I have some basic question on simple Linux administration related tasks. How can I make sure that the new user that got created does not have permissions to view certain files like /etc/shadow and more over which are the files that I need to protect? Do we have a predefined mechanism or a shell script that by default secures the system for all other users other than root for certain sensitive files.
Linux comes from Unix and Unix grew up in college computer labs, which means that it very quickly learned to protect itself from student pranks, hackers, and less admirable things.
Every file and directory in the Linux filesystem has a filesystem mask that indicates the permission to read, write, or execute (in the case of directories, to access). These 3 bits are themselves grouped into 3 categories: owner, group, and other.
/etc/shadow has by default the following mask: 400, which means that only the owner (root) can read it, and no else can do anything with it. So it's set up properly right out of the box.
/etc/shadow was, in fact created because people were reading /etc/passwd to get stuff like the list of valid userids, and they didn't want the really critical info to be in that file. So if a password is "x", the actual password is in /etc/shadow.
Ironically, this simple system is a lot less powerful than the ACL system that Windows NT and later versions support, but since security isn't something that can be slapped on top of an insecure system, Windows is infamous for security problems and Unix and Linux are not.
However, the 3-bits system is pretty crude, so more recently the selinux system was developed to allow more precise control of access. That, however, is a topic worthy of an entire book.
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