Useful Tips and Tricks - OS

Take Control of your Linux | sudoers file:

Wed, 08 Jul 2015 03:07:38 GMT

The sudoers file located at: /etc/sudoers, contains the rules that users must follow when using the sudo command.

If you have ever used used Ubuntu, you know that the root account is disabled. This is because the root password is not set in Ubuntu, you can assign one and use it as with every other Linux distribution. That anyway is another story. On normal Ubuntu Linux computers you need to use sudo to act as root.

I like using sudo, I'm not using Ubuntu anymore. The first thing I do when I install a new Linux is to usevisudo to edit the sudoers file. And I always give my account root rights, then I can run commands as root without switching users.

The best way to understand the sudo command, and the rules in sudoers file, the funny way is by this comics.

As you can see from this funny picture, using sudo command, makes the system obey any given order.

The two best advantages about using sudo command are:

Restricted privileges
Logs of the actions taken by users

I'm sure you are now fully aware of the advantages of using sudo command in a daily basis, how to use it?

In order to use sudo you first need to configure the sudoers file. The sudoers file is located at/etc/sudoers. And you should not edit it directly, you need to use the visudo command.

Once you enter visudo command, you will see something like this:

# /etc/sudoers
#
# This file MUST be edited with the 'visudo' command as root.
#
# See the man page for details on how to write a sudoers file.
# Defaults env_reset
# Host alias specification
# User alias specification
# Cmnd alias specification
# User privilege specification
root ALL=(ALL) ALL

Almost all lines are commented out, the one that matters in this sudoers file example is:

root ALL=(ALL) ALL

This line means: The root user can execute from ALL terminals, acting as ALL (any) users, and run ALL (any) command.

The first part is the user, the second is the terminal from where the user can use sudo command, the third part is which users he may act as, and the last one, is which commands he may run when using sudo.

sudoers examplesoperator ALL= /sbin/poweroff

The above command, makes the user operator can from any terminal, run the command power off.

You can also create aliases for: users -> User_Alias, run commands as other users -> Runas_Alias, host -> Host_Alias and command -> Cmnd_Alias

User_Alias OPERATORS = joe, mike, jude
Runas_Alias OP = root, operator
Host_Alias OFNET = 10.1.2.0/255.255.255.0
Cmnd_Alias PRINTING = /usr/sbin/lpc, /usr/bin/lprm

As you can see the alias OPERATORS includes the users joe, mike and jude, the alias OP includes the users root and operator, alias OFNET includes the network 10.1.2.0 (all the C class), and the command alias PRINTING includes the commands lpc and lprm.

So, a typical sudoers file may look like this:

If you want not to be asked for a password use this form:

go2linux ALL=(ALL) NOPASSWD: ALL

You may want to read sudoers man page

Considering that you are still reading here a bonus:

visudo command uses vi as the editor here some tips to use it:

Switch to root, (su root), then run visudo, (as above).
Find where it says "root ALL=(ALL) ALL".
Type "o" to insert a new line below it.
Now type what you want to insert, eg "username ALL=(ALL) ALL".
Hit esc to exit insert-mode.
Type ":x" to save and exit.

Can I change the default visudo editor?

Yes, changing the default visudo editor is easy.

And just because of your dedication, and still reading until here, I'll show you how to set nano or vim to use with visudo command as default editor.

Using vim with visudo

export VISUAL=vim; visudo

Using nano with visudo

export VISUAL=nano; visudo

source : http://bencane.com/

Understanding a little more about /etc/profile and /etc/bashrc

Tue, 07 Jul 2015 02:42:29 GMT

Recently I was working on an issue where an application was not retaining the umask setting set in the root users profile or /etc/profile. After looking into the issue a bit it seemed that the application in question only applied the umask setting that was set in /etc/bashrc and would not even accept the values being the applications own start scripts.

After doing a bit of researched I learned a little bit more about what exactly these files do, the differences between them and when they are executed. I figured this would be a good thing to share as it is not a topic that comes up very often.

What is /etc/profile used for?If you have been using Linux for a while you are probably familiar with the .profile or.bash_profile files in your home directory. These files are used to set environmental items for a users shell. Items such as umask, and variables such as PS1 or PATH.

The /etc/profile file is not very different however it is used to set system wide environmental variables on users shells. The variables are sometimes the same ones that are in the .bash_profile, however this file is used to set an initial PATH or PS1 for all shell users of the system.

/etc/profile.dIn addition to the setting environmental items the /etc/profile will execute the scripts within/etc/profile.d/*.sh. If you plan on setting your own system wide environmental variables it is recommended to place your configuration in a shell script within /etc/profile.d.

What is /etc/bashrc used for?Like .bash_profile you will also commonly see a .bashrc file in your home directory. This file is meant for setting command aliases and functions used by bash shell users.

Just like the /etc/profile is the system wide version of .bash_profile. The /etc/bashrc for Red Hat and /etc/bash.bashrc in Ubuntu is the system wide version of .bashrc.

Interestingly enough in the Red Hat implementation the /etc/bashrc also executes the shell scripts within /etc/profile.d but only if the users shell is a Interactive Shell (aka Login Shell)

When are these files used?The difference between when these two files are executed are dependent on the type of login being performed. In Linux you can have two types of login shells, Interactive Shells and Non-Interactive Shells. An Interactive shell is used where a user can interact with the shell, i.e. your typical bash prompt. Whereas a non-Interactive shell is used when a user cannot interact with the shell, i.e. a bash scripts execution.

The difference is simple, the /etc/profile is executed only for interactive shells and the /etc/bashrcis executed for both interactive and non-interactive shells. In fact in Ubuntu the /etc/profile calls the /etc/bashrc directly.

Interactive Shell vs Non-Interactive ShellTo show an example of an interactive shell vs a non-interactive shell I will add a variable into both/etc/profile and /etc/bash.bashrc on my Ubuntu system.

/etc/profile# grep TEST /etc/profile export TESTPROFILE=1

/etc/bash.bashrc# grep TEST /etc/bash.bashrc export TESTBASHRC=1

Interactive ShellThe below example is showing an interactive shell, in this case both the /etc/profile and/etc/bash.bashrc was executed.

# su - # env | grep TEST TESTBASHRC=1 TESTPROFILE=1

Non-Interactive ShellIn this example we are running a command through SSH that is non-interactive; because this is a non-interactive shell only the /etc/bash.bashrc file is executed.

# ssh localhost "env | grep TEST" root@localhost's password: TESTBASHRC=1

ConclusionIn my case the applications child processes are not recognizing the umask value set in /etc/profilebut do recognize the value in /etc/bashrc. This tells me that the subprocess is starting as a non-interactive shell. While the suggested route of modifying environmental variables is to add a shell script into /etc/profile.d in my case it is better to set the umask value in the /etc/bashrc.

ConnectBot -Secure Shell client available for the Android

Mon, 25 Feb 2013 09:42:30 GMT

ConnectBot is the most popular Secure Shell client available for the Android operating system. It lets users securely log in remotely to servers that run a secure shell daemon. This allows the user to enter commands from their android device and have the commands run on the remote server instead of the local android device.

ConnectBot at Google Play

Some of its features are

It supports login with a username and password to any arbitrary server on the local network or internet
Supports connections based on a public/private keypair instead of username/password for increased security
Allows frequently accessed hosts to be saved in a menu, so that they can quickly be re-connected to

Protecting your computer from cracking

Mon, 16 Feb 2009 23:54:40 GMT

You may be interested in how to prevent ordinary users from doing whatever they like, if you share your computer with other people. So this chapter describes how to improve the security of GRUB. One thing which could be a security hole is that the user can do too many things with GRUB, because GRUB allows one to modify its configuration and run arbitrary commands at run-time. For example, the user can even read /etc/passwd in the command-line interface by the command cat (see cat). So it is necessary to disable all the interactive operations. Thus, GRUB provides a password feature, so that only administrators can start the interactive operations (i.e. editing menu entries and entering the command-line interface). To use this feature, you need to run the command password in your configuration file (see password), like this: password --md5 PASSWORD

If this is specified, GRUB disallows any interactive control, until you press the key

and enter a correct password. The option --md5 tells GRUB that `PASSWORD' is in MD5 format. If it is omitted, GRUB assumes the `PASSWORD' is in clear text. You can encrypt your password with the command md5crypt (see md5crypt). For example, run the grub shell (see Invoking the grub shell), and enter your password: grub> md5crypt Password: ********** Encrypted: $1$U$JK7xFegdxWH6VuppCUSIb.

Then, cut and paste the encrypted password to your configuration file. Also, you can specify an optional argument to password. See this example: password PASSWORD /boot/grub/menu-admin.lst

In this case, GRUB will load /boot/grub/menu-admin.lst as a configuration file when you enter the valid password. Another thing which may be dangerous is that any user can choose any menu entry. Usually, this wouldn't be problematic, but you might want to permit only administrators to run some of your menu entries, such as an entry for booting an insecure OS like DOS. GRUB provides the command lock (see lock). This command always fails until you enter the valid password, so you can use it, like this:

title Boot DOS lock rootnoverify (hd0,1) makeactive chainload +1

You should insert lock right after title, because any user can execute commands in an entry until GRUB encounters lock. You can also use the command password instead of lock. In this case the boot process will ask for the password and stop if it was entered incorrectly. Since the password takes its own PASSWORD argument this is useful if you want different passwords for different entries.

Linux Distribtion Unbutu Pocket guide for free

Mon, 16 Feb 2009 21:11:16 GMT

Go to this link to find the download link of ur conveniece

http://www.ubuntupocketguide.com/download2.html