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Linux Security Tips

By Kapil Sharma

In this article I will explain  how to make your Linux box secure by taking  basic security measures. This article will enable anybody to tighten the security of a redhat Linux box.

BIOS Security
Always set a password on BIOS to disallow booting from floppy by changing the BIOS settings. This  will block undesired people from trying to boot your Linux system with a special boot disk and will  protect you from people trying to change BIOS feature like allowing boot from floppy drive or  booting the server without password prompt.

LILO Security
Add the three parameters in "/etc/lilo.conf" file i.e. time-out, restricted and  password. These options will ask for password if boot time options (such as "linux single") are passed to the boot loader.
Step 1
Edit the lilo.conf file (vi /etc/lilo.conf) and add or change the three options :
time-out=00   #change this line to 00
restricted   #add this line
password=<password>   #add this line and put your password

Step 2
The "/etc/lilo.conf" file should be readable by only root because it contains unencrypted passwords.
 [root@kapil /]# chmod 600 /etc/lilo.conf (will be no longer world readable).

Step 3
Update your configuration file "/etc/lilo.conf" for the change to take effect.
 [Root@kapil /]# /sbin/lilo -v (to update the lilo.conf file).

Step 4
One more security measure you can take to secure the "/etc/lilo.conf" file is to set it immutable, using the chattr command.
     *  To set the file immutable simply, use the command:
             [root@kapil /]# chattr +i /etc/lilo.conf
This will prevent any changes (accidental or otherwise) to the "lilo.conf" file.

For more information about lilo security, read my article on LILO.


Disable all special accounts
You should  delete all default users and group accounts that you don't use on your system like lp, sync, shutdown, halt, news, uucp, operator, games, gopher etc
To delete a user account :
[root@kapil /]# userdel LP

To delete a group:
[root@kapil /]# groupdel LP

Choose a Right password
You should follow the following guidelines before choosing the right password.

The password Length: The minimum acceptable password length by default when you install your Linux system is 5.  This is not enough and must be 8.  To do this you have to edit the login.defs file (vi /etc/login.defs) and change the line that read:

Enable shadow password support
You should enable the shadow password feature. You can use the "/usr/sbin/authconfig" utility to enable the shadow password feature on your system. If you want to convert the existing passwords and group on your system to shadow passwords and groups  then you can use the commands pwconv, grpconv respectively.
The root account
The "root" account is the most privileged account on a Unix system. When the administrator forgot to logout from the system root prompt before leaving the system then the system should automatically logout from the shell. To do that, you must set the special variable of  Linux named "TMOUT" to the time in seconds.
 Edit your profile file "vi /etc/profile" and add the following line somewhere after the line that read
 The value we enter for the variable "TMOUT=" is in second and represent 1 hours (60 * 60 =
3600 seconds). If you put this  line in your "/etc/profile" file, then the automatic logout after one hour of inactivity will apply for all users on the system. You can set this variable in user's individual ".bashrc " file to automatically logout them after a certain time.
 After this parameter has been set on your system, you must logout and login again (as root) for  the change to take effect.

Disable all console-equivalent access for regular users
You should disable all console-equivalent access to programs like shutdown, reboot, and halt for regular users on your server.
 To do this, run the following command:
[root@kapil /]# rm -f /etc/security/console.apps/<servicename>
 Where <servicename> is the name of the program to which you wish to disable console-equivalent access.

Disable & uninstall all unused services
You should  disable and uninstall all services that you do not use so that you have one less thing to worry about. Look at your "/etc/inetd.conf" file and  disable what you do not need by commenting them out (by adding a # at the beginning of the line), and then sending your inetd process a SIGHUP command to update it to the current "inetd.conf" file.  To do this:
Step 1
Change the permissions on "/etc/inetd.conf" file to 600, so that only root can read or write to it.
[Root@kapil /]# chmod 600 /etc/inetd.conf
Step 2
ENSURE that the owner of the file "/etc/inetd.conf" is root.

Step 3
Edit the inetd.conf file (vi /etc/inetd.conf) and disable the services like:
 ftp, telnet, shell, login, exec, talk, ntalk, imap, pop-2, pop-3, finger, auth, etc unless you plan to use it. If it's turned off it's much less of a risk.

Step 4
Send a HUP signal to your inetd process
[root@kapil /]# killall -HUP inetd

Step 5
Set "/etc/inetd.conf" file immutable,  using the chattr command so that nobody can modify that file
       *  To set the file immutable simply, execute the following command:
             [root@kapil /]# chattr +i /etc/inetd.conf
 This will prevent any changes (accidental or otherwise) to the "inetd.conf" file. The only person that can set or clear this attribute is the super-user root.  To modify the inetd.conf file you will need to unset the immutable flag:
*  To unset the immutable simply, execute the following command:
             [root@kapil /]# chattr -i /etc/inetd.conf

By using TCP_WRAPPERS  you can make  your server  secure against outside intrusion . The best policy is to deny all hosts by putting "ALL: ALL@ALL, PARANOID" in the "/etc/hosts.deny" file and then explicitly list  trusted hosts who are allowed to your machine in the "/etc/hosts.allow" file. TCP_WRAPPERS is controlled from two files and the search stops at the first match.

Step 1
Edit the hosts.deny file (vi /etc/hosts.deny) and add the following lines:
# Deny access to everyone.
 Which means all services, all locations is blocked, unless they are permitted access by entries in the allow file.

 Step 2
Edit the hosts.allow file (vi /etc/hosts.allow) and add for example, the following line:
As an example:
 For your client machine: is the IP address and the host name of one of your client allowed using ftp.
Step 3
The tcpdchk program is the tcpd wrapper configuration checker. It examines your tcp wrapper  configuration and reports all potential and real problems it can find.

 *  After your configuration is done, run the program tcpdchk.
             [Root@kapil /]# tcpdchk

Don't let system issue file to be displayed
 You should not  display your  system issue file when people log in remotely . To do this,  you can
change the telnet option in your "/etc/inetd.conf".
To do this change the line in "/etc/inetd.conf":

telnet  stream  tcp     nowait  root    /usr/sbin/tcpd  in.telnetd
to look like:
 telnet  stream  tcp     nowait  root    /usr/sbin/tcpd  in.telnetd -h
 Adding the "-h" flag on the end will cause the daemon to not display any system information and  just hit the user with a login: prompt.  I will recommend to use sshd instead.

Change the "/etc/host.conf" file
 The  "/etc/host.conf" file specifies how names are resolved.
 Edit the host.conf file (vi /etc/host.conf) and add the following lines:
 # Lookup names via DNS first then fall back to /etc/hosts.
order bind,hosts
# We have machines with multiple IP addresses.
multi on
# Check for IP address spoofing.
nospoof on

 The first option is to resolve the host name through DNS first and then hosts file.The multi option determines whether a host in the "/etc/hosts" file can have multiple IP addresses (multiple interface ethN).
 The nospoof option indicates to take care of not permitting spoofing on this machine.

Immunize the "/etc/services" file
You must immunize the "/etc/services" file to prevent unauthorized deletion  or addition of services.
           *  To immunize the "/etc/services" file, use the command:
             [root@kapil /]# chattr +i /etc/services

Disallow root login from different consoles
The "/etc/securetty" file allows you to specify which TTY devices the "root" user is allowed to login . Edit the "/etc/securetty" file to disable any tty that you do not need by commenting them out (# at the beginning of the line).
Blocking anyone to su to root
 The su (Substitute User) command allows you to become other existing users on the system.  If  you don't want anyone to su to root or restrict "su" command to certain users then add the following two lines to the top of your "su" configuration file in the "/etc/pam.d/" directory.
Step 1
Edit the su file (vi /etc/pam.d/su) and add the following two lines to the top of the file:
 auth sufficient /lib/security/ debug
auth required /lib/security/ group=wheel

Which means only members of the "wheel" group can su to root; it also includes  logging. You can add the users to the group wheel so that only those users will be allowed to su as root.

Shell logging
 The bash shell stores up to 500 old commands in the "~/.bash_history" file (where "~/" is your home directory) to make it easy for you to repeat long commands. Each user that has an account on the system will have this file "Bash_history" in their home directory. The bash shell should store less number of commands and delete it on logout of the user.
Step 1
The HISTFILESIZE and HISTSIZE lines in the "/etc/profile" file determine the size of old commands the "Bash_history" file for all users on your system can hold.  I would  highly recommend setting the HISTFILESIZE and HISTSIZE in "/etc/profile" file to a low value such as 30.
 Edit the profile file (vi /etc/profile) and change the lines to:
 Which mean, the "Bash_history" file in each users home directory can store 20 old commands
and no more.
 Step 2
The administrator should also add into the "/etc/skel/Bash_logout" file the
"rm -f  $HOME/Bash_history" line, so that each time a user logs out, its "Bash_history" file will be deleted.
 Edit the Bash_logout file (vi /etc/skel/Bash_logout) and add the following line:
 rm -f $HOME/Bash_history

Disable the Control-Alt-Delete keyboard shutdown command
To do this comment out the line (with a "#") listed below in your "/etc/inittab" file .
 To do this, edit the inittab file (vi /etc/inittab) and change the line:
 ca::ctrlaltdel:/sbin/shutdown -t3 -r now
To read:
#ca::ctrlaltdel:/sbin/shutdown -t3 -r now
 Now, for the change to take effect type in the following at a prompt:
 [root@kapil /]# /sbin/init q

Fix the permissions under "/etc/rc.d/init.d" directory for script files
 Fix the permissions of the script files that are responsible for starting and stopping all your normal  processes that need to run at boot time.  To do this:
 [root@kapil/]# chmod -R 700 /etc/rc.d/init.d/*
 Which means only root is allowed to Read, Write, and Execute scripts files on this directory.

Hide your system information
 By default, when you login to a Linux box, it tells you the Linux distribution name, version, kernel  version, and the name of the server. This is sufficient information for a crackers to get information about your server. You should just prompt users with a "Login:" prompt.
 Step 1
To do this, Edit the "/etc/rc.d/rc.local" file and Place "#" in front of the following lines as shown:

# This will overwrite /etc/issue at every boot.  So, make any changes you
# want to make to /etc/issue here or you will lose them when you reboot.
#echo "" > /etc/issue
#echo "$R" >> /etc/issue
#echo "Kernel $(uname -r) on $a $(uname -m)" >> /etc/issue
#cp -f /etc/issue /etc/
#echo >> /etc/issue

 Step 2
Then, remove the following files: "" and "issue" under "/etc" directory:
 [root@kapil /]# rm -f /etc/issue
[root@kapil /]# rm -f /etc/

Disable unused SUID/SGID programs
 A regular user will be able to run a program as root if it is set to SUID root. A system administrator should minimize the use of these SUID/GUID programs and disable the programs which are not needed.
Step 1
*  To find all files with the `s' bits from root-owned programs, use the command:
          [root@kapil]# find / -type f \( -perm -04000 -o -perm -02000 \) \-exec ls ­lg {} \;

     *  To disable the suid bits on selected programs above, type the following commands:
          [root@kapil /]# chmod a-s [program]

After following the above security guidelines, a system administrator can maintain a basic level of system security. Some of the above tasks are a continuous process. The system administrator has to continuously follow the above guidelines to keep system secure.

Written by: Kapil Sharma
[Kapil Sharma is a Linux and Internet security consultant. He has been working on various Linux/Unix systems and Internet Security for more than 2 years. He is maintaing a web site for providing free as well as commercial support for web, Linux and Unix solutions.]

Copyright © 2000, Kapil Sharma
Published in Issue 58 of Linux Gazette, October 2000

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