Remote Access with VNC

Virtual Network Computing (VNC) enables you to control a remote computer via a graphical desktop (as opposed to a remote shell access). VNC is platform-independent and lets you access the remote machine from any operating system.

openSUSE Leap supports two different kinds of VNC sessions: One-time sessions that "live" as long as the VNC connection from the client is kept up, and persistent sessions that "live" until they are explicitly terminated.

Note: Session Types

A machine can offer both kinds of sessions simultaneously on different ports, but an open session cannot be converted from one type to the other.

Important: Supported Display Managers

A machine can reliably accept VNC connections only if it uses a display manager that supports the XDMCP protocol. While gdm, lxdm, or lightdm support XDMCP, the KDE 5 default display manager sddm does not support it. When changing the default display manager, remember to log out of the current X session and restart the display manager with

 tux > sudo systemctl restart xdm.service

The vncviewer Client

To connect to a VNC service provided by a server, a client is needed. The default in openSUSE Leap is vncviewer, provided by the tigervnc package.

Connecting Using the vncviewer CLI

To start your VNC viewer and initiate a session with the server, use the command:

 tux > vncviewer thinkserver.freddythechick.uk:1

Instead of the VNC display number you can also specify the port number with two colons:

 tux > vncviewer thinkserver.freddythechick.uk::5901

Note: Display and Port Number The actual display or port number you specify in the VNC client must be the same as the display or port number picked by the vncserver command on the target machine. See Section 4.4, "Persistent VNC Sessions" for further info.

Connecting Using the vncviewer GUI

By running vncviewer without specifying --listen or a host to connect to, it will show a window to ask for connection details. Enter the host into the VNC server field like in Section 4.1.1, "Connecting Using the vncviewer CLI" and click Connect.

Figure 1: vncviewer

Notification of Unencrypted Connections

The VNC protocol supports different kinds of encrypted connections, not to be confused with password authentication. If a connection does not use TLS, the text "(Connection not encrypted!)" can be seen in the window title of the VNC viewer.

Remmina: the Remote Desktop Client

Remmina is a modern and feature rich remote desktop client. It supports several access methods, for example VNC, SSH, RDP, or Spice.

Installation

To use Remmina, verify whether the remmina package is installed on your system, and install it if not. Remember to install the VNC plugin for Remmina as well:

 sudo zypper in remmina remmina-plugin-vnc

Main Window

Run Remmina by entering the remmina command.

Figure 2: Reminna's Main Window

The main application window shows the list of stored remote sessions. Here you can add and save a new remote session, quick-start a new session without saving it, start a previously saved session, or set Remmina's global preferences.

Adding Remote Sessions

To add and save a new remote session, click in the top left of the main window. The Remote Desktop Preference window opens.

Figure 3: Remote Desktop Preference

Complete the fields that specify your newly added remote session profile. The most important are:

Name
Name of the profile. It will be listed in the main window.

Protocol
The protocol to use when connection to the remote session, for example VNC.

Server
The IP or DNS address and display number of the remote server.

User name, Password
Credentials to use for remote authentication. Leave empty for no authentication.

Colour depth, Quality
Select the best options according to you connection speed and quality.

Select the Advanced tab to enter more specific settings.

Tip: Disable Encryption

If the communication between the client and remote server is not encrypted, active Disable encryption, otherwise the connection fails.

Select the SSH tab for advanced SSH tunnelling and authentication options.

Confirm with Save. Your new profile will be listed in the main window.

Starting Remote Sessions

You can either start a previously saved session, or quick-start a remote session without saving the connection details.

Quick-starting Remote Sessions

To start a remote session quickly without proper adding and saving connection details, use the drop-down box and text field at the top of the main window.

Figure 4: Quick-Starting

Select the communication protocol from the drop-down box, for example 'VNC', then enter the VNC server DNS or IP address followed by a colon and a display number, and confirm with ↵ Enter.

Opening Saved Remote Sessions

To open a specific remote session, double-click it from the list of sessions.

Remote Sessions Window

Remote sessions are opened in tabs of a separate window. Each tab hosts one session. The toolbar on the left of the window helps you manage the windows/sessions, such as toggle full-screen mode, resize the window to match the display size of the session, send specific keystrokes to the session, take screenshots of the session, or set the image quality.

Figure 5: Remmina Viewing SLES 15 Remote Session

Editing, Copying and Deleting Saved Sessions

To edit a saved remote session, right-click its name in the Remmina's main window and select Edit. Refer to Section 4.2.3, "Adding Remote Sessions" for the description of the relevant fields.

To 'copy a saved remote session, right-click its name in the Remmina's main window and select Copy. In the Remote Desktop Preference window, change the name of the profile, optionally adjust relevant options, and confirm with Save.

To Delete a saved remote session, right-click its name in the Remmina's main window and select Delete. Confirm with Yes in the next dialog.

Running Remote Sessions from the Command Line

If you need to open a remote session from the command line or from a batch file without first opening the main application window, use the following syntax:

 tux > remmina -c profie_name.remmina

Remmina's profile files are stored in the .local/share/remmina/ directory in your home directory. To determine which profile file belongs to the session you want to open, run Remmina, click the session name in the main window, and read the path to the profile file in the window's status line at the bottom.

Figure 6: Reading Path to the Profile File

While Remmina is not running, you can rename the profile file to to a more reasonable file name, such as sle15.remmina. You can even copy the profile file to your custom directory and run it using the remmina -c command from there.

One-time VNC Sessions

A one-time session is initiated by the remote client. It starts a graphical login screen on the server. This way you can choose the user which starts the session and, if supported by the login manager, the desktop environment. When you terminate the client connection to such a VNC session, all applications started within that session will be terminated, too. One-time VNC sessions cannot be shared, but it is possible to have multiple sessions on a single host at the same time.

Procedure 1: Enabling One-time VNC Sessions

1. Start YaST > Network Services > Remote Administration (VNC).
2. Check Allow Remote Administration Without Session Management.
3. Activate Enable access using a web browser if you plan to access the VNC session in a Web browser window.
4. If necessary, also check Open Port in Firewall (for example, when your network interface is configured to be in the External Zone). If you have more than one network interface, restrict opening the firewall ports to a specific interface via Firewall Details.
5. Confirm your settings with Next.
6. In case not all needed packages are available yet, you need to approve the installation of missing packages.

Tip: Restart the Display Manager

YaST makes changes to the display manager settings. You need to log out of your current graphical session and restart the display manager for the changes to take effect.

Figure 7: Remote Administration

Available Configurations

The default configuration on openSUSE Leap serves sessions with a resolution of 1024x768 pixels at a colour depth of 16-bit. The sessions are available on ports 5901 for "regular" VNC viewers (equivalent to VNC display 1) and on port 5801 for Web browsers.

Other configurations can be made available on different ports, see Section 4.3.3, "Configuring One-time VNC Sessions".

VNC display numbers and X display numbers are independent in one-time sessions. A VNC display number is manually assigned to every configuration that the server supports (:1 in the example above). Whenever a VNC session is initiated with one of the configurations, it automatically gets a free X display number.

By default, both the VNC client and server try to communicate securely via a self-signed SSL certificate, which is generated after installation. You can either use the default one, or replace it with your own. When using the self-signed certificate, you need to confirm its signature before the first connection.

Initiating a One-time VNC Session

To connect to a one-time VNC session, a VNC viewer must be installed, see also Section 4.1, "The vncviewer Client".

Configuring One-time VNC Sessions

You can skip this section, if you do not need or want to modify the default configuration.

One-time VNC sessions are started via the systemd socket xvnc.socket. By default it offers six configuration blocks: three for VNC viewers (vnc1 to vnc3), and three serving a Java applet (vnchttpd1 to vnchttpd3). By default only vnc1 and vnchttpd1 are active.

To activate the VNC server socket at boot time, run the following command:

 sudo systemctl enable xvnc.socket

To start the socket immediately, run:

 sudo systemctl start xvnc.socket

The Xvnc server can be configured via the server_args option. For a list of options, see Xvnc --help.

When adding custom configurations, make sure they are not using ports that are already in use by other configurations, other services, or existing persistent VNC sessions on the same host.

Activate configuration changes by entering the following command:

 tux > sudo systemctl reload xvnc.socket

Important: Firewall and VNC Ports

When activating Remote Administration as described in Procedure 4.1, "Enabling One-time VNC Sessions", the ports 5801 and 5901 are opened in the firewall. If the network interface serving the VNC sessions is protected by a firewall, you need to manually open the respective ports when activating additional ports for VNC sessions. See Book "Security Guide", Chapter 15 "Masquerading and Firewalls" for instructions.

Persistent VNC Sessions

A persistent session can be accessed from multiple clients simultaneously. This is ideal for demonstration purposes where one client has full access and all other clients have view-only access. Another use case are trainings where the trainer might need access to the trainee's desktop.

Tip: Connecting to a Persistent VNC Session To connect to a persistent VNC session, a VNC viewer must be installed. Refer to Section 4.1, "The vncviewer Client" for more details.

There are two types of persistent VNC sessions:

• VNC Session Initiated using vncserver
• VNC Session Initiated using vncmanager

VNC Session Initiated using vncserver

This type of persistent VNC session is initiated on the server. The session and all applications started in this session run regardless of client connections until the session is terminated. Access to persistent sessions is protected by two possible types of passwords:

• A regular password that grants full access or
• An optional view-only password that grants a non-interactive (view-only) access.

A session can have multiple client connections of both kinds at once.

Procedure 2: Stating a Persistent VNC Session using vncserver

1. Open a shell and make sure you are logged in as the user that should own the VNC session.
2. If the network interface serving the VNC sessions is protected by a firewall, you need to manually open the port used by your session in the firewall. If starting multiple sessions you may alternatively open a range of ports. See Book "Security Guide", Chapter 15 "Masquerading and Firewalls" for details on how to configure the firewall.
   
   vncserver uses the ports 5901 for display :1, 5902 for display :2, and so on. For persistent sessions, the VNC display and the X display usually have the same number.
3. To start a session with a resolution of 1024x768 pixel and with a colour depth of 16-bit, enter the following command:

 vncserver -geometry 1024x768 -depth 16

The vncserver command picks an unused display number when none is given and prints its choice. See man 1 vncserver for more options.

When running vncserver for the first time, it asks for a password for full access to the session. If needed, you can also provide a password for view-only access to the session.

The password(s) you are providing here are also used for future sessions started by the same user. They can be changed with the vncpasswd command.

Important: Security Considerations

Make sure to use strong passwords of significant length (eight or more characters. Do no share these passwords.

To terminate the session shut down the desktop environment that runs inside the VNC session from the VNC viewer as you would shut it down if it was a regular local X session.

If you prefer to manually terminate a session, open a shell on the VNC server and make sure you are logged in as the user that owns the VNC session you want to terminate. Run the following command to terminate the session that runs on display :1: vncserver -kill :1.

Configuring Persistant VNC Sessions

Persistent VNC sessions can be configured by editing $HOME/.vnc/xstartup. By default this shell script starts the same GUI/window manager it was started from. In openSUSE Leap this will either be GNOME or IceWM. If you want to start your session with a window manager of your choice, set the variable WINDOWMANAGER:

 WINDOWMANAGER=gnome vncserver -geometry 1024x768
 WINDOWMANAGER=icewm vncserver -geometry 1024x768

Note: One Configuration for Each User

Persistent VNC sessions are configured in a single per-user configuration. Multiple sessions started by the same user will all use the same start-up and password files.

VNC Session Initiated Using vncmanager

Procedure 3: Enabling Persistent VNC Sessions

1. Start YaST > Network Services > Remote Administration (VNC).
2. Activate Allow Remote Administration With Session Management.
3. Active Allow access using a web browser if you plan to access the VNC session in a Web browser window.
4. If necessary, also check Open Port in Firewall (for example, when your network interface is configured to be in the External Zone). If you have more than one network interface, restrict opening the firewall ports to a specific interface via Firewall Details.
5. Confirm your settings with Next.
6. In case not all needed packages are available yet, you need to approve the installation of missing packages.

Tip: Restart the Display Manager

YaST makes changes to the display manager settings. You need to log out of your current graphical session and restart the display manager for the changes to take effect.

Configuring Persistent VNC Sessions

After you enable the VNC session management as described in Procedure 4.3, "Enabling Persistent VNC Sessions", you can normally connect to the remote session with your favourite VNC viewer, such as vncviewer or Remmina. You will be presented with login screen. After you log in, the 'VNC' icon will appear in the system tray of your desktop environment. Click the icon to open the VNC Session window. If it does not appear or if your desktop environment does not support icons in the system tray, run vncmanager-controller manually.

Figure 8: VNC Session Settings

There are several settings which influence the VNC session behaviour:

Non-persistent, private
This is equivalent to one-time session. Such session is not visible to others and will be terminated after you disconnect from it. Refer to Section 4.3, "One-time VNC Sessions" for more information.

Persistent, visible
The session is visible to other users and keeps running even after you disconnect from it.

Session name
Here you can specify the name of the persistent session so that it is easily identified when reconnecting.

No password required'
The session will be freely accessible without having to log in under user credentials.

Require user login
You need to log in with a valid user name and password to access the session. List the valid user names in the Allowed users text box.

Allow one client at time
Disables joining the session by multiple users at the same time.

Allow multiple clients at time
Allows multiple users to join the persistent session at the same time. Good for example for remote presentations or trainings.

Confirm with OK.

Joining Persistent VNC Sessions

After you set up a persistent VNC session as described in Section 4.4.2.1, "Configuring Persistent VNC Sessions", you can join it with your VNC viewer. After the your VNC client connects to the server, you will be prompted to choose whether you want to create a new session, or join the existing one:

Figure 9: Joining a Persistent VNC Session

After you click the name of the existing session, you may be asked for login credentials, depending on the persistent session settings.

Encrypted VNC Communication

If the VNC server is set up properly, all communication between the VNC server and the client is encrypted. The authentication happens at the beginning of the session, the actual data transfer only begins afterwards.

Whether for a one-time or a persistent VNC session, security options are configured via the -securitytypes parameter of the /usr/bin/Xvnc command located on the server_args line. The -securitytypes parameter selects both authentication method and encryption. It has the following options:

Authentications

None, TLSNone, X509None
No authentication.

VncAuth, TLSVnc, X509Vnc
Authentication using custom password.

Plain, TLSPlain, X509Plain
Authentication using PAM to verify user's password.

Encryptions

None, VncAuth, Plain
No encryption.

TLSNone, TLSVnc, TLSPlain
Anonymous TLS encryption. Everything is encrypted, but there is no verification of the remote host. So you are protected against passive attackers, but not against man-in-the-middle attackers.

X509None, X509Vnc, X509Plain
TLS encryption with certificate. If you use a self-signed certificate, you will be asked to verify it on the first connection. On subsequent connections you will be warned only if the certificate changed. So you are protected against everything except man-in-the-middle on the first connection (similar to typical SSH usage). If you use a certificate signed by a certificate authority matching the machine name, then you get full security (similar to typical HTTPS usage).

Tip: Path to Certificate and Key

With X509 based encryption, you need to specify the path to the X509 certificate and the key with -X509Cert and -X509Key options.

If you select multiple security types separated by comma, the first one supported and allowed by both client and server will be used. That way you can configure opportunistic encryption on the server. This is useful if you need to support VNC clients that do not support encryption.

On the client, you can also specify the allowed security types to prevent a downgrade attack if you are connecting to a server which you know has encryption enabled (although our vncviewer will warn you with the "Connection not encrypted!" message in that case).