It seems that my original post is to confusing, so I rewrote it to be more clear (sry, english is not my first language):
my webcontent is hosted by a different company then my DNS records/domains. Lets call the first contenthoster and the second domainhoster.
The contenthoster uses plesk (v12 on CentOS 7). I also got a domain there, but I don't want to use it. Lets call this one something.com
My "real" domain at the domainprovider is called realthing.com.
Up until now my working setup looked like this:
sub1.realthing.com pointed via A-record to IP-Address of something.com. That works.
sub2.something.com has content and is working (content is showing in a browser)
What I want to do now:
sub3.realthing.com point to sub2.something.com and then show the content of sub2.something.com.
I tried to do this with a CNAME entry but what happens is, that sub3.realthing.com shows the content of something.com.
Checking the reverse-DNS record with dnswatch.info shows the right setup (sub3.realthing.com CNAME sub2.something.com). So it seems that I need to do sth at the contentprovider side to enable apache to present the correct content. But I don't know how to do this with plesk.
Any help is much appreciated
Old post (same meaning): my domain there is some internal url (11-43-223-33.customer.company.com). I configured my real domain (mydomain.com) at my domain/dns provider to point to the IP-address (220.127.116.11) of the contenserver. That works.
Now what I want to do and until now am not able to accomplish: Using a subdomain (sub.mydomain.com) to point to a subdomain at my contenthoster (sub.11-43-223-33.customer.company.com). I used a CNAME entry for that (sub.mydomain.com CNAME sub.11-43-223-33.customer.company.com). Instead of showing the content of sub.11-43-223-33.customer.company.com when opening sub.mydomain.com in a browser I got presented with the content of mydomain.com.
As commented below, there is content behind sub.11-43-223-33.customer.company.com which I can access if I open that URL in a browser. So the subdomain at my contentprovider is up and running by itself.
Is it even possible to accomplish this with plesk? Do I need to add sth to the apache config so that it'll work? Any help is much appreciated