Linux Tutorial - Managing Group Access on Linux and UNIX



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All Linux users have a user ID and a group ID and a unique numerical identification number called a userid (UID) and a groupid (GID) respectively. Groups can be assigned to logically tie users together for a common security, privilege and access purpose. It is the foundation of Linux security and access. Files and devices may be granted access based on a users ID or group ID. This tutorial attempts to show how this is used.

File, directory and device (special file) permissions are granted based on "user", "group" or "other" (world) identification status. Permission is granted (or denied) for read, write and execute access.

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| |File, Directory and Device permissions: |[p|

|[p| |ic|

|ic|Modification of file, directory and device access is achieved with the chmod command. |] |

|] |Grant read permissions to a file which you own so that everyone may read it: | |

| |chmod ugo+r file-name | |

| |Grant read permissions on a directory: | |

| |chmod ugo+rx directory-name | |

| |Note that "execute" permission is required in order to read a directory. | |

| |Deny read access to a file by everyone except yourself: | |

| |chmod o-r file-name | |

| |Allow everyone in your group to be able to modify the file: | |

| |chmod 660 file-name | |

| |See chmod man page for more info. | |

| |Permissions may be viewd by issuing the command: ls -l file-name | |

| |File can be written by youself and members of the group. Others may only view it. | |

| |-rw-rw-r-- user group file-size date file-name | |

| |Directory is completely open for read/write: | |

| |drwxrwxrwx user group file-size date directory-name | |

| |File can only be accessed by owner (user): | |

| |-rwx------ user group file-size date file-name | |

| |[pic] | |

| |Groups and group members: | |

| | | |

| |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/bash | |

| | | |

| | | |

| |The 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". | |

| |Group Commands: | |

| |gpasswd: administer the /etc/group file | |

| |groupadd: Create a new group | |

| |groupmod: Modify a group | |

| |groupdel: Delete a new group | |

| |If using NIS, view the groups using the command: ypcat group | |

| |[pic] | |

| |Switching your default 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 - | |

| |[pic] | |

| |Changing group ownership: | |

| | | |

| |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. | |

| |[pic] | |

| |Default user groups: | |

| | | |

| |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. | |

| |[pic] | |

| |Pre-Configured system groups: | |

| | | |

| |The typical Linux installation will come with some exisitng standard groups: (See /etc/group) | |

| |Group ID | |

| |GID | |

| | | |

| |root | |

| |0 | |

| | | |

| |bin | |

| |1 | |

| | | |

| |daemon | |

| |2 | |

| | | |

| |sys | |

| |3 | |

| | | |

| |adm | |

| |4 | |

| | | |

| |tty | |

| |5 | |

| | | |

| |disk | |

| |6 | |

| | | |

| |lp | |

| |7 | |

| | | |

| |mem | |

| |8 | |

| | | |

| |kmem | |

| |9 | |

| | | |

| |wheel | |

| |10 | |

| | | |

| |mail | |

| |12 | |

| | | |

| |man | |

| |15 | |

| | | |

| |floppy | |

| |19 | |

| | | |

| |named | |

| |25 | |

| | | |

| |rpm | |

| |37 | |

| | | |

| |xfs | |

| |43 | |

| | | |

| |apache | |

| |48 | |

| | | |

| |ftp | |

| |50 | |

| | | |

| |lock | |

| |54 | |

| | | |

| |sshd | |

| |74 | |

| | | |

| |nobody | |

| |99 | |

| | | |

| |users | |

| |100 | |

| | | |

| |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. | |

| |[pic] | |

| |Grant use of a device to system users: | |

| | | |

| |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) | |

| |Grant mount privileges to system users | |

| |Create group cdrom . | |

| |Allow use of device by group cdrom . | |

| |Add user to group cdrom . | |

| |Grant privileges to system users to mount the device: | |

| |Manual method: This requires a change to the file /etc/fstab.The fourth column defines mounting options. By default only root| |

| |may mount the device (option owner ). To grant users the ability to mount the device, change the owner option to user . With | |

| |the user option only the user who mounted the device can unmount the device. To allow anyone to unmount the device, use the | |

| |option users . | |

| |Linuxconf GUI method: (Note: Linuxconf is no longer included with Red Hat Linux 7.3 and later) | |

| |RH 5.2: Start + Programs + Administration + Linuxconf . | |

| |RH 6.0: Select Gnome Start icon + System + Linuxconf . | |

| |Select Config + File systems + Access local drive . | |

| |Select the device /dev/cdrom | |

| |Select the tab Options . | |

| |Add the option User mountable to allow users to mount the CD-ROM. The user who mounted the CD must also be the one to unmount| |

| |the CD. OR Select the tab Misc. and add to Other options: users if you want to allow anyone to be able to unmount the CD | |

| |regardless of who mounted it. | |

| |For more information see the man pages for mount and fstab. | |

| |Create group cdrom : | |

| |Manual method: | |

| |Add the line cdrom:::root, to the file /etc/group where is the user to be granted use | |

| |of the CD-ROM. (For example: cdrom::25:root,user1") | |

| |OR | |

| |Add a group with the command: groupadd in this case groupadd cdrom . | |

| |Linuxconf GUI method: (Admin tool linuxconf is no longer included with Red Hat 7.3+.) | |

| |Start linuxconf. | |

| |Select Config + User Accounts + Normal + Group Definitions + Add . | |

| |Group Name: cdrom | |

| |Alternate Members (opt): root : (Add space delimited user ids here) | |

| |Accept | |

| |For more information see the man pages for groupadd, groupmod and groupdel. | |

| |Allow use of device by group cdrom . | |

| |Manual method: | |

| |Use the command: chown owner:group to assign the device to a user and group. For example: chown root.cdrom /dev/hdd | |

| |. (Use hdd if cdrom is the slave device on the 2nd IDE controller.) | |

| |Allow group access to the device: chmod 660 /dev/hdd | |

| |GUI method: | |

| |Start the File Manager and right click the file representing the cdrom device. Select Properties . Then select the tab | |

| |Permissions . Set the Owner to root and the Group to cdrom. Allow Read and Write privileges for the user and group by | |

| |selecting the appropriate buttons. | |

| |Add user to group cdrom : At this point, adding users to the group cdrom will grant them access to the device. | |

| |Manual method: The user id s specified in /etc/group is a comma separated list. | |

| |Use the command usermod -G . Be sure to list all groups as this is an absolute list | |

| |and not an addition. To list all groups to which a user is a member use the command groups . | |

| |Linuxconf GUI method: Step two allowed you to assign users to the group. If users still need to be assigned use the following| |

| |method: | |

| |After starting Linuxconf, select options Config + User Accounts + Normal + User Accounts . | |

| |Next to supplementary groups add the group cdrom. Groups should be delimited by spaces. | |

| |OR for a completely different method that steps 1 to 4, use the one step approach: | |

| |chmod 664 /dev/hdd : Allow read use to all users of the CD-ROM device (hdd is just the example, your device name can vary). | |

| |This method is quick, unelegant and can be used for your own desktop system but definitely don t do this on a server. | |

| |Using CD-ROM: | |

| |You must mount and un-mount each CD-ROM individually. Do not switch CDs without un-mounting and re-mounting the new CD. (The | |

| |GNOME desktop interface has features to do this for you. Covered later) | |

| |Command method: | |

| |mount -t iso9660 /dev/hdd /mnt/cdrom : This generates amount point for CD-ROM (or mount -t iso9660 /dev/cdrom /mnt/cdrom . | |

| |The device name /dev/cdrom is a symbolic link to the actual device) | |

| |Note: Only root user may execute the mount command. Users must use the tool usermount. | |

| |Desktop GUI method: | |

| |RH 5.2: Start + Programs + Administration + Disk Management . | |

| |RH 6.0/6.1: Select Gnome icon (located lower left corner) + System + Disk Management . | |

| |The gui tool can also be started using the shell command /usr/bin/usermount. | |

| |After mounting the CD-ROM one can view its contents from the directory /mnt/cdrom. | |

| |Use the command: cd /mnt/cdrom | |

| |OR | |

| |GNOME toollbar Start icon File manager and select the appropriate folders. | |

| |Group Interrogation and Verification: | |

| | | |

| |Check the group membership of a user: groups user-id | |

| |This will list all the groups to which user-id is a member. | |

| |Verification Commands: | |

| |pwck: verify integrity of password files | |

| |grpck: verify integrity of group files | |

| |User admin and other commands: | |

| | | |

| |useradd: Create a new user or update default new user information | |

| |usermod: Modify a user account | |

| |userdel: Delete a user account and related files | |

| |chage: change user password expiry information | |

| |pwconv: convert to and from shadow pass- words and groups. | |

| |pwunconv: convert to and from shadow pass- words and groups. | |

| |grpconv: creates gshadow from group and an optionally existing gshadow | |

| |grpunconv: creates group from group and gshadow and then removes gshadow | |

| |Links and Info: | |

| | | |

| |id man page - Use this command to display groups to which a user belongs. | |

| | |

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