Unable to configure YUM through a proxy server

All: 

I'm a Fedora newbie and I'm currently trying to configure YUM to go
through a proxy server. I've followed the direction via the Fedora site
but I'm still having no success here is the error: 

[root@linuxdesktop etc]#vi yum.conf 

[main]
Cachedir=/var/cache/yum
Keepcache=0
Debuglevel=2
Logfile=/var/log/yum.log
Pkgpolicy=newest
Distroverpkg=redhat-release
Tolerant=1
Exactarch=1
Obsoletes=1
Gpgcheck=1
Plugins=1
Metadata_expire=1800

# The proxy server entry

Proxy=http://<myproxyip>
Proxy_username=user
Proxy_password=password

# PUT YOU REPOS HERE OR IN separate files named file.repo
#in /etc/yum.repos.d

The error is: 

[Errno 4] Ioerror: <urlopen error (-2, 'Name or service not known')>
Trying other mirror. 
Error: Cannot open/read repomd.xml file for repository:core

Do you have any answers to help with this problem. 

Thanks, 

Chris Smith 
[email protected]
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