Occasionally, it’s necessary or desirable to make files immutable. For instance, your /etc/resolv.conf file might be being overwritten by your provider’s DHCP settings and you wish to use DNS servers other than those that are being set by your provider.

To determine if your /etc/resolv.conf file is immutable or not, use the following command:

ls -lo /etc/resolv.conf

The result will look like this if the file is mutable:

-rw-r--r--  1 root  wheel  - 38 Mar 25 13:58 /etc/resolv.conf

Or, the result will look like this is the file is immutable (notice the schg):

-rw-r--r--  1 root  wheel  schg 38 Mar 25 13:58 /etc/resolv.conf

To add the immutable flag to a file, use the following command:

chflags schg /path/to/your/file

To remove the immutable flag to a file, use the following command:

chflags noschg /path/to/your/file

Caveat: chflags does not work inside a jail. All flags must be set from the host server.