Change default DNS server in Arch Linux
Solution 1:
Arch Wiki explains: either use a resolv.conf.head file, or write-protect /etc/resolv.conf.
Solution 2:
The file /etc/resolv.conf generally should not be edited by hand. Most linux systems use a program called resolvconf that will automatically generate /etc/resolv.conf every time you connect to the LAN (as the post describes). Instead, you want to edit the file /etc/resolvconf.conf.
Check man resolvconf for more information.