yum不能升级了？

[root@localhost yum.repos.d]# yum update
Setting up Install Process
Setting up repositories
Cannot open/read repomd.xml file for repository: base
failure: repodata/repomd.xml from base: [Errno 256] No more mirrors to try.
Error: failure: repodata/repomd.xml from base: [Errno 256] No more mirrors to try.

我的软件库文件:

#released base
[base]
name=Red Hat Enterprise AS release 4 Updates
baseurl=http://ftp.chinalinuxpub.com/redhat/enterprise/os/i386/RedHat/
gpgcheck=1

#released updates
[update]
name=Red Hat Enterprise AS release 4 Updates
baseurl=http://ftp.chinalinuxpub.com/redhat/enterprise/updates/4AS/
gpgcheck=1
gpgkey=http://ftp.chinalinuxpub.com/redhat/enterprise/RPM-GPG-KEY-linuxpub

请问这是怎么回事?我参考这里的贴子制作的yu m :
http://www.chinalinuxpub.com/bbs/showthread.php?t=48338