Retrieving mirrorlist error

Hi,

I keep receiving this message from our Rockstor server!

Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=stock error was
14: curl#6 - “Could not resolve host: mirrorlist.centos.org; Unknown error”

How I can solve this issue?
Thank you,
Nicolas