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Recap

In the last class, you have learnt:

• Issues regarding deletion of files in a cyclic –

graph structure

• Advantages & disadvantage of a cyclic – graph

structure

• Understand the structure of general graph

directories & problems

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Objectives

On completion of this period, you would be able to

• Understand the concept of file protection

• Know types of access

• Understand Access Lists and Groups

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PROTECTION

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A computer file needs protection which is of two types

1. ReliabilityProtection from physical damageFile systems can be damaged by Hardware problems(errors in r/w)Power surges or failuresHead crashesDirt and temperatureBugs in file system software

2. SecurityProtection from improper access

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1. Reliability

• Provided by duplicate copies of files

• Take backup's at regular intervals

(daily/weekly/monthly)

A computer file needs protection which is of two types

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2. Security

• Protecting files from unauthorized access

• More important in a multi user system

• Provided by controlling access to files

A computer file needs protection which is of two types

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Need for protecting files

• Need for protection is due to the ability to

access files

• Two ways to tackle the problem

1. Prohibit access providing complete protection

2. Provide free access without protection

• Both approaches are too extreme for general

use

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Controlled Access

• Desirable

• Provided by limiting the types of file access that

can be made

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Types of Access

• Read : Read from the file

• Write : Write or rewrite the file

• Execute : Load file into memory and execute

• Append : Write new information at end of file

• Delete : Delete the file and free its space

• List : List the name and attributes of the file

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Protection Mechanisms

• Different mechanisms proposed

• Select mechanism depending on the application

Ex : A small computer used only by a few

members of a research group may not need the

same protection as a large corporate computer

used for research, finance and personnel

operations

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Access Lists and Groups

• A common approach to protection is provide

access based on user identity

• Associate with each file and directory an access list

• Access list specifies user name and types of

access allowed for each user

• Access to user is provided as per the access list

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Problem with access lists

• Length of the list

• If every user need to be given read access to a file,

we must list all users with read access

a) constructing such a list is tedious

b) The directory entry needs to be of

variable size, resulting in complicated

space management

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Classification of users

To condense the length of access list many systems recognize three classifications of users in connection with each file

• Owner : The user who created the file

• Group : A set of users sharing the file and need similar access

• Universe : All other users in the system constitute the universe

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Representing different users in UNIX

• Unix defines three fields of three bits each rwx, where

r controls read access

w controls write access

x controls execution

• A separate field is kept for the file owner, group users and other users

• 9 bits per file are needed

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A sample directory listing in Unix

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Consider the first field in the above listing -rwxrw-r--

• The first field describes the file’s protection

• A d as the first character indicates that the file is a

directory ; a – indicates an ordinary file

• 2nd,3rd & 4th bits owner’s access permissions

• 5th,6th & 7th bits group users’ permissions

• 8th,9th & 10th bits others’ permissions

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Other Protection Approaches

• Associate a password with each file

• Access to the file is controlled by the password

• Effective scheme in limiting access to a file if

passwords chosen randomly

passwords changed often

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Disadvantages of password protection

• If each file has a separate password, the no. of passwords a user needs to remember becomes large

• If all files have same password, then once it is discovered all files are accessible

• To deal with this problem, some systems allow a user to associate a password with a subdirectory rather than with a single file

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Summary

In this class, you have learnt:

• File protection

• Need for controlled access to files

• Types of access

• Access Lists

• User classification

• Password protection

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Frequently Asked Questions

1. Explain the concept of file protection

2. List the types of access to files

3. Explain access lists and groups

4. Explain disadvantages of password

protection

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Any questions ?

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Quiz

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1. File systems can be damaged by

a) Hardware problems

b) Power failures

c) Dirt

d) All of the above

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2. Which of following is desirable with regard to access to files?

a) Free access without protection

b) No access at all

c) Controlled access

d) None of the above

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3. Which is true about password protection ?

a) Passwords should be chosen randomly

b) Passwords must never be changed

c) Password should be changed often

d) Both a & c

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State whether the following are true or false

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State true or false

Reliability of files can be provided by duplicate copies

True

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State true or false

The problem with access lists is their length

True

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State true or false

Owner is the user who created the file

True

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State true or false

Unauthorized access to files is desirable

False

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