Novell LINUX ENTERPRISE DESKTOP 11 - SECURITY GUIDE 17-03-2009 Manual page 24

such programs (bind, postfix, ssh, etc.). The same should apply to software relevant
to local security.
• Change the /etc/permissions file to optimize the permissions of files crucial
to your system's security. If you remove the setuid bit from a program, it might
well be that it cannot do its job anymore in the intended way. On the other hand,
consider that, in most cases, the program will also have ceased to be a potential
security risk. You might take a similar approach with world-writable directories
and files.
• Disable any network services you do not absolutely require for your server to work
properly. This makes your system safer. Open ports, with the socket state LISTEN,
can be found with the program netstat. As for the options, it is recommended
to use netstat -ap or netstat -anp. The -p option allows you to see which
process is occupying a port under which name.
Compare the netstat results with those of a thorough port scan done from outside
your host. An excellent program for this job is nmap, which not only checks out
the ports of your machine, but also draws some conclusions as to which services
are waiting behind them. However, port scanning may be interpreted as an aggressive
act, so do not do this on a host without the explicit approval of the administrator.
Finally, remember that it is important not only to scan TCP ports, but also UDP
ports (options -sS and -sU).
• To monitor the integrity of the files of your system in a reliable way, use the program
AIDE (Advanced Intrusion Detection Environment), available on SUSE Linux
Enterprise Desktop. Encrypt the database created by AIDE to prevent someone
from tampering with it. Furthermore, keep a backup of this database available
outside your machine, stored on an external data medium not connected to it by a
network link.
• Take proper care when installing any third-party software. There have been cases
where a hacker had built a trojan horse into the tar archive of a security software
package, which was fortunately discovered very quickly. If you install a binary
package, have no doubts about the site from which you downloaded it.
12 Security Guide