[CentOS] yum behinf authenticated proxy

Hi,

I've just installed a CentOS 4.4 and i'm behind a proxy with authentication.

Does anyone know the parameters for the environment variable 
"http_proxy" so i can make a "yum update" ?

Usually i'd do a

$ export http_proxy="proxy.domaion.foo:3128"

but this is not the case of an authenticated proxy.

Any help would be appreciated.

Warm Regards,
Mário Gamito