Redirect your visitor to maintenance page is a must when you are upgrading your site. Now you can redirect your user to maintenance page easily by using .htaccess file. I guess you do not want your visitor to see an error page or 404 page during your server upgrade. So just follow the steps below to create a .htaccess file to redirect your visitor to maintenance page during upgrade:
1- Create a file and name it .htaccess with the content below:
Options +FollowSymlinks
RewriteEngine on
RewriteCond %{REQUEST_URI} !/maintenance.html$
RewriteRule $ /maintenance.html [R=302,L]
2- Once the file is created, put it at your root directory. (This will be in http://www.yourdomain.com/.htaccess)
3- Create another file name it maintenance.html (just put wat messeage you want to show to your visitor during the upgrade) and put it at the root directory too. (This will be in http://www.yourdomain.com/maintenance.html)
Now you can try to browse your site and it should now redirect you to maintenance.html
4- Since you are going to perform the upgrade, you have to able to browse the site. So you have to exclude your IP in the .htaccess file. Now your .htaccess file should look like below.
Options +FollowSymlinks
RewriteEngine on
RewriteCond %{REQUEST_URI} !/maintenance.html$
RewriteCond %{REMOTE_HOST} !^888.888.888.888
RewriteRule $ /maintenance.html [R=302,L]
Your visitor should be redirected to maintenance page by the .htaccess we set just now and you should be able to browse your site and perform your upgrade.
Note: .htaccess only works in apache webserver, if you do not know what web server you are on, your can consult with your server administrator for more details. This .htaccess using apache mod_rewrite module. Please ensure you have mod_rewrite installed and enabled.
From techiecorner
Maintenance HTML example: http://pastebin.com/c019Di2H