# nslookup yahoo.co.jp
Server:         192.168.26.2
Address:        192.168.26.2#53

Non-authoritative answer:
Name:   yahoo.co.jp
Address: 124.83.187.140
Name:   yahoo.co.jp
Address: 203.216.243.240

# curl yahoo.co.jp
The document has moved <A HREF="http://www.yahoo.co.jp/">here</A>.<P>
<!-- p11.f11.top.tnz.yahoo.co.jp uncompressed/chunked Fri Feb  8 10:40:54 JST 2013 -->

　etc…

　⇒ 失敗したら、DNS（nameserver） の設定を疑う

# vi /etc/resolv.conf

; generated by /sbin/dhclient-script
nameserver 192.168.80.1
search localdomain

# yum clean all
# yum update

それでもダメなら

# rm -rf /var/cache/yum
# yum check-update
# yum update