I have a network wordpress installation. The primary domain is domain1.com. I added a new subdirectory domain1.com/domain2 which works fine.
Now I wanted to map the domain1.com/domain2 to domain2.com.
Here is what I did:
Forwarded domain2.com to the IP address of my server.
I added an A record in the dns manager.
In the sites-available for domain1.com, I added an alias to the apache server to make domain2.com point to domain1.com
Now domain2.com displays the content of domain1.com (the address bar still displays domain2.com).
I activated domain mapping plugin in the network dashboard. IP is good in there.
I went to domain2's admin dashboard > Tools > Domain Mapping and added a new primary domain domain2.com.
Test domain2.com > It fails. No answer from the server. Test domain1.com/domain2 > It fails too. The failure is I get no answer from the server.
Go back to domain2's admin dashboard > tools > Domain mapping and change primary domain to domain1.com/domain2.
Now test domain2.com > It redirects to domain1.com/domain2 and works. Test domain1.com/domain2 and obviously it works.