当我尝试在 CentOS 6.8 上 yum install nss-pam-ldapd 时，我得到以下信息：
$ sudo yum install nss-pam-ldapd
Loaded plugins: fastestmirror, security
Setting up Install Process
No package nss-pam-ldapd available.
Error: Nothing to do
该怎么办？
$ yum repolist
Loaded plugins: fastestmirror, security
Loading mirror speeds from cached hostfile
repolist: 0