-files-YYYY-MM-DD.tar.gz -C /var/www/html
chown -R www-data:www-data /var/www/html
systemctl reload apache2

Before running: Restoring replaces everything changed
since the backup, including posts and uploads. Check the file names
first with ls /root/wp-backup.

Keep building

Keep WordPress, themes, and plugins updated from the Network Admin, limit
Super Admin access to people who need it, and back up the database and
wp-content regularly. Every site on the network shares one
database and one set of plugins, so test updates before applying them.

References:
Create a network
·
Before you create a network
·
Multisite administration