Don’t know if you were aware of it, also mirrorlist.nethserver.org has some issues:
# yum -y update
Plugin abilitati:changelog, fastestmirror, nethserver_events, presto,
Impostazione processo di aggiornamento
Loading mirror speeds from cached hostfile
Could not retrieve mirrorlist http://mirrorlist.nethserver.org/6.8/centos?release=6&repo=os&arch=x86_64 error was
14: PYCURL ERROR 22 - "The requested URL returned error: 404 Not Found"
Errore: Cannot find a valid baseurl for repo: centos-base
By the way, mirrorlist.nethserver.org resolves to packages.nethserver.org.