Ouch, are you really using rar? I don't think rar properly stores symbolic links, ownership and permissions. In
/etc, that would break many many things.
/etc/mtab is just one that happens to be a symbolic link to a read-only file, so you got an error for this one — but many other symbolic links were saved as regular files and while extracting them from the backup succeeded the end result is not a valid system. The worst problems would be from the permissions though — you can probably still boot with symbolic links replaced by their restored content (but then you'd run into problems whenever you install software) but not with broken permissions or ownership.
Use a native Unix tool such as tar, cpio or pax to back up system directories. Evem then, beware that some things won't work if you blithely extract a backup of
/etc from a different installation on Debian, because some services use dynamically-assigned user and group IDs; when you restore
/etc/group from a different bakcup, that will introduce an inconsistency between
/etc and permissions elsewhere. I'm not sure if there's a good solution to that one if you just want to restore
/etc as a whole.
You can't restore your rar backup. Reinstall the system, then extract the rar archive in a different directory. Figure out what files you modified on the original system (based on the dates, maybe) and copy only those. Don't copy any file you don't understand. In the user and group databases (
/etc/gshadow), copy only the entries for human users, let Debian manage the system users.
Going forward, a much better way to back up
/etc independently is to put it under version control. Etckeeper is great at that. Run
etckeeper init after installation. When you make some change in
etckeeper commit and enter a message to describe your change (your future self will thank you). Push a copy of the repository to your backup area. To restore a backup, initialize etckeeper on the new system, add the backup as an external repository and merge it into the local branch.