redhat下如何使用yum

我用yum update 出现 this system is not registered with RHN
我的yum 是yum-3.2.8-9.e15
我/etc/yum.conf配置
[main]
cachedir=/var/cache/yum
keepcache=0
debuglevel=2
logfile=/var/log/yum.log
distroverpkg=redhat-release
tolerant=1
exactarch=1
obsoletes=1
gpgcheck=1
plugins=1

# Note: yum-RHN-plugin doesn't honor this.
metadata_expire=1h

# Default.
# installonly_limit = 3

# PUT YOUR REPOS HERE OR IN separate files named file.repo
# in /etc/yum.repos.d

/etc/yum.repos.d/下原先有个文件夹哦rhel-debuginfo.repo
代码是：


[rhel-debuginfo]
name=Red Hat Enterprise Linux $releasever - $basearch - Debug
baseurl=ftp://ftp.redhat.com/pub/redhat/linux/enterprise/$releasever/en/os/$basearch/Debuginfo/
enabled=0
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release

后来我由加了

[base]
name=CentOS 4.4 for RHEL4-base
baseurl=http://mirror.be10.com/centos/5.0/os/$basearch/
http://ftp.iasi.roedu.net/mirrors/centos.org/centos/5.0/os/$basearch/
http://ftp.hostrino.com/pub/centos/5.0/os/$basearch/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5


[update]
name=CentOS 4.4 for RHEL4-updates
baseurl=http://mirror.be10.com/centos/5.0/updates/$basearch/
http://ftp.iasi.roedu.net/mirrors/centos.org/centos/5.0/updates/$basearch/
http://ftp.hostrino.com/pub/centos/5.0/updates/$basearch/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5


执行yum update 就出现一开始的错误，请高人指点。。。。