Check if immutable attribute is currently applied or not:
lsattr /etc/resolv.conf
Result
----i---------e------- /etc/resolv.conf
Remove it:
sudo chattr -i /etc/resolv.conf
Verify:
lsattr /etc/resolv.conf
Result
--------------e------- /etc/resolv.conf
Make changes to the resolv file for DNS nameservers:
nano /etc/resolv.conf
CTRL+X (save) > Y > ENTER
Apply the attribute again:
sudo chattr +i /etc/resolv.conf
Verify:
lsattr /etc/resolv.conf
Result
----i---------e------- /etc/resolv.conf
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