Page 305 - DCAP403_Operating System

Operating System

Notes Access to the root account should be limited for a couple of reasons. First, the more people with
root access, the more people who have complete control over the system. This makes access
control diffi cult.
Also, the more people that have root access, the more fingers get pointed. There are people who

are going to deny having done something wrong. Often this results in a corrupt system, as there
are everyone has the power to do everything, someone did something that messed up the system
somehow and no one will admit. Sound familiar?

The fewer people that have root, the fewer fingers need to be pointed and the fewer people can
pass the buck. Not that what they did was malicious, mistakes do happen. If there are fewer
people with root access and something goes wrong, tracking down the cause is much easier.
Rather than several users all having the root password, some people think that it is safer to create
several users all with the UID of root. Their belief is that since there are several lognames, it’s
easier to keep track of things. Well, the problem in that thinking is that the system keeps track of
track of users by the UID. There is no way to keep these users separate, once they log in.

Another security precaution is to defi ne secure terminals. These are the only terminals that the
root user can login from. In my opinion, it is best to only consider directly connected terminals as
“secure”. That is, the root user can log into the system console, but not across the network. To get

access as root across the network, a user must first login under their own account and then use
su. This also provides a record of who used the root account and when.
If the system is connected to the Internet, such as for a HTTP or FTP server, then security is a
primary consideration.
One way of avoiding compromising your system is to have your WWW server connected to the
Internet, but not to your internal network. Should someone be able to break into the WWW server,
the worst that can happen is that the WWW server is down for a day or so as you reload from
backups. If the intruder had access to the internal network, your livelihood could be threatened.
One very common attack is the dictionary attack. Here the hacker uses common words, encrypts

them using the same as the password taken from the password file and then the two are compared.

Remember that the /etc/passwd file is readable by everyone and the seed is contained within the
encrypted password is always the first two characters.

Although this seems to be a major security hole, it is very effective if you use passwords that are
not easy to guess. The reason is that the encryption goes only one way.

13.11 Summary

Linux is a modular Unix-like operating system. Linux operates in two modes - the Kernel mode
(kernel space) and the User mode (user space). The Linux kernel is a monolithic kernel. Any
application that runs on a Linux system is assigned a process ID or PID. This is a numerical
representation of the instance of the application on the system. There are generally two types
of processes that run on Linux. Interactive processes are those processes that are invoked by a
user and can interact with the user. Interactive processes can be classified into foreground and

background processes. The foreground process is the process that you are currently interacting
with, and is using the terminal as its stdin (standard input) and stdout (standard output). A
background process is not interacting with the user and can be in one of two states - paused or
running. All processes are derived from the init process and can trace their roots back to init.
All processes run partially in user mode and partially in system mode. Virtual memory is used
in Linux. It uses the Buddy algorithm to effectively allocate and deallocate blocks of pages. Its
developers and users use the web to exchange information ideas, code, and Linux itself is often
used to support the networking needs of organizations. This unit describes how Linux supports
the network protocols known collectively as TCP/IP.

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