Sitecore set the default domain for the login page

By default, the Sitecore login page will automatically use the Sitecore domain. Which works well for accounts such as sitecore\admin as you only need to enter admin to login. However if you are integrated with active directory, you may want to default the domain to the active directory one.

This can be achieved by modifying a site line in the sites section of the web.config

<site name="login" virtualFolder="/sitecore/login" physicalFolder="/sitecore/login" enableAnalytics="false" database="core" domain="sitecore" disableXmlControls="true" />