How to Fix "Xlib: No protocol specified"

While installing Oracle database I was getting this error. I searched various blogs and forums and
found out many solutions but lastly I got one solution which worked for me.

But well I will post all the possible ways which you can try (as per my knowledge) to overcome this issue.

Error:

Xlib: connection to ":0.0" refused by server
Xlib: No protocol specified

Specify the display protocol by logging into the user account you are getting the error

Solution 1:

$ export DISPLAY=localhost:0.0
$ xhost +

Solution 2:

Syntax:
$ export DISPLAY=(machine_name):0.0
For example:
$ export DISPLAY=10.10.20.42:0.0
$ xhost +

Solution 3 (which worked for me)
Login to the user account where you are getting the above error

$ xauth $DISPLAY
xauth: creating  new authority file /home/oracle/.Xauthority
xauth: (argv):1: unknown command ":0.0"

The best way to check whether your Xlib display protocol is working or not is by using xclock command

$ xclock

After running this command you should see a small clock on your screen.

Solution 4
In case the above solution does not works
Login as root

# xauth $DISPLAY
xauth: creating new authority file /home/root/.Xauthority

# xauth list $DISPLAY
test.example.com/unix:0 MIT-MAGIC-COOKIE-1 f601e6a29ea688786765434c5c6325071

Now copy the above output and su to the user you are facing issue

# su - oracle
$ xauth add test.example.com/unix:0 MIT-MAGIC-COOKIE-1 f601e6a29ea688786765434c5c6325071
xauth: creating new authority file /home/oracle/.Xauthority

Now test if your DISPLAY is working

$xclock

You should get a small clock output on the screen