Problems with domain mapping, SSL and redirect loop

Greetings,

So, we’re experiencing some strange issues with our multisite, regarding mapped domains and SSL.

The site is set up as follows, with subdomains:

Main site:
domain.no

Subsites -> mapped to:
site1.domain.no -> site1.no
site2.domain.no -> site2.no
site3.domain.no -> site3.no

We have a SSL certificate that includes all of our domains, except the «internal» ones (site1.domain.no). I did however try with a self generated certificate, but the redirect issue remained, so I concluded that the certificate is not at fault.

The issue is that https://site1.no redirects to https://site1.no, resulting in an endless loop. http://site.no works just fine, and I can access «ordinary», non-Wordpress files on the site, like https://site1.no/info.php without any problems, but as soon as I try to view a WordPress page shit hits the fan. There is nothing in our vhost- or .htaccess-files that should be causing the redirect, and seeing that non-Wordpress files work, the issue should not be related to how our server is configured.

I’ve tried mapping the following domains in the Domain Mapping options without luck:

site1.no:443
https://site1.no

There is however one thing that solves the issue completely: If I add the following line to the top of wp-config.php, the redirect loop disappears and everything works as it should:

$_SERVER['HTTP_HOST'] = $_SERVER['SERVER_NAME'];

Can anyone explain why this works, and does anyone have any idea what the problem might be?

Kind regards,
Runar

http://wordpress.org/plugins/wordpress-mu-domain-mapping/