This is step to redirect domain.com to www.domain.com for Ubuntu and Apache2 environment :
Requirement
1) enable mod vhost alias
a2enmod vhost_alias
2) reconfigure virtual host
<VirtualHost 10.9.192.212>
ServerName domain.com
ServerAlias domain.com
ServerAdmin web@test.my
Redirect / http://www.domain.com/
</VirtualHost>
<VirtualHost 10.9.192.212>
ServerName www.domain.com
DocumentRoot /home/domain.com/www
</VirtualHost>
3) Restart the apache’s services
Then you will success to redirect domain.com to www.domain.com