yum clean all

-- yum update 안될때
vi /etc/resolv.conf 파일 확인
첫줄에 nameserver 8.8.8.8 삽입



yum clean all 후

[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

edit network interface. For my case, I used eth0 in CentOS 6.X In CentOS 7 you can use the "ensxxx" interface.
vim /etc/sysconfig/network-scripts/ifcfg-eth0


update NM_CONTROLLED to no
ONBOOT=no => yes 톰캣구동안됨
NM_CONTROLLED=no 
BOOTPROTO=dhcp or none  (DHCP install 가능)
DNS1=8.8.8.8
DNS2=4.2.2.2


restart the network interface
ifdown eth0
ifup eth0