should I be saving this file somewhere else?
Yes, the /etc/dns.cfg file is the live file that is being used by the system. But the hard copy of that file is stored in /boot/etc/dns.cfg
So modify the /etc/ file and then copy it to /boot/etc/ to survive a reboot. This same procedure does NOT apply to the /etc/passwd or /etc/restrict.cfg files because they are copied to /boot/etc/ by the system any time the /etc/ version is changed in any way. But the procedure does apply to some other files like the /etc/cron/root is copied to /boot/etc/cron.cfg and /etc/dhcp.cfg is copied to /boot/etc/dhcp.cfg and /etc/dyndns.cfg is copied to /boot/etc/dyndns.cfg and /etc/portfw.cfg is copied to /boot/etc/
Those are probably the primary files you may be dealing with. Just remember the /etc/ is the ram disk and /boot/etc/ is always the hard copy that is copied to /etc/ at boot time. One of the reasons for doing this is that the system runs a LOT faster because all of the files needed are directly in memory rather than having to spin the disk to read them for every operation.
If you are afraid that you might make a mistake. The chances are high that you will never learn anything.