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openSUSE 11.1 Reference Guide
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38.2 Some General Security Tips and Tricks

To handle security competently, it is important to keep up with new developments and stay informed about the latest security issues. One very good way to protect your systems against problems of all kinds is to get and install the updated packages recommended by security announcements as quickly as possible. SUSE security announcements are published on the list [email protected]. It is a first-hand source of information regarding updated packages and includes members of SUSE's security team among its active contributors. You can subscribe to this list on page

The mailing list [email protected] is a good place to discuss any security issues of interest. Subscribe to it on the same Web page.

[email protected] is one of the best-known security mailing lists worldwide. Reading this list, which receives between 15 and 20 postings per day, is recommended. More information can be found at

The following is a list of rules you may find useful in dealing with basic security concerns:

  • According to the rule of using the most restrictive set of permissions possible for every job, avoid doing your regular jobs as root. This reduces the risk of getting a cuckoo egg or a virus and protects you from your own mistakes.

  • If possible, always try to use encrypted connections to work on a remote machine. Using ssh (secure shell) to replace telnet, ftp, rsh, and rlogin should be standard practice.

  • Avoid using authentication methods based on IP addresses alone.

  • Try to keep the most important network-related packages up-to-date and subscribe to the corresponding mailing lists to receive announcements on new versions of 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 openSUSE. 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.

    SUSE's RPM packages are gpg-signed. The key used by SUSE for signing is:

    ID:9C800ACA 2000-10-19 SUSE Package Signing Key <[email protected]>
    Key fingerprint = 79C1 79B2 E1C8 20C1 890F 9994 A84E DAE8 9C80 0ACA

    The command rpm --checksig package.rpm shows whether the checksum and the signature of an uninstalled package are correct. Find the key on the first CD of the distribution and on most key servers worldwide.

  • Check your backups of user and system files regularly. Consider that if you do not test whether the backup works, it might actually be worthless.

  • Check your log files. Whenever possible, write a small script to search for suspicious entries. Admittedly, this is not exactly a trivial task. In the end, only you can know which entries are unusual and which are not.

  • Use tcp_wrapper to restrict access to the individual services running on your machine, so you have explicit control over which IP addresses can connect to a service. For further information regarding tcp_wrapper, consult the manual pages of tcpd and hosts_access (man 8 tcpd, man hosts_access).

  • Use SuSEfirewall to enhance the security provided by tcpd (tcp_wrapper).

  • Design your security measures to be redundant: a message seen twice is much better than no message at all.

  • If you use suspend to disk, consider to configure the suspend image encryption using the script. The program creates the key, copies it to /etc/suspend.key, and modifies /etc/suspend.conf to use encryption for suspend images.

openSUSE 11.1 Reference Guide
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  Published under the terms fo the GNU General Public License Design by Interspire