我需要包裹rar。但是每当我尝试安装它时都会收到此错误：
Reading package Lists... Done
Building dependency tree
Reading state information... Done
Package rar ist not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or is only available form another source
E: package 'rar' has no installation candidate.
我必须做什么才能得到rar？