301 permanent redirect abc.com to www.abc.com

It is in your .htaccess file of your Drupal installation. Look for these lines:

 # To redirect all users to access the site WITH the 'www.' prefix,
  # (http://example.com/... will be redirected to http://www.example.com/...)
  # uncomment the following:
  # RewriteCond %{HTTP_HOST} .
  # RewriteCond %{HTTP_HOST} !^www\. [NC]
  # RewriteRule ^ http%{ENV:protossl}://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

uncomment the lines so it looks like:

 # To redirect all users to access the site WITH the 'www.' prefix,
  # (http://example.com/... will be redirected to http://www.example.com/...)
  # uncomment the following:
   RewriteCond %{HTTP_HOST} .
   RewriteCond %{HTTP_HOST} !^www\. [NC]
   RewriteRule ^ http%{ENV:protossl}://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

Hope that helps.