I have a log in page which is the home page so is located at www.domainname.com
when someone goes to this page it adds a returnurl to the string so they are at:
Yes, we can use the above solution. One problem here is when we are using SEO implementation. To achieve your requirement, you can use URLRewrite.
http://weblogs.asp.net/scottgu/archive/2010/04/20/tip-trick-fix-common-seo-problems-using-the-url-rewrite-extension.aspx