[Yum] Newbie YUM and Proxy questions.

[WING-LOK LEE]

I am new working at the present job.  All network traffic at this site is 
controlled by a proxy server.  Most people just work with Microsoft systems 
and probably just browsers as well.
I am building a Linux cluster running Fedora 5.  I would like to keep things 
up to date with YUM.  I got the browsers working.  I can use wget to get 
files with http_proxy set in my .profile.  I couldn't get ftp to work.  When 
I try to run "yum update", I got:
Loading "installonlyn" plugin
Setting up Update Process
Setting up repositories
livna                                                                [1/7]
livna                     100% |=========================|  951 B    00:00
flash                                                                [2/7]
flash                     100% |=========================|  951 B    00:00
core                                                                 [3/7]
http://download.fedora.redhat.com/pub/fedora/linux/core/5/x86_64/os/repodata/repomd.xml: 
[Errno 4] IOError: <urlopen error (-2, 'Name or service not known')>
Trying other mirror.
Cannot open/read repomd.xml file for repository: core
failure: repodata/repomd.xml from core: [Errno 256] No more mirrors to try.
Error: failure: repodata/repomd.xml from core: [Errno 256] No more mirrors 
to try.

It looks like I got livna and flash but cannot get anything from 
fedora.redhat.com.  Can anyone knowledgable tell me what I am doing wrong?

Lee