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Unit 11: User Accounts

Adding and removing groups can also be done by modifying each individual user account. notes
Start Linuxconf by typing linuxconf at the shell prompt.

Open [Config] → [Users accounts] → [Normal] → [User accounts].
If you have more than 15 accounts on the system, Linuxconf will provide you with a filter
screen.
On the User accounts screen, select a user that you wish to update. You will be presented with
the User information screen.
Add or remove the desired groups from the, Supplementary groups field. Each group should be
separated by a space ‘’ ‘’ character.
Once you’ve made all the changes you’d like, select Accept at the bottom of the screen.
This will automatically update the group definitions. Repeat the process for each user.

setuid and setgid programs

setuid and setgid (short for set user ID upon execution and set group ID upon execution,
respectively) are Linux access rights flags that allow users to run an executable with the permissions
of the executable’s owner or group. They are often used to allow users on a computer system to
run programs with temporarily elevated privileges in order to perform a specific task. While the
assumed user id or group id privileges provided are not always elevated, at a minimum they are
specific.
setuid and setgid are needed for tasks that require higher privileges than those which a common
user has, such as changing his or her login password. Some of the tasks that require elevated
privileges may not immediately be obvious, though — such as the ping command, which must
send and listen for control packets on a network interface.

setgid()

setgid sets the effective group ID of the current process. If the caller is the superuser, the real and
saved group ID’s are also set. If the process has appropriate privileges, setgid() shall set the real
group ID, effective group ID, and the saved set-group-ID of the calling process to gid.
If the process does not have appropriate privileges, but gid is equal to the real group ID or the
saved set-group-ID, setgid() shall set the effective group ID to gid; the real group ID and saved
set-group-ID shall remain unchanged.
The setgid() function shall not affect the supplementary group list in any way.
Any supplementary group IDs of the calling process shall remain unchanged.

Upon successful completion, 0 is returned. Otherwise, -1 shall be returned and errno set to
indicate the error.

setuid()

If the process has appropriate privileges, setuid() shall set the real user ID, effective user ID, and
the saved set-user-ID of the calling process to uid.

If the process does not have appropriate privileges, but uid is equal to the real user ID or the
saved set-user-ID, setuid() shall set the effective user ID to uid; the real user ID and saved set-
user-ID shall remain unchanged.

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