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The preservation of identity that was mentioned as a desired property of a security policy for a distributed environment. This is applicable both to user IDs and to group IDs. Here is a system design that addresses both of these problems.
There are multiple commands that access or change group information
There are also several "exotic" commands:
Modification of file, directory and device access is achieved with the chmod command.
Permissions may be viewed by issuing the command: ls -l file-name
Users are members of a default group. Red Hat Linux will add new users to a group of the same group name as the user name. The default group is specified in the file /etc/passwd
user-name:x:user-number:group-number:comment section:/home-directory:default-shell user1:x:500:500:Greg:/home/user1:/bin/bashThe user id has a user system number associated with it and this is defined in /etc/passwd. The group has a group system number associated with it and this is defined in /etc/group
group-name:x:group-number:user1,user2 user1:x:500: user2:x:501: floppy:x:19:user1 accounting:x:600:user2 apache:x:48:User "user1" is a member of default group "user1" and also a member of group "floppy".
If using NIS, view the groups using the command: ypcat group
Use the command newgrp group-name to switch your default used in file creation or directory access. This starts a new shell. Exit to return to the previous group id. Use the ps command to see if more than one shell is active.
For example "user2" would like to create a file in the accounting directory which can be read my members of his group. First switch the default group with the command: newgrp accounting
To return to your default group issue the "exit" command. If confused, issue the "ps" command. There should only be one instance of bash, else you are in the alternate group and not the default group.
Use the command newgrp group-name file-name to change the group associated with a file. You must be a member of the group to execute the command sucessfully. (or be root)
The newgrp command logs a user into a new group by changing a user's real and effective group ID. The user remains logged in and the current directory is unchanged. The execution of newgrp always replaces the current shell with a new shell, even if the command terminates with an error (unknown group).
Any variable that is not exported is reset to null or its default value. Exported variables retain their values. System variables (such as PS1, USER, PATH and HOME), are reset to default values unless they have been exported by the system or the user.
With no operands and options, newgrp changes the user's group IDs (real and effective) back to the group specified in the user's password file entry. This is a way to exit the effect of an earlier newgrp command.
A password is demanded if the group has a password and the user is not listed in /etc/group as being a member of that group. The only way to create a password for a group is to use passwd(1), then cut and paste the password from /etc/shadow to /etc/group. Group passwords are antiquated and not often used.
Gives new login as if logged in as group member: newgrp -
If the user creates a file, the default group association is the group id of user. If he wishes to change it to another group of which he is a member issue the command: chgrp new-group-id file-name
If the user is not a member of the group then a password is required.
Users are assigned upon user creation, a User Private Group (UPG) which is a unique group ID of the same name as the user ID. This allows for a fine atomic level of group permissions to be assigned for tighter and simpler default security.
The typical Linux installation will come with two dozens existing standard groups: (See /etc/group)
This is only a partial listing of the default groups. There will also be a default set of member user ID's associated with most of the groups.
The first example will be of granting access to a device, the CD-ROM. This is generally not done for regular users on a server. Server access to a CD-ROM is limited to root by default. (This example may also be applied to the diskette. Group: floppy, first floppy device: /dev/fd0)
OR for a completely different method that steps 1 to 4, use the one step approach:
Desktop GUI method:
After mounting the CD-ROM one can view its contents from the directory /mnt/cdrom.
This will list all the groups to which user-id is a member.
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