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Security Tips
Need a quick security fix? Whether its sudo or lilo, tcp or icmp, you'll find short and straight-to-the point guidance in our listing of Security Tips.


Password protection with LILO
Posted by Eric Lubow   
Password protect your linux install with LILO. Edit your /etc/lilo.conf.
At the end of each linux image that you want to secure, put the lines:
read-only
restricted
password = MySecurePassword
Ensure you rereun /sbin/lilo so the changes take effect.

 
Chattr
Posted by Eric Lubow   
There are files that get changed very infrequently. For instance, if your system won't have any users added anytime soon then it may be sensible to chattr immutably the /etc/password and /etc/shadow files. Only the superuser or a process possessing the CAP_LINUX_IMMUTABLE capability can set or clear this attribute.
chattr +i /etc/passwd /etc/shadow

 
Disallow ICMP
Posted by Eric Lubow   
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

 
World writable files
Posted by Eric Lubow   
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

 
Setuid and Setgid access
Posted by Eric Lubow   
Setuid and Setgid files have the potential to be very hazardous if they are accessible 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

 
Superuser check
Posted by Eric Lubow   
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

 
Passwordless account
Posted by Eric Lubow   
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

 
Default umask
Posted by Eric Lubow   
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.

 
UsePrivilegeSeparation and StrictModes
Posted by Eric Lubow   
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

 
Avoid Accidental Reboot
Posted by Eric Lubow   
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

 
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