Asked By
jonny singh
10 points
N/A
Posted on - 10/05/2011
How does a UNIX file system control access rights on files for different groups of users & these users interfaces of files system for manipulation files?
what is Trojan horses and how does Trojan horses enter in the OS and start attack?
Answered By
sherob
0 points
N/A
#86783
How does unix access files for different users
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Hi,
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Ø  To answer the first part of your question UNIX controls file access by using permission and access control lists.
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With UNIX permission, different users and user groups are granted different levels of access to a file or document. There are three categories of users to whom Permission for file access is granted, these are:
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· user – who basically owns the file or document
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· group – this is the group to which the user or owner belongs
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· others – these are persons who neither falls into a group or is a user (owner).
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There are also three things these users are allowed or permitted to do to each file based on the category which they fall into, these are; read access, write access and execute access. These basically outline who can read or write to and from a file, and who can execute an action. For directories it determines which users can access, delete or list the content.
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An access control list (ACL) is just a list of permission(s) that can be executed on a file, and specifies which users and/or systems are permitted to access the file and what operation said user is allowed to perform. Each entry in an ACL gives a subject (name of person) and what operation they can perform on the file. Example, if a file has the ACL (Jake, delete), this would be granted Jake the permission or ability to delete the file.
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Ø  The second part of the question, Trojan horse is typically a malicious (harmful) code found inside programs or data which are thought to be harmless, as it appears to carry out a desired function for the user. However, what it does it takes control of your system and does whatever damage it choose to such as stealing information, destroying your system and ruining the file allocation table stored on your hard drive. The most insidious of all Trojan horses is the ones that claim that they have rid your computer of viruses but instead introduces new ones to your system.
How does unix access files for different users
UNIX operating system allows selective definition of users who have access to a particular file or directory. For each file or directory, there are three classes of users:
    owner-user who created the file or directory,
    more user-group is defined as a group and each file belonging to a group,
    other-other users of the system who are not the owner nor the group members.
Also, each file or directory has three modes of access:
    read-only file can be read, the directory can only list.
    write-permitted modification of the file, the directory can create new files or delete existing.
    execute-file is excellent. When the directory is allowed to copy, as a working definition of directories and so on.
Compared to the previously described classes and approaches for each file or directory's mode is defined:
rwx rwx rwx
owner of the other groups
If it says "-" Access is denied. There are other permissions:
    t (sticky bit) – great version remains in memory
    s (ie, UID set. GID) – set the identification number of users or groups
    l (file locking) – locked files
Chmod command it is possible to change the access file.
