YUM : Proxy authentication cofiguration

You are connected through proxy server which asks for authentication (i.e. username & password). In this case you have to make some configuration to work with YUM. So simply follow these instructions:
open your terminal as root and type:

#gedit .bashrc hit Enter

you can use vim, emac instead of gedit.
Type the following at the end of file:

export http_proxy="http://username:pw@proxyserver:port" 

hit Enter, then save file. close gedit and restart your computer

username & pw is your proxy username & password respectively. proxyserver:port is address of your proxy server with port e.g. export http_proxy="http://mac:jack@10.1.1.18:80"