In managed networks where there is an HTTP proxy that regulates which workstations and servers can access internal or external networks with for example HTTP protocol, yum must be configured appropriately in order to use the proxy resources to be able to access to software repositories when needed.
In yum.conf you can use the following three directives:
proxy
URL of proxy server that yum should use.
proxy_username
Username to use with the proxy server.
proxy_password
Password for this proxy server.
Remember that YUM may use available repositories via HTTP and FTP so it is not necessarily an HTTP proxy server.
Relevant excerpts from a /etc/yum.conf example configuration:
proxy=http://proxy.domain.com:3128
proxy_username=dragon
proxy_password=secret
