10 Tips To Start Securing Your Linux System

A while back I had been asked to write a few quick tips that as an administrator, one would find helpful. They published in one form or another and are now available here. There are MANY more, but these are just a few. Enjoy.

  1. Users who may be acting up or aren’t listening can still be controlled. Using a program called ‘skill’ (signal kill) which is part of the ‘procps’ package.
       Halt/Stop User eric: skill -STOP -u eric
       Continue User eric: skill -CONT -u eric
       Kill and Logout User eric: skill -KILL -u eric
       Kill and Logout All Users: skill -KILL -v /dev/pts/*
  2. Make use of security tools out there to test your server’s weaknesses. Nmap is an excellent port scanning tool to test to see what ports you have open. On a remote machine, type the command:
       # nmap -sTU <server_ip>

       Starting nmap 3.70 ( http://www.insecure.org/nmap/ ) at 2006-08-10 13:51 EST
       Interesting ports on eric (
       (The 3131 ports scanned but not shown below are in state: closed)
       PORT    STATE         SERVICE
       22/tcp  open          ssh
       113/tcp open          auth
       Nmap run completed -- 1 IP address (1 host up) scanned in 221.669 seconds
  3. On a production server that is in a common area (although this should not be the case, some situations are inevidable). To avoid an accidental CTRL-ALT-DEL reboot of the machine, do the following to remove the necessary
    lines from the /etc/inittab file:

       # sed -i 's/ca::ctrlaltdel:/#ca::ctrlaltdel:/g' /etc/inittab
  4. Two SSH configuration options that can be set to improve security should be checked on your production server. UsePrivilegeSeparation is an option, when enabled will allow the OpenSSH server to run a small (necessary) amount of code as root and the of the code in a chroot jail environment. StrictModes checks to ensure that your ssh files and directories have the proper permissions and ownerships before allowing an SSH session to open up. The
    directives should be set in the /etc/ssh/sshd_config as follows:

       UsePrivilegeSeparation yes
       StrcitModes yes
  5. The default umask (usermask) on most systems should be 022 to ensure that files are created with the permissions 0644 (-rw-r–r–). To change the default umask setting for a system, edit /etc/profile to ensure that you umask is appropriate for your setup.
  6. Some users like to have a passwordless account. To check this you need to look at the /etc/shadow account with the following command line:
    awk -F: '$2 == "" { print $1, "has no password!" }' /etc/shadow
  7. Just in case someone else who has access to the superuser account decided to alter the password file and potentially make themselves a superuser. This is a method to check:
       awk -F: '$3 == 0 { print $1, "is a superuser!" }' /etc/passwd
  8. Setuid and Setgid files have the potential to be very hazardous if they are accessilbe by the wrong users on the system. Therefore it is handy to be able to check with files fall into this category.
       find /dir -xdev -type f -perm +ug=s -print
  9. World writable files can be left around by users wanting to make things easier for themselves. It is necessary to be careful about who can write to which files. To find all world writable files:
       find /dir -xdev -perm +o=w ! \( -type d -perm +o=t \) ! -type l -print
  10. Some attackers, prior to attacking a host, (or users nmaping a host) will check to see if the host is alive. They do this by ‘ping’ing the host. In order to check if the host is up, they will use an ICMP echo request packet.
    To disallow these types of packets, use iptables:

       iptables -A INPUT -p icmp --icmp-type echo-request -j DROP