Warning: No xauth data; using fake authentication data for X11 forwarding

Either you have changed your hostname or any similar activity on your system that's why other system is not identifying you.

Solution :
Run this command on terminal

xauth add :0 . `mcookie`

To check whether your hostname is added in the list or not, run

xauth list

Now ssh into other system, you will not get this error.

Received disconnect from IP: 2: Too many authentication failures for root

Received disconnect from : 2: Too many authentication failures for root

You are getting this error because your public keys are not saved on the server and it is not prompting for password so the public key authentication is failed, it throws you back on the command prompt with error.

The solution is simple. Either you need to save your public keys on server in authorized_keys file or set PubkeyAuthentication=no in your command. Now it will ask for password.

ssh -o PubkeyAuthentication=no server-IP

Example :
If you want to loginto server whose IP is 192.168.0.10, Run

ssh -o PubkeyAuthentication=no 192.168.0.10

Understanding Linux File Permissions

Linux File Permissions :

Understanding Linux File & Directory Permissions :

Linux File and directory Permission mechanism
	Read	Write	Execute
Owner	400	200	100
Group	40	20	10
Others	4	2	1

If you want to give Read, Write & Execute permission to Owner, Groups & Others, it means full permission.

400+200+100+40+20+10+4+2+1 = 777

If you do not want to give Write permission to Groups & Others, it means 

400+200+100+40+10+4+1 = 755

The best thing of this calculation, you can not make sum of a number in two different ways .

It means 644 can be made in this way only that is 400+200+40+4

The meaning of 644 is Read & Write permission to Owner and Read permission to Group and Others only.

To give 777 to a directory :

# chmod -R 777 /path/of/the/directory

where -R means Recursive, it means give this permission to all sub-directories and files.

To give 777 to a file :

# chmod 777 /path/of/the/file

Now you can create & give permissions by your own.