I want to redirect a url to a domain mapped url

I want to redirect olddomain.com to the mapped newdomain.com (mapped from newdomain.main.com) how do I go about doing this on my multisite install on main.com. The current mapped domain is working perfectly, the domain that I want to redirect has the dns pointing to the same server as the mapped domain. It does not have any content it's just the url.

Use case: when a user goes to olddomain.com I want the user to see the domain mapped website newdomain.com

I am obviously not entering exact url to protect client identities showing up in search results. I can private email if needed.

Thanks.