Most of you probably know it already, but just in case, here goes the explanation. You can connect two computers and simulate what would happen if you had two monitors instead: when the mouse leaves the screen in one computer, it “enters” the other computer on the opposite side of the screen. For example, if you have two laptops on your table and you move the mouse to the right, after reaching the border of the screen you see it coming out on the left side of the other computer.

The magic is done by using a remote desktop protocol like VNC, which lets you send keystrokes and mouse events, but discarding the framebuffer updates. That is, a program that connects to the remote desktop, but instead of showing you the remote screen, it monitors mouse movements locally and when the mouse reaches the screen limits, it captures the mouse and starts sending mouse events to the remote system. Similarly, when the mouse reaches the opposite limit on the remote system, it stops capturing the mouse so you can use it on the local system.

Since I use Linux my choice was x2vnc, the Linux cousin of the more famous Win2VNC, also by the same author. But unlike Win2VNC which is now actively maintained at SourceForge, x2vnc seems to have been abandoned.

x2vnc supports SSH tunneling, which is just perfect since VNC is a plaintext-only protocol and insecure by design. However there’s no easy way to tell x2vnc to which port to connect or which username to login as – it defaults always to the current local user and port 22.

I also had a problem with my other laptop, which had Windows 7. When the mouse cursor leaves the screen, it is “parked” to a corner of the screen so it doesn’t show (it’d be confusing to see two mouse cursors as you wouldn’t know which one is active). But the corner chosen by x2vnc is always the lower right corner, causing Windows 7 to hide all active windows… very annoying.

So I did what any other geek would do in my situation – branch it!

The patched code is now at Google Code Github. The new command line switches I added were:

-sshuser: Log in to the SSH tunnel using the given username.

-sshport: Connect to the given TCP port number instead of the SSH default (22).

-restingx and -restingy: Tell x2vnc where to park the mouse cursor. A value of 0 means left or top, a value of -1 means right or bottom. Then for example, -restingx 0 -restingy -1 means the top right corner of the screen.

Enjoy!

Download

Source code: https://github.com/MarioVilas/x2vnc/archive/master.zip

Let’s see the description from it’s webpage:

Exegesis
--------

So you have an ssh public authentication key and you 'lost' the
private key.  Did you generate that key in the last two years on
Debian or Ubuntu GNU/Lunix?  Yes?  Ok, great.  

$ cat id_dsa.pub
ssh-dss AAAAB3NzaC1kc3MAAACBAIW0doTjIKPNwAjHogbLhXNxNlwdvHHKzFPgZ
5cpwF4a2e8YYlEyXo8gyoub5c2s0f8B61ZNkowc9tcN+Iy1aiE2LBloxds3IwWNpZ
8KnJruCX/mYbltUp3CNJP/8gmeL41akUddPJ5wg6pYjDY5z7Kd9lojhqKOn3qSPXZ
JDJXJAAAAFQDZMKlBeKVX9/FCO5auyzPHxn6QnwAAAIBULtChrN1rGfAjIU8VZwQa
rQNunGFDfstWNOcx0lvAm2DkQCVCFT8DUXlibHWQJJbeMk3DfOl02ItIAhMvTTAPM
rb8vtFsB3Fcw7KAuK0cAJaY3R2S6/tBbWXch7SaaOQ4dxa+8hmEl54icW/me0H6Z0
SEDYEm3j8pnAUnPAu/pgAAAIALkFjo4rsTTcSyW841Gdy+rhsH4St3dd4ZXiTdDVh
wCbpBqSqiYxZO/gBHdCDAIs2uD8+GElpv7Q5Hx0g5JYLoBCpa1O8R2UAZMapZORRE
umPRs6buJ4GMf33S5f/WSqdFaMo1+/67VkvUS/9Drtb7Mz3aI/QUIh1H3gfT0xFIm
A== lamer@gnubuntu

First you'll need the fingerprint.

$ ssh-keygen -l -f ./id_dsa.pub
1024 b2:f0:f6:47:19:64:ff:8e:8f:90:75:bd:57:6c:71:0c ./id_dsa.pub

Now look for that fingerprint in the generated fingerprint database
files.  You can just use 'grep' for this.

$ grep b2:f0:f6:47:19:64:ff:8e:8f:90:75:bd:57:6c:71:0c dsa_1024_32_le.out
b2:f0:f6:47:19:64:ff:8e:8f:90:75:bd:57:6c:71:0c 25191 dsa 1024 32 0

Oh, it's your lucky day!  You're on the list.

The fingerprint database files have the following format:

  fingerprint pid key_type key_bits arch big_endian

  pid        The process id of the ssh-keygen which originally generated the key
  key_type   Either 'dsa' or 'rsa' depending on the type of key
  key_bits   The size of the key.  1024 and 2048 are common.
  arch       Either 32 or 64 depending on the processor which the key was created on
  big_endian Is 1 if the key was generated on a big endian box or 0 otherwise

So, the key we matched is a 1024 bit DSA key, generated on a 32 bit little endian
processor.  That sounds about right.

$ ./exegesis
Usage: ./exegesis [options]
Options:
  -B            Select big endian target (default is little endian target).
  -A            Selecet 64 bit target (default is 32 bit target)
  -o <file>     Output file.
  -t (dsa|rsa)  Type of key(s) to generate (default is rsa)
  -b bits       Key size to generate in bits (default is 1024 bits)
  -g            Generate all keys for a range of pids (all pids by default)
  -r start,end  Specify a pid range to generate (default is 1,32768)
  -p pid        Generate a key for a chosen pid value

$ ./exegesis -t dsa -b 1024 -p 25191
-----BEGIN DSA PRIVATE KEY-----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-----END DSA PRIVATE KEY-----

Whoah?! Is that really the private key?  Let's compare it to the
original key generated with ssh-keygen

$ ./exegesis -t dsa -b 1024 -p 25191 > key.out
$ md5sum id_dsa key.out
0aa477a9a01c6724708f9f362bcf0f7d  id_dsa
0aa477a9a01c6724708f9f362bcf0f7d  key.out

Generating Databases
--------------------

$ ./exegesis -g -t dsa -b 1024 -o dsa_1024_32_le.out

Unlike inferior competing products, Exegesis models the backdoored PRNG
in Debian OpenSSL.  It uses a version of the OpenSSL random number and
key generating code that can be configured to behave like any of the
hardware platforms that affect the generated random numbers.

This means you can generate databases for each different relevant hardware
configuration without actually needing to run it on those architectures.

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

WARNING: Generating your own databases takes a very long time and may
         cause side effects such as acute boredom and drowsiness.

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Since we know you're anxious to get started recovering all those misplaced
private SSH keys, this release of Exegesis conveniently includes, right out
of the box, ten starter databases at no extra charge!

$ md5 keysets/*
MD5 (keysets/dsa_1024_32_be.out) = d422aa60e3d6180ec65adb7179ebe43d
MD5 (keysets/dsa_1024_32_le.out) = d6f1e5f4d5dd9e84a05de47cc9e0e81a
MD5 (keysets/dsa_1024_64_le.out) = 89d34fe52f083c7e0c2297c2d8439bbc
MD5 (keysets/dsa_2048_32_le.out) = b81ca4cd84613c0fa19056036153fc62
MD5 (keysets/dsa_2048_64_le.out) = f914df33f27a11d7b2ab06446c6c13ec
MD5 (keysets/rsa_1024_32_be.out) = f5a13ffcbc63206d1c90850e2ad2e052
MD5 (keysets/rsa_1024_32_le.out) = 082b47d57e1d77366ce3795359926440
MD5 (keysets/rsa_1024_64_le.out) = 18c80767c00db8130da8a77f7e81f448
MD5 (keysets/rsa_2048_32_le.out) = 977b88495603c860abbd48a47847065a
MD5 (keysets/rsa_2048_64_le.out) = dcdd098089281388e1c3bc935dec5b7e

ps:

This product includes software developed by the OpenSSL Project for use in the OpenSSL Toolkit (http://www.openssl.org/)

Download