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x11vnc encryption: ssl works better with the realVNC client than ssh tunneling

Posted by jpluimers on 2020/05/08

“Unencrypted connection” “This connection is unencrypted. Would you like to continue?”

When you run realVNC to an x11vnc server, even over an ssh tunneled connection, it will produce errors like the screenshots on the right (from an Android device) and below (from a Mac).

Before I had the realVNC client on the Mac, the Android message totally put me on the wrong foot. I tried searching x11vnc encryption, of which almost all results – especially the Google Search abstracts – will talk about ssh tunneling. So tried to setup the client to use the SSH endpoint, but it refused because it doesn’t talk SSH.

So then I installed a desktop realVNC client (in this case on my Mac) and got this message:

“Unencrypted connection” “The connection to this VNC Server will not be encrypted”

Then it occurred to me that maybe the VNC server itself could do encryption as well and would not need an SSH tunnel after all. And it does even in the first hit:

x11vnc command line options

x11vnc command line options

So below is a very long quote from them on which I’ve based further searching and found these links and steps:

Basically the old command-line was this:

x11vnc -shared -forever -usepw -display :0 -ultrafilexfer

And the new one this:

x11vnc -shared -forever -usepw -display :0 -ultrafilexfer -ssl -bg

Alternatively: use SSH capable VNC viewers

These viewers are capable of SSH:

[WayBack] x11vnc command line options

-ssl [pem]             Use the openssl library (www.openssl.org) to provide a
                       built-in encrypted SSL/TLS tunnel between VNC viewers
                       and x11vnc.  This requires libssl support to be
                       compiled into x11vnc at build time.  If x11vnc is not
                       built with libssl support it will exit immediately when
                       -ssl is prescribed.  See the -stunnel option below for
                       an alternative.

                       The VNC Viewer-side needs to support SSL/TLS as well.
                       See this URL and also the discussion below for
                       ideas on how to enable SSL support for the viewer:
                       http://www.karlrunge.com/x11vnc/faq.html#faq-ssl-tun
                       nel-viewers .  x11vnc provides an SSL enabled Java
                       viewer applet in the classes/ssl directory (-http or
                       -httpdir options.)  The SSVNC viewer package supports
                       SSL tunnels too.

                       If the VNC Viewer supports VeNCrypt or ANONTLS (vino's
                       encryption mode) they are also supported by the -ssl
                       mode (see the -vencrypt and -anontls options for more
                       info; use -sslonly to disable both of them.)

                       Use "-ssl /path/to/mycert.pem" to specify an SSL
                       certificate file in PEM format to use to identify and
                       provide a key for this server.  See openssl(1) for more
                       info about PEMs and the -sslGenCert and "-ssl SAVE"
                       options below for how to create them.

                       The connecting VNC viewer SSL tunnel can (at its option)
                       authenticate this server if it has the public key part
                       of the certificate (or a common certificate authority,
                       CA, is a more sophisticated way to verify this server's
                       cert, see -sslGenCA below).  This authentication is
                       done to prevent Man-In-The-Middle attacks.  Otherwise,
                       if the VNC viewer simply accepts this server's key
                       WITHOUT verification, the traffic is protected from
                       passive sniffing on the network, but *NOT* from
                       Man-In-The-Middle attacks. There are hacker tools
                       like dsniff/webmitm and cain that implement SSL
                       Man-In-The-Middle attacks.

                       If [pem] is empty or the string "SAVE" then the
                       openssl(1) command must be available to generate the
                       certificate the first time.  A self-signed certificate
                       is generated (see -sslGenCA and -sslGenCert for use
                       of a Certificate Authority.)  It will be saved to the
                       file ~/.vnc/certs/server.pem.  On subsequent calls if
                       that file already exists it will be used directly.

                       Use "SAVE_NOPROMPT" to avoid being prompted to
                       protect the generated key with a passphrase.  However in
                       -inetd and -bg modes there will be no prompting for a
                       passphrase in either case.

                       If [pem] is "SAVE_PROMPT" the server.pem certificate
                       will be created based on your answers to its prompts for
                       all info such as OrganizationalName, CommonName, etc.

                       Use "SAVE-<string>" and "SAVE_PROMPT-<string>"
                       to refer to the file ~/.vnc/certs/server-<string>.pem
                       instead (it will be generated if it does not already
                       exist).  E.g. "SAVE-charlie" will store to the file
                       ~/.vnc/certs/server-charlie.pem

                       Examples: x11vnc -ssl SAVE -display :0 ...
                                 x11vnc -ssl SAVE-someother -display :0 ...

                       If [pem] is "TMP" and the openssl(1) utility
                       command exists in PATH, then a temporary, self-signed
                       certificate will be generated for this session.  If
                       openssl(1) cannot be used to generate a temporary
                       certificate x11vnc exits immediately.  The temporary
                       cert will be discarded when x11vnc exits.

                       If successful in using openssl(1) to generate a
                       temporary certificate in "SAVE" or "TMP" creation
                       modes, the public part of it will be displayed to stderr
                       (e.g. one could copy it to the client-side to provide
                       authentication of the server to VNC viewers.)

                       NOTE: In "TMP" mode, unless you safely copy the
                       public part of the temporary Cert to the viewer for
                       authenticate *every time* (unlikely...), then only
                       passive sniffing attacks are prevented and you are
                       still open to Man-In-The-Middle attacks.  This is
                       why the default "SAVE" mode is preferred (and more
                       sophisticated CA mode too).  Only with saved keys AND
                       the VNC viewer authenticating them (via the public
                       certificate), are Man-In-The-Middle attacks prevented.

                       If [pem] is "ANON" then the Diffie-Hellman anonymous
                       key exchange method is used.  In this mode there
                       are *no* SSL certificates and so it is not possible
                       to authenticate either the VNC server or VNC client.
                       Thus only passive network sniffing attacks are avoided:
                       the "ANON" method is susceptible to Man-In-The-Middle
                       attacks.  "ANON" is not recommended; instead use
                       a SSL PEM you created or the default "SAVE" method.

                       See -ssldir below to use a directory besides the
                       default ~/.vnc/certs

                       If your x11vnc binary was not compiled with OpenSSL
                       library support, use of the -ssl option will induce an
                       immediate failure and exit.  For such binaries, consider
                       using the -stunnel option for SSL encrypted connections.

                       Misc Info: In temporary cert creation mode "TMP", set
                       the env. var. X11VNC_SHOW_TMP_PEM=1 to have x11vnc print
                       out the entire certificate, including the PRIVATE KEY
                       part, to stderr.  There are better ways to get/save this
                       info.  See "SAVE" above and "-sslGenCert" below.

–jeroen

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