How to fix/suppress "apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1." warning message?

Warning message Before the fix:

"apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1. Set the 'ServerName' directive globally to suppress this message"


Fix: 
execute the following command in terminal

echo "ServerName localhost" | sudo tee -a /etc/apache2/apache2.conf

Now restart apache server by executing the following command

sudo /etc/init.d/apache2 restart


Message after the fix:
[ ok ] Restarting web server: apache2.

How to resolve "no suitable module for running kernel found" error in virtualbox instalation in debian linux?

1. Check wheather you machine is installed with "linux-headers-, linux-image-"
by issueing the follwing command in terminal.

  $dpkg -l | grep linux

In my case my kernel version is "3.16.0-4-686-pae" so the output of the above command would be something like as follows, 

Missing linux headers package

If you don't find any linux-headers or linux-images then you must install them first.

2. To find the your kernel version use the following command
  $uname -a

[ My output -> Linux hostname 3.16.0-4-686-pae #1 SMP Debian 3.16.36-1+deb8u2 (2016-10-19) i686 GNU/Linux ]

now install the required packages specified in step 1.

3.  Uninstall your previous virutalbox package and reinstall it.

  $sudo apt-get remove --purge virtualbox
  $sudo apt-get instlal virtualbox

4.  Restart you machine.

How to find filesystem type in linux machine?

There are many ways to find the filesystem of a partition in linux (provided the partition mounted)
Following are the command line tools run these commands as is in terminal.

1.  cat /etc/fstab

2.  cat /etc/mtab

3.  mount

4.  df -T

Note:  You should know the partition number (like /dev/sda2, /dev/sda4 etc.,) or the partition label (Like mypendrive, 8GB_drive etc.,) or the UUID number.

Youtube videos not working asking for flash plugin | How to install adobe-flash player in (Debian based)Linux?

1.  Download the adobe-flash player from -> https://get.adobe.com/flashplayer/

2.  Select ".tar.gz for other linux" then click on "Download now"
        Note:  If you are using debian or rpm based linux then select your option accordingly.

3.  Unpack the download file (Right-click on the file and select -> "Extract here")
        (Note:  There will be a readme.txt file inside the extracted directory (like install_flash_player_11_linux.i386).  This file contains the detailed instructions for installation for all the flavours of Linux.)

4.  copy the "libflashplayer.so" file into following directories

        a. /usr/lib/flashplugin-nonfree/
        b. /usr/lib/mozilla/plugins/
        If you have chromium browser then copy the file into
        c. /usr/lib/chromium/plugins/
       
5.    Test the flash plugin -> Close the browsers (if already opened) then start again.
        Now, open youtube and check the videos will play like charm.

How to enable ssh access for root user in Linux (debian based)?

1. Edit the /etc/ssh/sshd_config file as sudo(or root) user
        $sudo vim /etc/ssh/sshd_config
2. Modify the "PermitRootLogin" value as "yes" under "Authenticaton" Section
        After editing your sshd_config file's Authentication section will look like

PermitRootLogin yes

3. Save the changes (esc :wq)
4. Restart the ssh service
        $sudo /etc/init.d/ssh restart
       
5.  Now check root login via ssh from a remote machine
        (From a another/remote machine)
        $ssh root@
       
Note:
The parameter PermitRootLogin can take any of the following values namely,
    yes, without-password, forced-commands-only, no
   
   
PermitRootLogin
 "yes"                                    =    ssh root login allowed

 "without-password"         = password authentication is disabled for root.

 "forced-commands-only" = root login with public key authentication will be allowed,
                                                     but only if the command option has been specified. All
                                                     other authentication methods are disabled for root.

 "no"                                        = root is not allowed to log in.