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Linux Tutorial - Security - File Access
  Passwords ---- The Root Account  

File Access

Although this password protection stops most attempts to gain unauthorized access to the system, many security issues involve users that already have accounts. Unchecked, curious users could access payroll information and find out what their boss gets paid. Corporate spies could steal company secrets. Disgruntled workers could wreak havoc by destroying data or slowing down the system.

Once logged in, Linux (among other UNIX dialects) provides a means of limiting the access of "authorized" users. This is in the form of file permissions, which we already talked about. File permissions are one aspect of security that most people are familiar with in regard to UNIX security. In many cases, this is the only kind of security other that user accounts.

As we talked about earlier, each file has an owner, whether or not some user explicitly went out there and "claimed" ownership. Its a basic characteristic of each file and is imposed upon them by the operating system. The owner of the file is stored, along with other information, in the inode table in the form of a number. This number corresponds to the User ID (UID) number from /etc/passwd.

Normally, files are initially owned by the user who creates them. However, there are many circumstances that would change the ownership. One of the obvious ways is that the ownership is intentionally changed. Only the owner of the file and root can change its ownership. If you are the owner of a file, you can, in essence, "transfer ownership" of the file to someone else. Once you do, you are no longer the owner (obviously) and have no more control over that file.

Another characteristic of a file is its group. Like the owner, the file's group is an intrinsic part of that files characteristics. The file's group is also stored in the inode as a number. The translation from this number to the group name is made from the /etc/group file. As we talked about in the section on users, the concept of a group has only real meaning in terms of security. That is, who can access which files.

What this means is that only "authorized" users can access files in any of the three manners: read, write and execute. It makes sense that normal users cannot run the fdisk utility, otherwise they would have the ability to re-partition the hard disk, potentially destroying data. It also makes sense that normal users do not have write permission on the /etc/passwd file, otherwise they could change it so that they would have access to the root account. Since we talked about it in the section on shell basics and on users, there is no need to go into more details here.

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Copyright 2002-2009 by James Mohr. Licensed under modified GNU Free Documentation License (Portions of this material originally published by Prentice Hall, Pearson Education, Inc). See here for details. All rights reserved.
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