[CentOS] Yum showing error

Fri Apr 25 07:05:26 UTC 2008
JohnsonKoilraj <johnson.raj at wipro.com>

     I am using centos 5. yum saying following error..

[root at zeus Desktop]# yum grouplist
Loading "installonlyn" plugin
Setting up Group Process
Setting up repositories
Could not retrieve mirrorlist
http://mirrorlist.centos.org/?release=5&arch=i386&repo=os error was
[Errno 14] HTTP Error 407: Proxy Authentication Required
Error: Cannot find a valid baseurl for repo: base

my system is behind corporate filter. Is there any where to use yum.
help me out in this. in some forum it says we need NTLM to handle


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