How to map secure.domain.com to www.domain.com or domain.com with “WordPress MU Domain Mapping” on the primary multisite domain?

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In the end I ended using mod_rewrite through .htaccess. The following is what I am using and so far is working well.

RewriteCond %{HTTP_HOST} ^secure\.domain\.net$
RewriteRule (.*) http://www.domain.net/$1 [P,L]