The following method can be used when a user login to a linux machine, an email will be sent with IP information.

For this to work, you need to have the program mutt installed.

Then add the following to the user’s .bashrc file, which is located in /home/$user/.bashrc

echo `who` | /usr/bin/mutt -s "SSH Alert" email@address.here

For example, if you want an email sent everytime the user root login, edit the file /home/root/.bashrc and add the above code.

There are two ways of achieving passwordless authentication on a Linux Box.

On Debian/Ubuntu you can just type:

  $ cd $HOME
~$ ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key:

Press Enter each question and you will have a pair of keys ready to use.

You will be asked for a passphrase but if you do not want to insert anything just leave it blank.

This procedure will create one private and one public key.

$ ls .ssh/
id_rsa  id_rsa.pub  known_hosts
The private key must be secured on your box whereas the public key can be copied across
~$ ssh-copy-id -i .ssh/id_rsa.pub 192.168.1.30

At this time you will be asked for the password but once the key has been copied you will be able to ssh just perfectly.

As additional security you could lock the account to prevent someone without key to ssh into the box. In order to do that

me@mybox:~$ ssh 192.168.1.30
me@server:~$ su -
password:
root@server:~# usermod -L me

usermod -L will lock the account “me” preventing anyone using password credentials for that account. You will only be able to access with you trusted ssh-key. If you don’t want anymore the account locked just type:

root@server:~# usermod -U me

-U flag will unlock the account

ON SOME OTHER DISTROS:

the “ssh-copy-id” utility doesn’t exist so you will have to copy it manually.
If you can copy and paste then

~$ cat .ssh/id_rsa.pub
ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAv4k0ChLXCfpF+o/4HcqAqYEivRSHHYsTlXfT4I0jmOAI+MKjVTB/CtKqq4h7KMyXrUUo7vtceac4i2FRSm6PdsWksJXsYxkOj+ZXXD2fOnJIDKfIr41URcZH4qmztYO+/9YYcQudPzNlt9tLx5jrkhI7sLy56OmKRwfrxq+UY7ebt+j7y5DmevJP0u7bzREPUA/rcVoPxH0/u015O2BcaJmNoxR1pNfMC3Oefn1eAkodo6fOa3vHHo7WhSpDL/42xsBWPnOAAEDM9tmOUyCJDc8l4Mzm+TindqY2yL2GPspabaEAV3rfuF9O4Ywe+tVIPc2/YXo9XvQxyXZqHxtcw==== me@mybox

Paste the line into the remote server’s .ssh/authorized_keys file.

REMEMBER that it’s just one line so if during copy and paste you get some new line characters that key won’t work.

If you can’t copy and paste and want to have the confidence the key has been correctly copied just scp the id_rsa.pub to the server

me@mybox:~$ scp .ssh/id_rsa.pub 192.168.1.30:/home/me/
me@server:~$ ssh 192.168.1.30
me@server:~$ cat id_rsa.pub >> .ssh/authorized_keys

For more info

man usermod
man ssh-keygen

Doing visudo you get nano instead of your favorite text editor?
Mine is vim therefore I issue:

DEBIAN/UBUNTU way

# update-alternatives --config editor
There are 4 alternatives which provide `editor'.
  Selection    Alternative
-----------------------------------------------
          1    /usr/bin/vim.tiny
          2    /bin/ed
*+        3    /bin/nano
          4    /usr/bin/vim.basic
Press enter to keep the default[*], or type selection number:

Selecting 4 I’m ready to use my full syntax colors when I edit any file.

OTHER DISTROS
Edit your .bashrc file and add the following:

EDITOR=vim
export EDITOR

Next login you will have your VIM working.
If you want to have it immediately and only for this session just type

# export EDITOR=vim

and press enter.

I’ve come across multiple ways of doing it but so far the best way is the following (I assume you are on i386 platform and you have access to the net):
1. Download boot.img.gz from Debian website
2. Download the net-install image choosing i386.
3. Plug your USB drive into a Linux PC, open the shell and type

$ dmesg

Last few lines should be like the following:

 sd 7:0:0:0: [sdb] Attached SCSI removable disk
 sd 8:0:0:0: [sdb] 4030464 512-byte logical blocks: (2.06 GB/1.92 GiB)
 sd 8:0:0:0: [sdb] Write Protect is off

Now we know it has been mapped as sdb

4. Use zcat to load the boot.img.gz onto your USB drive

# zcat boot.img.gz > /dev/sdb

CAUTION!!! this will destroy the entire data on the USB drive, make sure you have done the backup.
If you get an error ensure the following:
- You are root (don’t use sudo)
- The USB drive it’s not mounted, if it is umount it before issuing the above command.

5. Now mount the USB drive and copy the net-inst.iso image on it.

All done! Now plug it into the box you want to setup and enjoy the old fashion Debian installer

This tip might comes handy when you do a system check and you want to make sure you don’t check the same file twice.

Let’s pretend that our “file1″ is a conf file that needs review. As you can see the output of the command issued below shows that the file was last edited in June.
Today I want to check the file without editing it and make sure next time I won’t check it again:

$ ls -l
total 0
-rw-r--r-- 1 luca luca 290 2009-06-29 16:33 file1

Touch is an excellent tool in this case:

$ touch file1
$ ls -l
total 0
-rw-r--r-- 1 luca luca 0 2009-08-29 19:43 file1

The Modification Time has changed and so has the access time.

If you want to change just the modification time leaving the access time untouched try with the -m option

$ touch -m file1
$ ls -l
total 0
-rw-r--r-- 1 luca luca 0 2009-08-29 19:46 file1
$ stat file1
[..]
Access: 2009-08-29 19:43:45.000000000 +0100
Modify: 2009-08-29 19:46:15.000000000 +0100
Change: 2009-08-29 19:46:15.000000000 +0100

And -a is just for the Access Time.

Another interesting option is -t. It lets you set the time and the date with whatever you like. This is often used to do fishy things

$ touch -t 200701012301 file1
$ stat file1
[..]
Access: 2007-01-01 23:01:00.000000000 +0000
Modify: 2007-01-01 23:01:00.000000000 +0000
Change: 2009-08-29 19:52:26.000000000 +0100

It is pretty easy when it comes to gaining root password to any Linux System as long as you have physical access.

This is how to change Fedora’s ROOT Password.

When your box starts up, You will see the GRUB screen…

What you need to do is, press the a Key instead of Enter

You will see something like the following…

kernel /vmlinuz-2.6.9-1.667 ro root=LABEL=/ acpi=on rhgb quiet

Add 1 at the end of that… So it looks like the following…

kernel /vmlinuz-2.6.9-1.667 ro root=LABEL=/ acpi=on rhgb quiet 1

This will make the box boot into Runlevel 1

You simply type

passwd

Enter the new ROOT password and reboot the machine.

Same thing goes for Any Linux System as welll… All you got to do is, add the following kernel argument…

init=/bin/bash

And then just change the password…