[root@centos6test ~]# yum update
Loaded plugins: fastestmirror, refresh-packagekit
Determining fastest mirrors
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=6&arch=i386&repo=os
error was 14: PYCURL ERROR 6 - "" Error: Cannot find a valid baseurl for repo: base

Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=6&arch=x86_64&repo=os error was 14: PYCURL ERROR 6 - "Couldn't resolve host 'mirrorlist.centos.org'" Error: Cannot find a valid baseurl for repo: base

vim /etc/sysconfig/network-scripts/ifcfg-eth0

ONBOOT=no
NM_CONTROLLED=no
BOOTPROTO=dhcp
DNS1=8.8.8.8
DNS2=4.2.2.2

ifdown eth0
ifup eth0

proxy=http://xxx.xxx.xx.xx:8080

proxy_username=username
proxy_password=password

[base]
name=CentOS-$releasever - Base
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6

nameserver 8.8.8.8
nameserver 8.8.4.4

# yum update