由于2020年11月30日CentOS官方已经停止对CentOS6版本的维护，
CentOS6要使用yum命令都需要修改yum配置
现在我们直接修改配置文件

修改文件
输入命令

vi /etc/yum.repos.d/CentOS-Base.repo

将内容
将所有的mirrorlist注释掉
将所有的baseurl注释
并加入baseurl=https://mirrors.tuna.tsinghua.edu.cn/centos-vault/6.8/os/x86_64/
再将所有的gpgkey注释
并加入gpgkey=https://mirrors.tuna.tsinghua.edu.cn/centos-vault/6.8/os/x86_64/RPM-GPG-KEY-CentOS-6
注意：有大量重复操作

修改后文件内容：

[base]
name=CentOS-$releasever - Base
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
baseurl=https://mirrors.tuna.tsinghua.edu.cn/centos-vault/6.8/os/x86_64/
gpgcheck=1
#gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
gpgkey=https://mirrors.tuna.tsinghua.edu.cn/centos-vault/6.8/os/x86_64/RPM-GPG-KEY-CentOS-6

#released updates
[updates]
name=CentOS-$releasever - Updates
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/
baseurl=https://mirrors.tuna.tsinghua.edu.cn/centos-vault/6.8/updates/x86_64/
gpgcheck=1
#gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
gpgkey=https://mirrors.tuna.tsinghua.edu.cn/centos-vault/6.8/os/x86_64/RPM-GPG-KEY-CentOS-6

#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/extras/$basearch/
baseurl=https://mirrors.tuna.tsinghua.edu.cn/centos-vault/6.8/extras/x86_64/
gpgcheck=1
#gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
gpgkey=https://mirrors.tuna.tsinghua.edu.cn/centos-vault/6.8/os/x86_64/RPM-GPG-KEY-CentOS-6

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/
baseurl=https://mirrors.tuna.tsinghua.edu.cn/centos-vault/6.8/centosplus/x86_64/
gpgcheck=1
enabled=0
#gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
gpgkey=https://mirrors.tuna.tsinghua.edu.cn/centos-vault/6.8/os/x86_64/RPM-GPG-KEY-CentOS-6

#contrib - packages by Centos Users
[contrib]
name=CentOS-$releasever - Contrib
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=contrib&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/contrib/$basearch/
baseurl=https://mirrors.tuna.tsinghua.edu.cn/centos-vault/6.8/contrib/x86_64/
gpgcheck=1
enabled=0
#gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
gpgkey=https://mirrors.tuna.tsinghua.edu.cn/centos-vault/6.8/os/x86_64/RPM-GPG-KEY-CentOS-6

再输入

vi /etc/yum.conf

在文件后追加 sslverify=false

最后直接使用yum命令下载ssh命令

yum -y install openssh-clients

即可解决 ssh-copy-id: command not found 问题

参考
https://blog.csdn.net/qq_39946015/article/details/111086184
https://blog.csdn.net/whz2017/article/details/104969784