If you installed UltraVNC, which is free and open-source software, to provide Virtual Network Computing (VNC) connectivity from/to a system for remote administration capabilities, but haven't enabled the UltraVNC server service to allow the system itself to be managed remotely, you can enable that service by taking the following steps:

1. Run the uvnc_settings.exe program, which you can find in the directory where you installed UltraVNC. You will need to provide the login credentials for an account with administrator-level access on the system.
2. Click on the Service tab then click on the Install Service button, which will install the software as a service, i.e. as a function provided by a server, on the system.
3. Once the service is installed, you can click on the Start Service button to run the service.
4. You can set the password to be used for VNC connections by clicking on the Security tab. You can set two passwords. The "VNC Password" will allow remote control of the system whereas the "View-Only Password" will only allow viewing the display on the system, which is helpful if you want to show someone what is happening on the system without giving the person the ability to control the system. You can then click on the OK button to close the UltraVNC settings window.

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C:\Users\Collen>netstat -a | find "5900"
  TCP    0.0.0.0:5900           Speleologist:0         LISTENING

C:\Users\Collen>

I used windump, a free packet capture tool for Microsoft Windows systems, similar to the tcpdump tool found on OS X/macOS and Linux systems, to verify that the system was seeing the incoming connections to network port 5900.

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Right-click on it and select Admin Properties. At the Admin Properties window, you can type a password in the "View-Ony Password" that can be used to allow remote users access to the system via VNC. It will only check the first 8 characters of a password.After making any password change, click on the Apply button.

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• Default configuration uses 2048-bit RSA keys and 256-bit AES keys.
• RSA public-key cryptography supports 512-, 1024-, 2048-, and 3072-bit keys.
• Configurable choice of symmetric ciphers and keys:
  • AES: Supports 128-, 192-, and 256-bit keys.
  • Blowfish: Supports 56-, 128-, 192-, 256-, and 448-bit keys.
  • IDEA: Supports 128-bit keys.
  • CAST5: Supports 56- and 128-bit keys.
  • ARC4: Supports 56-, 128-, 192-, and 256-bit keys.
• Classic interface for older UltraVNC versions 1.0.8.2 or below uses 2048-bit RSA keys and 128-bit AES keys.
• All versions are threadsafe, allowing the UltraVNC server to host multiple simultaneous viewers.

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To get Windows Defender to accept RealVNC, select "always allow" for the action and then click on "Apply Actions".

When you are configuring RealVNC during installation, under the Connections tab in RealVNC, you can set the ports or retain the default ports that will be used for listening for connections, if you are installing the server portion of the software so you can connect remotely to the system you are installing it on. The default values are shown below:

Accept connections on port: 5900
Disconnect idle clients after (seconds): 3600

Serve Java viewer via HTTP on port: 5800

Under Access Control in the Connections section, you can check "Only accept connections from the local machine", if you wnat to require access to be through an SSH connection.

If you install the server portion of the software to run as a service on the Windows system, but don't want it to start automatically, click on the Windows Start button, select Run and type services.msc. Find "VNC Server Version 4" within the services list, double-click on it and change the startup type from "automatic" to "manual", then click on OK.

References:

1. Malicious Software Encyclopedia: RemoteAccess:Win32/RealVNC
   Published: September 12, 2006
   Microsoft Corporation

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From a Unix workstation running Solaris, I entered the command ssh -L 5901:localhost:5900 administrator@192.168.0.5 to establish an SSH connection to the system with port 5901 on the Unix workstation being forwarded to port 5900 on the Windows SBS 2003 server. Port 5900 is the default port for a VNC server and the one I used on the Windows system. However, when I tried to connect to the Windows system with vncviewer by connecting to port 5901 on the Unix system, which then should be forwarded over the SSH connection to port 5900 on the Windows system, I saw the message "Local loop-back connections are disabled."

bash-2.03$ vncviewer localhost:1

VNC viewer for X version 4.0 - built Jun 14 2004 12:04:05
Copyright (C) 2002-2004 RealVNC Ltd.
See http://www.realvnc.com for information on VNC.

Tue Nov 22 16:08:49 2005
 CConn:       connected to host localhost port 5901
 CConnection: Server supports RFB protocol version 3.3
 CConnection: Using RFB protocol version 3.3

Tue Nov 22 16:08:50 2005
 main:        Local loop-back connections are disabled.

Doing a Google search on the error, I learned I needed to have the registry key HKEY_LOCAL_MACHINE\SOFTWARE\ORL\WinVNC3 on the Windows system set to a DWORD value of "1". Since I had a command line prompt on the Windows system via the SSH connection, I used the reg query command to check the current value of that registry key. Sure enough, its current value was "0".

C:\Documents and Settings\Administrator>reg query HKEY_LOCAL_MACHINE\SOFTWARE\OR
L\WinVNC3                                                                       
                                                                                
HKEY_LOCAL_MACHINE\SOFTWARE\ORL\WinVNC3                                         
    ConnectPriority    REG_DWORD    0x0                                         
    DebugMode    REG_DWORD    0x0                                               
    DebugLevel    REG_DWORD    0x2                                              
    LoopbackOnly    REG_DWORD    0x0                                            
    EnableHTTPDaemon    REG_DWORD    0x1                                        
    EnableURLParams    REG_DWORD    0x0                                         
    AllowLoopback    REG_DWORD    0x0                                           
    AuthRequired    REG_DWORD    0x1                                            
                                                                                
HKEY_LOCAL_MACHINE\SOFTWARE\ORL\WinVNC3\Default                                 

Fortunately, the reg command can also be used to modify the registry from the command line.

C:\Documents and Settings\Administrator>reg add HKEY_LOCAL_MACHINE\SOFTWARE\ORL\
WinVNC3 /v AllowLoopback /t REG_DWORD /d 1                                      
Value AllowLoopback exists, overwrite(Yes/No)? yes                              
The operation completed successfully.                                           

The REG ADD command uses the following syntax:

REG ADD KeyName [/v ValueName | /ve] [/t Type] [/s Separator] [/d Data] [/f]


In this case, the parameters used have the following meaning:

/v     The value name, in this case "AllowLoopback", to be added under the selected key.

/t     Allows one to specify the data type, which can be REG_SZ, REG_MULTI_SZ, REG_EXPAND_SZ, REG_DWORD, REG_BINARY, or REG_NONE. If none is specified, REG_SZ is assumed. In this case REG_DWORD is needed.

/d     The data to assign to the registry ValueName being added, which is "1" in this case.

Another reg query HKEY_LOCAL_MACHINE\SOFTWARE\ORL\WinVNC3 shows the value was changed.

C:\Documents and Settings\Administrator>reg query HKEY_LOCAL_MACHINE\SOFTWARE\OR
L\WinVNC3 

HKEY_LOCAL_MACHINE\SOFTWARE\ORL\WinVNC3                                         
    ConnectPriority    REG_DWORD    0x0                                         
    DebugMode    REG_DWORD    0x0                                               
    DebugLevel    REG_DWORD    0x2                                              
    LoopbackOnly    REG_DWORD    0x0                                            
    EnableHTTPDaemon    REG_DWORD    0x1                                        
    EnableURLParams    REG_DWORD    0x0                                         
    AllowLoopback    REG_DWORD    0x1                                           
    AuthRequired    REG_DWORD    0x1                                            
                                                                                
HKEY_LOCAL_MACHINE\SOFTWARE\ORL\WinVNC3\Default

After modifying the registry value, you then must stop and restart the VNC Server service.

C:\Documents and Settings\Administrator>net stop "VNC Server"                   
The VNC Server service is stopping..                                            
The VNC Server service was stopped successfully.                                
                                                                                
                                                                                
C:\Documents and Settings\Administrator>net start "VNC Server"                  
The VNC Server service is starting.                                             
The VNC Server service was started successfully.    

You should then be able to use vncviewer localhost:1 to access the remote system with VNC. After the message about the protocol version, you should see a VNC Authentication window open.

bash-2.03$ vncviewer localhost:1

VNC viewer for X version 4.0 - built Jun 14 2004 12:04:05
Copyright (C) 2002-2004 RealVNC Ltd.
See http://www.realvnc.com for information on VNC.

Tue Nov 22 17:07:42 2005
 CConn:       connected to host localhost port 5901
 CConnection: Server supports RFB protocol version 3.3
 CConnection: Using RFB protocol version 3.3

In this case, I was then able to enter the VNC authorization password and then hit Ctrl-Alt-Del on the Solaris system to enter the user name and password for the Windows SBS 2003 server.

References:

1. Cygwin ssh (OpenSSH), and Win98 - Use "crypt newpassword" to add a password into /etc/passwd

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