yum and white-listed based proxy

Hello,
here we've got about ten servers running CentOS to host and
development/test environment.
All servers are on our corporate LAN and should access Internet trough
corporate proxy.
Proxy limits access and permits only white-listed HTTP sites (not sure
about FTP protocol).
I need to install software or just update servers, because yum gets
software by dynamically generated list (a querystring in the URL) I
don't know wich sites to add in the white-list (= no update/software.
Is there a workaround for this problem ?
Can I force yum to download packages just from some white-listed URLs ?

Thanks in advance,

Filippo