How To Ubuntu » Security

What is the sudo command?

Eli — Sat, 05 Dec 2009 16:27:54 +0000

So, you’re reading an Ubuntu how to for something, and you come across the sudo command. What in the world is it, and why do you need it? The sudo command is used to allow a regular user to execute a command that requires root privileges.

In addition to the user name you provided at installation, Ubuntu creates a user called root, except it does not give it a password. This user has the power to do anything to your system. It can delete all of your system files, modify them, anything it wants to. For you to login as this root user, you must first assign it a password with which to login. This is pretty much never necessary for most users to do, so I won’t be explaining how to do that here.

There are certain tasks you may need to perform that require the root user’s level of authority. These include installing software, making changes to system configuration files, updating the system, etc. This is where the sudo command comes into play. That command tells Ubuntu that you want to execute whatever follows it, with the authority of the root user, which as you’ll remember has complete authority over your system. This is a safety feature. What this accomplishes is only giving you the root authority for the execution of that command, and not giving you all the powers of root, thereby turning you loose on your system with way too much power. It’s much better this way.

For example…

Installing software from the command line…
sudo apt-get install conky

You’ll then have to enter your password, and Ubuntu will execute the command. Without the sudo command, Ubuntu will deny you permission to run that command.
This same basic thing is happening when you are required to enter your password on the desktop. As in when you update your system by clicking on the update notification icon in the taskbar, and selecting install updates from the window that opens.

This is just one more thing that makes Ubuntu so nice and secure.

Shortcut to root command line access while logged in as un-privileged user

Eli — Tue, 01 Sep 2009 03:27:03 +0000

Let’s say you’re logged into your Ubuntu machine as an unprivileged user, meaning that you don’t have permission to use the sudo command on the command line. As such, you cannot update the system, install a program, or perform other tasks in the terminal that require the sudo command without logging out and then back in as the administrative user, or using the User Switcher. This annoys me. So, how do I execute commands in the terminal that require the sudo command while still logged in as the unprivileged user??

First off, you’ll need ssh. To make sure you have this installed, login to your administrative account (a step that will soon become unnecessary), open a terminal, and enter…

sudo apt-get install openssh-server openssh-client

After this install is finished you can logout, and back in as the unprivileged user.Now you can open a terminal and enter…

ssh [administrative username]@127.0.0.1

You’ll be asked for your password, this is the administrative account’s password, enter it and press enter. Now, you’ll have a command line prompt for the administrative user, as if you had logged in as that user and opened a terminal. You can now update the system, or make whatever change you need to make that requires you to enter your sudo password.

A nice time saving trick.

How to lock your screen from the command line

Eli — Thu, 11 Dec 2008 23:35:29 +0000

This could be very useful to you if you’re working in the terminal and need to step away for a moment. You can achieve a locked screen, just like when you do it the graphical way, with this command entered at the command line, or cli…

gnome-screensaver-command --lock

This will instantly launch your screensaver, and lock your computer’s screen. Your password will be required to gain access to your system once this is activated.

That command is pretty long, and typing it out would probably get on my nerves. You can use an alias to effectively shorten it. Here’s how we’ll do that, also at the command line…

Let’s say we want to be able to type in lock, to lock the screen. If I just type in lock, I get nothing. So I need to create an alias for this command. An alias is just like what it sounds like. It’s assigning another name to a certain command. At the command line enter…

gedit .bashrc

There’s a certain section of this file that talks about aliases. You can add this line of text to that section of the file…

alias lock='gnome-screensaver-command --lock'

This tells Ubuntu that when you type in lock, it executes gnome-screensaver-command –lock. Click the Save button and close the text editor. The next time you login, the change will have taken effect. Try it out and see how you like it.

Here’s what it should look like…

Blocking websites using the hosts file

Eli — Thu, 27 Nov 2008 03:11:32 +0000

Let’s say you want to block certain websites. Maybe to keep your kids off of myspace, for security reasons, or to block advertising from being loaded. You can do this by means of your hosts file. By modifying that little file, you can keep certain websites from being accessed from your computer. Here’s how to do it…

First of all, you must be logged in as a user with administrative privileges.

Then you need to open a text editor with administrative rights. Here’s a simple way to do it.

Hit ALT+F2 to open a ‘Run Application’ dialog box.

Type in… gksudo gedit /etc/hosts

Press Enter, & type in your password, and the file will open.

Add one line to the end of the file for each site you want to block, like this…

0.0.0.0 www.example.com

Note that there is a space between the 0.0.0.0 & the web site address. This will tell you computer to look for www.example.com at the ip address of 0.0.0.0. It won’t be there of course, and they won’t be able to get to the website. This can be done with any site, and with any number of sites. Just keep adding lines to that file. Once you’re finished modifying the file, click the Save button. The changes will take effect upon your next boot up, and the changes will affect every user on the computer. You can even find pre-made hosts file online that will block a lot of advertising that pops up while online.

Here’s what the file should look like if you want to block MySpace & Worldcat…

If you type a line that begins with a “#”, the computer will ignore that line, like my line “# Blocked web sites by How To Ubuntu”

How to lock your screen

Eli — Fri, 21 Nov 2008 04:30:41 +0000

Locking your screen when you’re away from your computer is very important when there are other people around. You don’t want anyone messing around on your computer. In Ubuntu you can lock your screen very easily. Once your screen is locked, it will immediately start the screensaver. When someone tries to use the mouse or keyboard they are prompted for a password. If they don’t have the password, they can’t access your machine. This password is the same one you use to login with. Here’s how to lock your screen…

The quickest and easiest way to do this is to simply hit CTRL+ALT+L. This will automatically lock your screen, and start the screensaver. If, however, you don’t want to use that keyboard combination, there are other ways to do this. In this case, you’ll need to bring up this box, and choose the Lock Screen option.

You can bring this box up a few different ways. You can also hit CTRL+ALT+DEL, or you can go to the System menu and select Quit, or you can click on the Exit icon on the taskbar. If someone tries to wake your computer up from the screensaver, this is what they’ll see…

If they don’t have your password, they won’t get into your system. They can login as a different user, and this won’t affect what you’re doing. When that other user logs out, the machine will just go back to yourscreensaver. Another nice little feature is the ability for someone to leave you a message when your machine is locked. All they have to do is click the Leave Message box after they wake it from the screensaver. They type their message, hit Save, and leave. When you come back & login, that message is displayed in the lower right hand corner of the screen until you close it. Very cool!

Notice the non-darkened area

Zoomed in view

[digg=http://digg.com/linux_unix/How_to_lock_your_screen_in_Ubuntu]

Install rkhunter, root kit detection software

Eli — Fri, 16 May 2008 22:31:31 +0000

A rootkit is a program (or combination of several programs) designed to take control of a computer system, without authorization. It assumes administrative, or root level control of the system. This control isn’t taken for your benefit, it’s malicious. Antivurs software might not get these little buggers, so root kit detection software comes in. This one, rkhunter, will search for them, find them, and then do whatever you want with them. Here’s how to get it. Open a terminal and enter…

code

sudo apt-get install rkhunter

That’ll download & install the software. It gets run from the command line, and you can see how to use it by executing the help feature…

code

rkhunter

You will also want to update it’s definitions file…

code

sudo rkhunter – -update

Now you have an up to date, root kit detection application.