yum update で以下のようなエラーが出て直らない。
Trying other mirror.
以下サイトで解決。設定ファイルを下の内容で丸々置き換えることで直りました。それ以上の原因追及はしたほうがいいけれどしていません。
→どなたか原因がわかった人がいればおしえてください。
name=CentOS-$releasever - Base
baseurl=http://ftp.riken.jp/Linux/caos/centos/$releasever/os/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=http://ftp.riken.jp/Linux/caos/centos/RPM-GPG-KEY-CentOS-5
#gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5
#released updates
[updates]
name=CentOS-$releasever - Updates
baseurl=http://ftp.riken.jp/Linux/caos/centos/$releasever/updates/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates
#baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=http://ftp.riken.jp/Linux/caos/centos/RPM-GPG-KEY-CentOS-5
#gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5
#packages used/produced in the build but not released
[addons]
name=CentOS-$releasever - Addons
baseurl=http://ftp.riken.jp/Linux/caos/centos/$releasever/addons/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=addons
#baseurl=http://mirror.centos.org/centos/$releasever/addons/$basearch/
gpgcheck=1
gpgkey=http://ftp.riken.jp/Linux/caos/centos/RPM-GPG-KEY-CentOS-5
#gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5
#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
baseurl=http://ftp.riken.jp/Linux/caos/centos/$releasever/extras/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras
#baseurl=http://mirror.centos.org/centos/$releasever/extras/$basearch/
gpgcheck=1
gpgkey=http://ftp.riken.jp/Linux/caos/centos/RPM-GPG-KEY-CentOS-5
#gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5
#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
baseurl=http://ftp.riken.jp/Linux/caos/centos/$releasever/centosplus/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus
#baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=1
gpgkey=http://ftp.riken.jp/Linux/caos/centos/RPM-GPG-KEY-CentOS-5
#gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5

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インフラ構築する前に必読です。
正直、ちょっと難しい。。。