Problems with Subdomain on separate server

Apache doesn't take Servername and outputs IP

[berserka]

Hi Folks,

this is my first post here and I must admit that I'm quite a noob regarding Apache configuration.

So here is my problem:

I want to run a subdomain on a different server as the main domain:
[domain.com...] => Server A (I don't have access to this server)
[sub.domain.com...] => Server B (The server I work with)

The Subdomain (http://sub.domain.com and [sub.domain.com)...] is pointed to the IP adress of Server B, which works fine, but I can't get Apache to output the Subdomain as Servername. It always shows the IP in the adress bar.

I checked with a phpinfo() script and it seems that http_host is the IP adress and not the subdomain.

Here is the relevant part of my configuration in httpd.conf (Apache2 on Suse):

Listen 80
UseCanonicalName On
ServerName www.sub.domain.com
NameVirtualHost *
<VirtualHost *>
 ServerName www.sub.domain.com
 ServerAlias sub.domain.com
 DocumentRoot "/var/lib/srv/www/htdocs"
 DirectoryIndex index.php
 <Directory "/var/lib/srv/www/htdocs">
 Options ExecCGI
 AllowOverride None
 Order allow,deny
 Allow from all
 </Directory>
</VirtualHost>

Is this ok? Am I missing anything important?
Are there any tricks or special settings for hosting a subdomain on a separate server?

Thanks in advance,

berserka