--- 1 ---

$ sudo yum install yum-plugin-downloadonly

$ sudo yum install --downloadonly --downloaddir=/tmp <package-name>

（默认路径 /var/cache/yum/x86_64/7/base/packages）

--- 2 ---

$ sudo yum install yum-utils

$ sudo yumdownloader nmap-ncat

$ ls
nmap-ncat-6.40-7.el7.x86_64.rpm