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| Red Hat Linux 8.0: The Official Red Hat Linux Security Guide |
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| Prev | Chapter 4. Workstation Security | Next |
Administrative Controls When administering a home machine, the user will have to perform some
tasks as the root user or by acquiring effective root privileges via a
setuid program, such as sudo
or su. A setuid program is one that operates with
the user ID (UID) of the owner of program rather
than the user operating the program. Such programs are denoted by a
lower case s in the owner section of a
long format listing.
For a system administrator, however, choices must be made as to how much
administrative access each users within the organization should have to
their machine. Through a PAM module called
pam_console.so, some activities normally reserved
only for the root user, such as rebooting and mounting removable media
are allowed for the first user to log in at the physical console (see
the chapter titled Pluggable Authentication Modules
(PAM) in the Official Red Hat Linux Reference Guide for more on the
pam_console.so module). However, other important
system administration tasks such as altering network settings, configuring
a new mouse, or mounting network devices are impossible without
administrative access. As a result system administrators must decide how
much to trust the users on their network.
Allowing Root Access If the users within an organization are a trusted, computer-savvy group,
then allowing them root access may not be a bad thing. Allowing
root access users means that minor issues like adding
devices or configuring network interfaces can be handled
by the individual user, leaving system administrators free to
deal with network security and other important issues.
On the other hand, giving root access to individual users can
lead to the following issues (to name a few):
Machine Misconfiguration — Users
with root access can misconfigure their machines and require
assistance or worse, open up security holes without knowing it.
Run Insecure Services — Users with
root access may run insecure servers on their machine, such as FTP
or telnet, potentially putting usernames and passwords at risk as
they pass over the network in the clear.
Running Email Attachments As Root —
Although rare, email viruses that effect Linux do exist. The only time they
are a threat, however, is when they are run by the root user.
Disallowing Root Access If an administrator is uncomfortable allowing users to log in as root
for these or other reasons, the root password should be kept secret
and access to runlevel one or single user mode should be disallowed
through boot loader password protection (see the Section called Boot Loader Passwords for more on this topic).
Table 4-1
shows ways an administrator can further ensure that root logins are disallowed:
Table 4-1. Methods of Disabling the Root Account | Method | Description | Effects | Does Not Effect |
|---|
| Disabling root SSH logins. | Edit the /etc/ssh/sshd_config file
and set the PermitRootLogin parameter to
no.
| | Prevents root access via the OpenSSH suit
of tools. The following programs are prevented from
accessing the root account: | | · ssh | | · scp | | · sftp |
| | Since this only effects the OpenSSH suite
of tools, no other programs are effected by this setting. |
| | Changing the root shell. | Edit the /etc/passwd file and change
the shell from /bin/bash to
/sbin/nologin. | | Prevents access to the root
shell and logs the attempt. | | The following programs are prevented from accessing
the root account: | | · login | | · gdm | | · kdm | | · xdm | | · su | | · ssh | | · scp | | · sftp |
| | Programs that do not require a
shell, such as FTP clients, mail clients, and many setuid
programs. | | The following programs are not
prevented from accessing the root account: | | · sudo | | · FTP clients | | · Email clients |
| | Disabling root access via any console device (tty). | An empty /etc/securetty file
prevents root login on any devices attached to the computer. | | Prevents access to the root account
via the console or the network. The following
programs are prevented from accessing the root
account: | | · login | | · gdm | | · kdm | | · xdm | | · Other network services that open a tty |
| | Programs that do not log in as root, but
perform administrative tasks through through setuid or other
mechanisms. | | The following programs are not
prevented from accessing the root account: | | · su | | · sudo | | · ssh | | · scp | | · sftp |
| | Use PAM to limit root access to services. | Edit the file for the target service in the
/etc/pam.d/ directory. Make sure the
pam_listfile.so is required
for authentication. See the Section called Disabling Root Using PAM for more details.
| | Prevents root access to network
services that are PAM aware. | | The following services are prevented from accessing
the root account: | | · FTP clients | | · Email clients | | · login | | · gdm | | · kdm | | · xdm | | · ssh | | · scp | | · sftp | | · Any PAM aware services |
| | Programs and services that are not
PAM aware. |
|
Disabling the Root Shell If the administrator does not wish for users to log in directly as
root, he can set the root account's shell to
/sbin/nologin in the
/etc/passwd file. This will prevent access to
the root account through commands that require a shell, such as
the su and the ssh commands.
 | Important |
|---|
| | Programs that do not require access to the shell, such as email
clients or the sudo command, can still access
the root account.
|
Disabling Root Logins To further enforce this, he can disable root logins at the console
by editing the /etc/securetty file. This file
lists all devices the root user is allowed to log into. If the file
does not exist at all, the root user can log in through any
communication device on the system, whether it by via the console or
a raw network interface. This is dangerous because if configured in
this way, a user could telnet into his machine as root, sending his
password in plain text over the network. By default, Red Hat Linux's
/etc/securetty file only allows the root user
to log at the console physically attached to the machine. To prevent
root from logging in, remove the contents of this file by typing the
following command:
 | Warning |
|---|
| | A blank /etc/securetty file does
not prevent the root user from logging in
remotely using the OpenSSH suite of tools because the console is not
opened until after authentication.
|
Disabling Root SSH Logins To prevent root logins via the SSH protocol, you will need to edit
the SSH daemon's configuration file:
/etc/ssh/sshd_config. Change the line that
says:
To read as follows: Disabling Root Using PAM PAM allows great flexibility in denying specific accounts via the
/lib/security/pam_listfile.so module. This
allows the administrator to point the module at a list of users that
are not allowed to log in. Below is an example of how the module is
used for the FTP service in the /etc/pam.d/ftp
PAM configuration file (the \
character at the end of the first of the first line is
not necessary if the directive is all on one
line):
auth required /lib/security/pam_listfile.so item=user \
sense=deny file=/etc/ftpusers onerr=succeed |
This tells PAM to consult the file
/etc/ftpusers and deny any user listed access
to the service. The administrator is free to change the name of this
file and can keep separate lists for each service or use one central
list to deny access to multiple services.
If the administrator wants to deny access to multiple services, he
can add a similar line to the PAM configuration services, such as
/etc/pam.d/pop and
/etc/pam.d/imap for mail clients or
/etc/pam.d/ssh for SSH clients.
For more information about PAM, see the chapter titled
Pluggable Authentication Modules (PAM) in the
Official Red Hat Linux Reference Guide.
Limiting Root Access Often, rather than completely deny access to the root user, the
administrator may wish to allow access only via setuid programs,
such as su or sudo.
The su Command When a user types the command su she is
prompted for the root password and, after authentication, given a
root shell prompt.
Once logged in via the su command, the user
is the root user and has absolute
administrative access to the system. In addition, once a user has
attained root, it is possible in some cases for them to use the
su command to change to any other user on the
system without being prompted for a password.
Because this program is so powerful, administrators may wish to
limit who has access to the command.
One of the simplest ways to do this is to add users to the special
administrative group called wheel.
To do this, type the following command as root:
usermod -G wheel username |
To use the User Manager for this purpose,
go to the (on the Panel) =>
=> or type the command
redhat-config-users at a shell prompt. Select the
Users tab, select the user from the user list,
and click Properties from the button menu (or
choose =>
from the pull-down menu).
Then select the Groups tab and click on the
wheel group, as shown in Figure 4-2.
Next open the PAM configuration file for su,
/etc/pam.d/su, in a text editor and remove the
comment [#] from the following line:
auth required /lib/security/pam_wheel.so use_uid |
Doing this will permit only members of the administrative group
wheel to use the program.
 | Note |
|---|
| | The root user is part of the
wheel group by default.
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The sudo Command The sudo command offers another approach for
giving trusted users administrative access. When a trusted user
precedes an administrative command with sudo, he
is prompted for his password. Then, once
authenticated and assuming that the command is permitted, the
administrative command is executed as if by the root user.
The basic format of the sudo command is as
follows:
In the above example, command would be
replaces by a command normally reserved for the root user, such
as mount.
| Important |
|---|
| | Users of the sudo command should take extra
care to log out when they walk away from their machine since
sudoers can use the command again without being asked for a
password until a five minute period has passed. This setting can
be altered via the configuration file,
/etc/sudoers.
|
The sudo command allows for a high degree of
flexibility. For instance, only users listed in the
/etc/sudoers configuration file are allowed to
use the sudo command and the command is executed
in their shell, not root's. This means the root
shell can be completely disabled, as shown in the Section called Disabling the Root Shell.
The sudo command also provides a comprehensive
audit trail. Each successful authentication is logged to the file
/var/log/messages and command that was issued
along with the issuer's user name is logged to the file
/var/log/secure.
Another advantage of the sudo command is that an
administrator can allow different users access to
specific commands based on their needs.
All commands executed via sudo are recorded
in the /var/log/secure file, as well as all
attempts to use the sudo command. Administrators wanting to edit the sudo
configuration file, /etc/sudoers, should use
the visudo.
To give someone full administrative privileges, type
visudo and add a line similar to the following
in the user privilege specification section:
This example states that the user,
juan, can use
sudo from any host and execute any command.
The example below illustrates the granularity possible when
configuring sudo:
%users localhost=/sbin/shutdown -h now |
This example states that any user can issue the command
/sbin/shutdown -h now as long as they issue it
from the console.
The man page for sudoers has a detailed
listing of options for this file.
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Disclaimer: For authoritative source or latest update to this
documentation, please refer to http://www.redhat.com/docs/manuals/linux/ |
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Quotes: Don't walk in front of me, I may not follow. Don't walk behind me, I may not lead. Walk beside me and just be my friend.
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