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Chapter 8. File System Security. File Protection Schemes. Login passwords Encryption File Access Privileges. Figure 8.1 The process of encryption and decryption. File Access Rights. Types of Users: Owner Group All/Other Types of Permissions: Read Write Execute Types of Files

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chapter 8

Chapter 8

File System Security

file protection schemes
File Protection Schemes
  • Login passwords
  • Encryption
  • File Access Privileges
file access rights
File Access Rights
  • Types of Users:
    • Owner
    • Group
    • All/Other
  • Types of Permissions:
    • Read
    • Write
    • Execute
  • Types of Files
    • Directories
    • Other files
directory permissions
Directory Permissions
  • read = list files in the directory
  • write = add new files to the directory
  • execute = access files in the directory
changing the access rights
Changing the Access Rights

Purpose – to set/change permissions in files

  • chmod [options] octal-mode filelist
  • chmod [options] symbolic-mode filelist

Options

  • -R recursively process subdirectories
figure 8 2 position of file type and access privilege bits for linux files as seen by ls l command
Figure 8.2  Position of file type and access privilege bits for LINUX files(as seen by “ls –l” command)
figure 8 3 position of access privilege bits for linux files as specified in the chmod command
Figure 8.3  Position of access privilege bits for LINUX files as specified in the chmod command
default file access rights
Default File Access Rights
  • umask is a bitmap which tells which permissions to deny by default on new files
  • 022 = 000 010 010 (deny write for g+o) rwx r-x r-x (new files permissions)
  • umask with no parameters returns the current mask value
  • umask newmask - sets new mask
  • umask command usually used in a startup file
suid bit
SUID Bit
  • A special permission bit that allows executable files to run using the privileges of the owner of the files rather than the user of the file
  • Can be set using commands:

chmodu+sfilelist

chmod 4xxx filelist

  • Shows up in ls - l in place of the user x bit as an s if the file is executable - (rwsrwxrwx)
  • Very dangerous to use
sgid bit
SGID Bit
  • A special permission bit that allows executable files to run using the privileges of the owner’s group rather than the user of the file
  • Set using the commands

chmod g+s filelist

chmod 2xxx filelist

sticky bit
Sticky Bit
  • A special bit that can be used as follows:
  • For a file: it directs the operating system to keep the program in memory if possible after it finishes execution (Early versions of UNIX)
  • For a directory: it sets it up such that only the owner of the directory can delete (or rename) files from the directory, even if other users have write privilege (tmp)
  • Can be set using the chmod command using the options:chmod +t filelist
  • Shows up in “ls –l” as a t - (rwxrwxrwt)
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