1.yum介绍
yum(全称为 Yellow dog Updater, Modified)是一个在Fedora和RedHat以及SUSE中的Shell前端软件包管理器。基於RPM包管理，能够从指定的服务器自动下载RPM包并且安装，可以自动处理依赖性关系，并且一次安装所有依赖的软体包，无须繁琐地一次次下载、安装。yum提供了查找、安装、删除某一个、一组甚至全部软件包的命令。
2.背景
虚拟机安装了Centos6.8，通过yum源安装软件报错，发现无法安装
[root@hadoop03 usr]# yum install -y openssh-clients
已加载插件：fastestmirror
设置安装进程
Loading mirror speeds from cached hostfile
YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
Eg. Invalid release/repo/arch combination/
removing mirrorlist with no valid mirrors: /var/cache/yum/x86_64/6/base/mirrorlist.txt
错误：Cannot find a valid baseurl for repo: base
3.方案
3.1 方案一
根据错误查看网上材料，大部分解决方案，是要求更新/etc/yum.repos.d/CentOS-Base.repo镜像源（比如阿里云镜像），操作如下
#1.首先备份系统自带yum源配置文件/etc/yum.repos.d/CentOS-Base.repo
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
#2、进入yum源配置文件所在的文件夹
cd /etc/yum.repos.d/
#3、查看CentOS系统版本
lsb_release -a
#4、下载ailiyun的yum源配置文件到/etc/yum.repos.d/
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-6.repo
-bash: wget: command not found提示 “-bash: wget: command not found”。由于yum源本身的问题，无法通过yum源下载wget
yum install wget -y
已加载插件：fastestmirror
设置安装进程
Loading mirror speeds from cached hostfile
YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
Eg. Invalid release/repo/arch combination/
removing mirrorlist with no valid mirrors: /var/cache/yum/x86_64/6/base/mirrorlist.txt
错误：Cannot find a valid baseurl for repo: base
3.2 方案二
先把之前的repo挪到备份，然后下面两个二选一
源一：替换为官方Vault源(海外服务器用)
curl -o /etc/yum.repos.d/CentOS-Base.repo https://www.xmpan.com/Centos-6-Vault-Official.repo
源二：阿里云Vault镜像(国内服务器用)
curl -o /etc/yum.repos.d/CentOS-Base.repo https://www.xmpan.com/Centos-6-Vault-Aliyun.repoyum源下载wget正常，其他问题迎刃而解
yum install wget -y
已加载插件：fastestmirror
设置安装进程
Loading mirror speeds from cached hostfile
base | 3.7 kB 00:00
base/primary_db | 4.7 MB 00:00
extras | 3.4 kB 00:00
extras/primary_db | 29 kB 00:00
updates | 3.4 kB 00:00
updates/primary_db | 12 MB 00:00
解决依赖关系
--> 执行事务检查
---> Package wget.x86_64 0:1.12-10.el6 will be 安装
--> 完成依赖关系计算
依赖关系解决
===================================================================================================================================================================================================================================
软件包 架构 版本 仓库 大小
===================================================================================================================================================================================================================================
正在安装:
wget x86_64 1.12-10.el6 base 484 k
事务概要
===================================================================================================================================================================================================================================
Install 1 Package(s)
总下载量：484 k
Installed size: 1.8 M
下载软件包：
wget-1.12-10.el6.x86_64.rpm | 484 kB 00:00
warning: rpmts_HdrFromFdno: Header V3 RSA/SHA1 Signature, key ID c105b9de: NOKEY
Retrieving key from http://mirrors.aliyun.com/centos-vault/RPM-GPG-KEY-CentOS-6
Importing GPG key 0xC105B9DE:
Userid: "CentOS-6 Key (CentOS 6 Official Signing Key) <centos-6-key@centos.org>"
From : http://mirrors.aliyun.com/centos-vault/RPM-GPG-KEY-CentOS-6
运行 rpm_check_debug
执行事务测试
事务测试成功
执行事务
正在安装 : wget-1.12-10.el6.x86_64 1/1
Verifying : wget-1.12-10.el6.x86_64 1/1
已安装:
wget.x86_64 0:1.12-10.el6
完毕！