需求：
假如需要下载某些rpm软件包，但这些rpm软件包已经很难找到下载资源并且很多是存在依赖包，一个一个下载很麻烦，但很多网络yum源提是可以直接在线安装，如果用户需要是把rpm软件包和依赖包一并下载，这时，使用网络yum源下载rpm软件包最为方便。
Yum下载软件包有两种方法：（本教程以阿里云Yum源为下载RPM软件包）
Linux System Environment
[root@yumserver ~]cat /etc/redhat-release #==》系统版本
CentOS Linux release 7.5.1804 (Core)
[root@yumserver ~]uname –r #==》内核版本
3.10.0-862.el7.x86_64
[root@yumserver ~] uname -m #==》系统架构
x86_64
[root@yumserver ~]echo $LANG #==》系统字符集
en_US.UTF-8
方法一、使用yum源的downloadonly插件（Yum工具自带此插件）
提示：此方法会把RPM软件包和相关的依赖包一并下载，但本地系统不会安装下载的软件，推荐使用此方法。
1、配置阿里云Yum源
curl -o /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
curl -o /etc/yum.repos.d/epel.repo http://mirrors.aliyun.com/repo/epel-7.repo
yum makecache
2、使用yum源命令下载createrepo软件包并指定路径
[root@yumserver ~]# mkdir -p /tmp/yum
[root@yumserver ~]# yum -y install createrepo --downloadonly --downloaddir=/tmp/yum
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
* base: mirrors.aliyun.com
* extras: mirrors.aliyun.com
* updates: mirrors.aliyun.com
Resolving Dependencies
--> Running transaction check
---> Package createrepo.noarch 0:0.9.9-28.el7 will be installed
--> Processing Dependency: python-deltarpm for package: createrepo-0.9.9-28.el7.noarch
--> Processing Dependency: deltarpm for package: createrepo-0.9.9-28.el7.noarch
--> Running transaction check
---> Package deltarpm.x86_64 0:3.6-3.el7 will be installed
---> Package python-deltarpm.x86_64 0:3.6-3.el7 will be installed
--> Finished Dependency Resolution
Dependencies Resolved
===================================================================================================
Package Arch Version Repository Size
===================================================================================================
Installing:
createrepo noarch 0.9.9-28.el7 base 94 k
Installing for dependencies:
deltarpm x86_64 3.6-3.el7 base 82 k
python-deltarpm x86_64 3.6-3.el7 base 31 k
Transaction Summary
===================================================================================================
Install 1 Package (+2 Dependent packages)
Total download size: 207 k
Installed size: 558 k
Background downloading packages, then exiting:
(1/3): createrepo-0.9.9-28.el7.noarch.rpm | 94 kB 00:00:00
(2/3): deltarpm-3.6-3.el7.x86_64.rpm | 82 kB 00:00:00
(3/3): python-deltarpm-3.6-3.el7.x86_64.rpm | 31 kB 00:00:00
---------------------------------------------------------------------------------------------------
Total 1.8 MB/s | 207 kB 00:00:00
exiting because "Download Only" specified
[root@yumserver ~]# ls -lrth /tmp/yum
total 212K
-rw-r--r-- 1 root root 83K Jul 4 2014 deltarpm-3.6-3.el7.x86_64.rpm
-rw-r--r-- 1 root root 32K Jul 4 2014 python-deltarpm-3.6-3.el7.x86_64.rpm
-rw-r--r-- 1 root root 94K Aug 10 2017 createrepo-0.9.9-28.el7.noarch.rpm
方法二：利用Yum工具缓存功能
提示：此方法会把RPM软件包和相关的依赖包一并下载，并且本地系统会把下载的软件包一并安装。
1、配置阿里云Yum源
curl -o /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
curl -o /etc/yum.repos.d/epel.repo http://mirrors.aliyun.com/repo/epel-7.repo
yum makecache
2、/etc/yum.conf配置文件参数说明
[root@yumserver ~]# cat /etc/yum.conf
[main]
cachedir=/var/cache/yum/$basearch/$releasever ##指定yum缓存的RPM软件包
keepcache=0 ##1保存已安装的RPM软件包，0不保存
debuglevel=2 ##调试级别（0-10）
logfile=/var/log/yum.log ##记录yum下载的软件包记录
exactarch=1
obsoletes=1
gpgcheck=1 ##是否密钥检查
plugins=1
installonly_limit=5
bugtracker_url=http://bugs.centos.org/set_project.php?project_id=23&ref=http://bugs.centos.org/bug_report_page.php?category=yum
distroverpkg=centos-release
3、修改/etc/yum.conf配置文件将keepcache=0改成keepcache=1，保存退出
[root@yumserver ~]# sed -i 's/keepcache=0/keepcache=1/g' /etc/yum.conf
[root@yumserver ~]# cat /etc/yum.conf | grep keepcache
keepcache=1
[root@yumserver ~]# systemctl daemon-reload
[root@yumserver ~]# yum makecache
[root@yumserver ~]# rpm -qa | grep createrepo
[root@yumserver ~]# yum -y install createrepo
[root@yumserver ~]# ls -lrth /var/cache/yum/x86_64/7/base/packages/
total 212K
-rw-r--r-- 1 root root 83K Jul 4 2014 deltarpm-3.6-3.el7.x86_64.rpm
-rw-r--r-- 1 root root 32K Jul 4 2014 python-deltarpm-3.6-3.el7.x86_64.rpm
-rw-r--r-- 1 root root 94K Aug 10 2017 createrepo-0.9.9-28.el7.noarch.rp
[root@yumserver ~]# rpm -qa | grep createrepo
createrepo-0.9.9-28.el7.noarch