Please create a web.config file in your root directory if it's not already there. And then put the following code into your web.config file.
1: 301 redirect domain.com to www.domain.com
<?xmlversion="1.0"encoding="UTF-8"?><configuration><system.webServer><rewrite><rules><rulename="Canonical Host Name"stopProcessing="true"><matchurl="(.*)"/><conditions><addinput="{HTTP_HOST}"pattern="^domain\.com$"/></conditions><actiontype="Redirect"url="http://www.domain.com/{R:0}"redirectType="Permanent"/></rule></rules></rewrite></system.webServer></configuration>
2: 301 redirect all domains to one domain name
<?xmlversion="1.0"encoding="UTF-8"?><configuration><system.webServer><rewrite><rules><rulename="Enforce canonical hostname"stopProcessing="true"><matchurl="(.*)"/><conditions><addinput="{HTTP_HOST}"negate="true"pattern="^www\.domain\.com$"/></conditions><actiontype="Redirect"url="http://www.domain.com/{R:0}"redirectType="Permanent"/></rule></rules></rewrite></system.webServer></configuration>
