このエラーメッセージに関する原因と対処に関して説明します。
エラーメッセージ(英語)：
cannot yum update
対処
関連リンク
nat - Yum update not working on CentOS 6.2 minimal install ...
https://serverfault.com/questions/399597/yum-update-not-working-on-centos-6-2-minimal-install
RHUI: clients below RHEL 6.4 unable to download content if RHUI ...
https://access.redhat.com/solutions/1261993
Recently Active 'yum' Questions - Page 5 - Unix & Linux Stack Exchange
https://unix.stackexchange.com/questions/tagged/yum?page=5&sort=active
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https://unix.stackexchange.com/questions/tagged/yum?page=3&sort=newest
Internal VPC S3 Endpoints in AWS
http://lyndonswan.tumblr.com/