When administering a home machine, the user must 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 program's owner 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, as in the following example:
-rwsr-xr-x 1 root root 47324 May 1 08:09 /bin/su
For the system administrators of an organization, however, choices must
be made as to how much administrative access 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 that logs in at the physical console (see
the chapter titled Pluggable Authentication Modules
(PAM) in the Red Hat Enterprise Linux Reference Guide for more about
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 not possible
without administrative priveleges. As a result, system administrators
must decide how much access the users on their network should receive.
If the users within an organization are a trusted, computer-savvy group,
then allowing them root access may not be an issue. Allowing
root access by users means that minor activities, like adding
devices or configuring network interfaces, can be handled
by the individual users, 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:
Machine Misconfiguration — Users
with root access can misconfigure their machines and require
assistance or worse, open up security holes without knowing it.
Running 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 affect Linux do exist. The only time they
are a threat, however, is when they are run by the root user.
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 (refer to Section 4.2.2 Boot Loader Passwords for more on this topic.)
Table 4-1
shows ways an administrator can further ensure that root logins are disallowed:
Method
Description
Effects
Does Not Affect
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
Disabling root SSH logins.
Edit the /etc/ssh/sshd_config file
and set the PermitRootLogin parameter to
no.
Prevents root access via the OpenSSH suite
of tools. The following programs are prevented from
accessing the root account:
� ssh
� scp
� sftp
This only prevents root access to the
OpenSSH suite of tools.
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.[a]
Prevents root access to network
services that are PAM aware.
The following services are prevented from accessing
the root account:
To prevent users from logging in directly as root, the system
administrator can set the root account's shell to
/sbin/nologin in the
/etc/passwd file. This prevents 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.
To further limit access to the root account, administrators 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 via the console or a raw network
interface. This is dangerous as a user can login into his machine as
root via Telnet, which sends his password in plain text over the network. By
default, Red Hat Enterprise Linux's /etc/securetty file only
allows the root user to login 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:
echo > /etc/securetty
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.
PAM, through the /lib/security/pam_listfile.so
module, allows great flexibility in denying specific accounts. This
allows the administrator to point the module at a list of users who
are not allowed to log in. Below is an example of how the module is
used for the vsftpd FTP server in the
/etc/pam.d/vsftpd PAM configuration file (the
\ character at the end of the first
line in the following example is not necessary
if the directive is on one line):
This tells PAM to consult the file
/etc/vsftpd.ftpusers and deny access to the
service for any user listed. 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, a
similar line can be added 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, refer to the chapter titled
Pluggable Authentication Modules (PAM) in the
Red Hat Enterprise Linux Reference Guide.
Upon typing the su command, the user is prompted
for the root password and, after authentication, is 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
become root, it is possible 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 within an
organization 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>
In the previous command, replace
<username> with the username you
want to add to the wheel group.
To use the User Manager for this purpose,
go to the Main Menu Button (on the Panel) =>
System Settings => Users &
Groups or type the command
system-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 File =>
Properties from the pull-down menu).
Then select the Groups tab and click on the
wheel group, as shown in Figure 4-2.
Figure 4-2. Groups Pane
Next, open the PAM configuration file for su
(/etc/pam.d/su) in a text editor and remove the
comment [#] from the following line:
The sudo command offers another approach to
giving users administrative access. When trusted users precede an
administrative command with sudo, they are
prompted for their own 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:
sudo <command>
In the above example, <command> would be
replaced 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 before walking away from their machines since
sudoers can use the command again without being asked for a
password within a five minute period. 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 the user's shell, not a root shell. This means
the root shell can be completely disabled, as shown in Section 4.4.2.1 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 the command 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.
Administrators wanting to edit the sudo
configuration file, /etc/sudoers, should use
the visudo command.
To give someone full administrative privileges, type
visudo and add a line similar to the following
in the user privilege specification section:
juan ALL=(ALL) ALL
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 it is issued
from the console.
The man page for sudoers has a detailed
listing of options for this file.
