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 (refer to Section 2.1.2.2, “Boot Loader Passwords” for more information on this topic.)

The following are four different ways that an administrator can further ensure that root logins are disallowed:

Changing the root shell

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.

Table 2.2. Disabling the Root Shell

Effects Does Not Affect

Prevents access to the root shell and logs any such attempts. The following programs are prevented from accessing the root account:

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)

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, because a user can log in to their machine as root via Telnet, which transmits the password in plain text over the network.

By default, Red Hat Enterprise Linux's /etc/securetty file only allows the root user to log in at the console physically attached to the machine. To prevent the root user from logging in, remove the contents of this file by typing the following command at a shell prompt as root:

echo > /etc/securetty

To enable securetty support in the KDM, GDM, and XDM login managers, add the following line:

auth [user_unknown=ignore success=ok ignore=ignore default=bad]

pam_securetty.so to the files listed below:

/etc/pam .d/gdm

/etc/pam .d/gdm -autologin /etc/pam .d/gdm -fingerprint /etc/pam .d/gdm -password /etc/pam .d/gdm -sm artcard /etc/pam .d/kdm

/etc/pam .d/kdm -np /etc/pam .d/xdm

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.

Table 2.3. Disabling Root Logins

Effects Does Not Affect

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 setuid or other mechanisms. The following

programs are not prevented from accessing the root account:

su sudo ssh scp sftp

Disabling root SSH logins

To prevent root logins via the SSH protocol, edit the SSH daemon's configuration file, /etc/ssh/sshd_config, and change the line that reads:

#PermitRootLogin yes to read as follows:

PermitRootLogin no

Table 2.4. Disabling Root SSH Logins

Effects Does Not Affect

Prevents root access via the OpenSSH suite of tools. The following programs are

prevented from accessing the root account:

ssh scp sftp

Programs that are not part of the OpenSSH suite of tools.

Using PAM to limit root access to services

PAM, through the /lib/security/pam _listfile.so module, allows great flexibility in denying specific accounts. The administrator can use this module to reference a list of users who are not allowed to log in. To limit root access to a system service, edit the file for the target service in the /etc/pam .d/ directory and make sure the pam _listfile.so module is required for authentication.

The following 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 is not necessary if the directive is on a single line):

auth required /lib/security/pam_listfile.so item=user \ sense=deny file=/etc/vsftpd.ftpusers onerr=succeed

This instructs PAM to consult the /etc/vsftpd.ftpusers file and deny access to the service for any listed user. The administrator can 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 files, such as /etc/pam .d/pop and /etc/pam .d/im ap for mail clients, or /etc/pam .d/ssh for SSH clients.

For more information about PAM, refer to the chapter titled Using Pluggable Authentication Modules (PAM) in the Red Hat Enterprise Linux Managing Single Sign-On and Smart Cards guide.

Table 2.5. Disabling Root Using PAM

Effects Does Not Affect

Prevents root access to network services that are PAM aware. The following services are prevented from accessing the root account:

login gdm kdm xdm ssh scp sftp FTP clients Email clients

Any PAM aware services

Programs and services that are not PAM aware.