Check List for Linux Security

Linux is an amazing operating system considering how it was originally created. It was a modest program written for one person as a hobby - Linus Torvald of Finland. It has grown into a full-fledge 32-bit operating system. It is solid, stable and provides support for an incredible number of applications. It has very powerful capabilities and runs very fast and rarely crashes. Unfortunately Linux machines are broken almost every day. This happens not because it is an insecure operating system. It contains all the necessary tools to make it very secure. But the truth is. It hasn't become significantly more secure with the increase in popularity. On the other hand, our understanding of the hackers methods and the wide variety of tools and techniques available contributed to help system administrators to secure their Linux computers. Our goal in this article is to list the most critical situations, and how to prevent an invasion with simple measures. 1- Weak passwords By far the first and most used method used by hackers to try penetrating a Linux system is cracking a password, preferently of the user root. Usually they will target a common user first, and then, using his/her access to the operating system, try to get a privileged access cracking the root password. Good password policy, and good passwords are absolutely critical to the security on any computer. Some common mistakes when selecting a password: A- use "password" as password. B- use the name of the computer. C- a well-know name from science, sports or politics. D- reference to movies. E- anything that is part of the user web site. F- references associated with the account. The latest version of Linux offer shadowed passwords. If a cracker can see an encrypted password, crack it would a simple task. So, instead of storing the password in the passwd file, they are now stored in the shadow file which is readable only for root. Before a hacker can crack a password he needs to figure out an account name. So, simple accounts names must be avoided as well. Another security measure is to apply a "no login" to the account in the passwd file. This must be done to all the accounts that don't need to log in to the system. Examples are: apache, mysql, ftp and other. Limit which terminals root may log in from. If the root account is allowed to log in only in certain terminals that are considered secure, it will be almost impossible for a hacker to penetrate the system. This can be done listing the allowed terminals on /etc/security. The login program will consider insecure any terminal that is not listed on this file, which is readable, only by root. 2- Open Network Ports Any Linux default installation will provide the Operating System with tons of software and services. Several of them are not necessary or even wanted by the administrator. Removing these software and services will close the path to several attacks and improve security. The /sbin/chkconfig program can be used to stop services from automatically starting at run levels 3, 4 and 5. Log in as root and type /sbin/chkconfig --list to view all the services set to start automatically. Select the ones you don't need and type /sbin/chkconfig 345 name_of_service off. You must do that to all services you don't want to keep running. Also, the xinetd server can be used to disable other services as well. 3- Old Software Versions Everyday vulnerabilities are found in programs, and most of them are fixed constantly. It is important, and sometimes critical, to keep up with the changes. There are mailing lists for every Linux distribution where one can have security related information's, and the latest vulnerabilities found. Some place to watch for security holes are: http://www.debian.org/MailingLists/ http://www.suse.com/us/private/support/security/index.html http://www.linuxtoday.com/ http://www.securityfocus.com/ Firewalls - Discuss the design, construction, operation, and maintenance of firewall systems. http://www.risks.org/ http://www.insecure.org/ Jair Santos is a Software Engineer for over 10 years. He is the manager of the Technical Support Department of Cliconnect Internet Telephony -