Check if at root domain

[eluser]TheFuzzy0ne[/eluser]

Code:

<IfModule mod_rewrite.c>
Options +FollowSymLinks
RewriteEngine on
RewriteBase /

RewriteCond %{HTTP_HOST} !^rent\.
RewriteCond %{HTTP_HOST} !^sales\.
RewriteRule ^$ index.php?/select_site [QSA,L]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php?/$1 [QSA,L]
</IfModule>

Hopefully this will work for you.

You can set the base_url dynamically by adding something like this in your config.php.

Code:

$config['base_url'] = $_SERVER['HTTP_HOST'];

The above code is untested.