[Plugin: WordPress MU Domain Mapping] 302 redirection with alias

I want to build a WordPress multisite with differents domains :

• mainblog.com (I installed wordpress on this url)
• mainblog.com/blog1
• mainblog.com/blog2

This works well.

I made 2 alias :

• site1.com/blog > mainblog.com folder
• site2.com/blog > mainblog.com folder

I installed WordPress MU Domain Mapping and mapped the domains :

• site1.com/blog on mainblog.com/blog1
• site2.com/blog on mainblog.com/blog2

When I type site1.com/blog or site2.com/blog, I redirect (302) on mainblog.com. Have you an idea ?