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CIS/CSE 643: Computer Security (Syracuse University)

Unix Security Overview: 1

Unix Security Overview

1

User and Group

  • Users

  • root: super user (uid = 0)

  • daemon: handle networks.

  • nobody: owns no files, used as a default user for unprivileged operations. Web browser can run with this mode.

  • User needs to log in with a password. The encrypted password is stored in /etc/shadow.

  • User information is stored in /etc/passwd, the place that was used to store passwords (not anymore). The following is an example of an entry in this file. john:x:30000:40000:John Doe:/home/john:/usr/local/bin/tcsh

  • Groups

  • Sometimes, it is more convenient if we can assign permissions to a group of users, i.e. we would

like to assign permission based on groups.

  • A user has a primary group (listed in /etc/passwd), and this is the one associated to the files

the user created.

  • Any user can be a member of multiple groups.

  • Group member information is stored in /etc/group

% groups

uid

(display

the

groups

that

uid

belongs

to)

  • For systems that use NIS (Network Information Service), originally called Yellow Page (YP),

we can get the group information using the command ypcat.

%

ypcat

group

(can

display

all

the

groups

and

their

members)

2

File Permissions

  • File Permissions

    • The meaning of the permission bits in Unix. Owner (u), Group (g), and Others (o). Readable (r), Writable (w), and Executable (x). Example: -rwxrwxrwx (777)

  • Permissions on Directories:

    • r: the directory can be listed.

    • w: can create/delete a file or a directory within the directory.

September 7, 2009

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