VNC Server for CentOS-7

Read about installing tigervnc-server on CentOS7.

yum install tigervnc-server xorg-x11-fonts-Type1

Copy the VNC Server config file.
Use port :3 which is 5903 instead of :0 or 5900

cp /lib/systemd/system/vncserver@.service /etc/systemd/system/vncserver@:3.service

Replace <USER> with the user name in the service file.
cat /etc/systemd/system/vncserver@:3.service

[Unit]
Description=Remote desktop service (VNC)
After=syslog.target network.target

[Service]
Type=forking

# Clean any existing files in /tmp/.X11-unix environment
ExecStartPre=/bin/sh -c '/usr/bin/vncserver -kill %i > /dev/null 2>&1 || :'
ExecStart=/usr/sbin/runuser -l mihai -c "/usr/bin/vncserver %i"
PIDFile=/home/mihai/.vnc/%H%i.pid
ExecStop=/bin/sh -c '/usr/bin/vncserver -kill %i > /dev/null 2>&1 || :'

[Install]
WantedBy=multi-user.target

VNC connection is not encrypted. We must use a ssh tunnel.
Open both ports ssh 22 and vnc 5903

iptables -A INPUT -p tcp --dport 22   -j ACCEPT
iptables -A INPUT -p tcp --dport 5903 -j ACCEPT

If you do this on a headless server then install X11, XFCE and lightdm as on CentOS-8.
You have to start XFCE once for the <USER> to create the default desktop.

Set the VNC password for the user.

[mihai@centos: ~]$ vncserver

You will require a password to access your desktops.

Password:
Verify:
Would you like to enter a view-only password (y/n)? n
A view-only password is not used

New 'centos:1 (mihai)' desktop is centos:1

Creating default startup script /home/mihai/.vnc/xstartup
Creating default config /home/mihai/.vnc/config
Starting applications specified in /home/mihai/.vnc/xstartup
Log file is /home/mihai/.vnc/centos:1.log

[mihai@centos: ~]$

By default VNC server will start GNOME Desktop session.
Read here how to switch to XFCE session.

mv ~/.vnc/xstartup ~/.vnc/xstartup.bak
cat > ~/.vnc/xstartup << EOF
#!/bin/bash
xrdb $HOME/.Xresources
startxfce4 &
EOF

Grant executable privilages

chmod +x ~/.vnc/xstartup

Enable the VNC service.

systemctl daemon-reload
systemctl start vncserver@:3.service
systemctl enable vncserver@:3.service

It may be needed to reboot the server once.

Download the VNC viewer for Linux from realvnc website and install it to /opt

su
mkdir -pv /opt/vnc-viewer
mv VNC-Viewer-*-Linux-x64 /opt/vnc-viewer/
cp -v /home/mihai/Code/git-main/script/realvnc/vnc-viewer.png /opt/vnc-viewer 
chown -Rv root:root /opt/vnc-viewer
chcon -Rv -u system_u -t usr_t /opt/vnc-viewer

cat > /usr/share/applications/realvnc.desktop << EOF
[Desktop Entry]
Name=RealVNC
GenericName=VNC Viewer
Comment=Control remote computer
Exec=/opt/vnc-viewer/VNC-Viewer-6.19.1115-Linux-x64
Terminal=false
Icon=/opt/vnc-viewer/vnc-viewer.png
Type=Application
Categories=Network;
Version=1.0
X-Desktop-File-Install-Version=0.23
EOF

Create the ssh tunnel

ssh -L 5903:$DIGITALOCEAN:5903 -l mihai $DIGITALOCEAN

or connect using a private key

ssh -L 5903:$DIGITALOCEAN:5903 -i ~/.ssh/id_rsa_digitalocean -l mihai $DIGITALOCEAN

Start RealVNC

Add a new connection

To connect from a Windows client read here about How to Tunnel VNC over SSH.
You will need PuTTY and RealVNC for this.

See which VNC port is currently open

nmap localhost
PORT     STATE SERVICE
22/tcp   open  ssh
25/tcp   open  smtp
5903/tcp open  vnc-3
6003/tcp open  X11:3

Kill a VNC instance

[sammy@centos-server: ~]$ vncserver -kill :1
Killing Xvnc process ID 1323

Troubleshoot a VNC service that refuses to start

systemctl start vncserver@:3.service
Job for vncserver@:3.service failed. See "systemctl status vncserver@:3.service" and "journalctl -xe" for details.

journalctl -xe
May 01 15:32:17 centos-server runuser[9207]: pam_unix(runuser-l:session): session opened for user sammy by (uid=0)
May 01 15:32:17 centos-server runuser[9207]: Warning: centos-server:3 is taken because of /tmp/.X11-unix/X3
May 01 15:32:17 centos-server runuser[9207]: Remove this file if there is no X server centos-server:3
May 01 15:32:17 centos-server runuser[9207]: A VNC server is already running as :3
May 01 15:32:21 centos-server runuser[9207]: New 'centos-server:1 (sammy)' desktop is centos-server:1
May 01 15:32:21 centos-server runuser[9207]: Starting applications specified in /home/sammy/.vnc/xstartup
May 01 15:32:21 centos-server runuser[9207]: Log file is /home/sammy/.vnc/centos-server:1.log
May 01 15:32:21 centos-server runuser[9207]: pam_unix(runuser-l:session): session closed for user sammy
May 01 15:32:21 centos-server systemd[1]: Can't open PID file /home/sammy/.vnc/centos-server:3.pid (yet?) after start: No such file or directory
May 01 15:32:21 centos-server systemd[1]: Failed to start Remote desktop service (VNC).

Removing temporary (old) directories

cd /tmp/.X11-unix/
rmdir X3