[Yum] yum behind firewall,how to setup proxy?

[Hou Xiang ZHU]

hi there:
I am trying to use yum to update my FC4 system, but didn't succeed!
looks like it is because I am behind a firewall, I need proxy to access outside web, and maybe an MS proxy, is there any ways to use yum behind a such proxy? how do I know what kind of proxy I am behind?

I export https_proxy with username and passwd but the following error
occured. actually I tried a few formats of setting this env variable, I am not sure I setup correct or not. my corporate proxy need a user name and passwd, in user name there is a +space+ " " in my user name. so could any one tell me exactly which variable and how to and where to set it? 

-----------------------------------------------------------
[root at zhufedora ~]# yum update
Repository livna-testing is listed more than once in the configuration
Setting up Update Process
Setting up repositories
livna-stab: ###### 1/8Cannot open/read repomd.xml file for repos itory:
livna-stable
failure: repodata/repomd.xml from livna-stable: [Errno 256] No more
mirrors to try.
Error: failure: repodata/repomd.xml from livna-stable: [Errno 256] No
more mirrors to try.
[root at zhufedora ~]#
-----------------------------------------------------------

any clues? I know our company may have strict firewall than others. I heard only port 8080 is opened.

thank you!
Houxiang