Hardening consulting

Better late than never. Four years ago I was giving a talk on FreeRDS at the XDC 2014 and I was announcing that we would opensource the project at the end of the year. The opensourcing is finally here but in April 2018, you can observe a kind of delay !


About

We were late

So obviously lots of things have occured since 2014. First the project's name has changed: it was initially FreeRDS but the name was owned by one of the project's member. And our fork has diverged a lot, so there was the necessity for a new name. We thought at fireRDS,

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After watching a video on meson, it made me want to play with this software to see how it was in practise. So I did a first shot on a OGON subproject that uses cmake as build system.


Lovely CMake

I often hear that everybody hates cmake, but lots of projects use it anyway. So most probably it's for bad reasons. I'm not an exception, and as soon as I have to touch these lovely CMakeFile.txt, I always feel dirty, or at least I never have the impression to have done some nice job. This happens even when everything goes as I wanted. Not even talking of when things go wrong, with epic debugging sessions. I must be missing the cmake pĥilosophy because everytime I suspect a behaviour, cmake does it the opposite way. So everytime there's some cmake involved I'm reticent to go in that work.

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The LCA (inux Conference Australia) is one of my favorite conference, with talks of good technical level. Some years ago, it's a talk by Daniel Stone that lead me to start playing with Wayland.

A selection of my preferred talks:

  • The talk by Keith Packard on VR helmet in X, it was very interesting. A nice trip in the Xorg world;

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Let's start this new year (best wishes) with some RDP stuff, in this post I will talk of an implementation to resize the window in xfreerdp.

Context

FreeRDP already had the smart-sizing option, it allows to see the window with a given aspect ratio: 100, 150 ou 180 percent.

With the MS-RDPEDISP specification, the client can send its monitors layout to the server in real time, that allows the server to react when an output is plugged or unplugged, or when the resolution changes. You can experiment that with mstsc in fullscreen mode, if you change resolution while the mstsc window is iconized, when you uniconize, it is supposed to resize.

You can also use that specification to make the RDP window resizable: you announce a monitor that has the size of the client window.

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For some project I had the opportunity to look at H264 decoding and the hardware decoding using VAAPI. An ideal excuse to write a post about that subject...

What is VAAPI ?

Taken from Wikipedia:

The main motivation for VA API is to enable hardware-accelerated video decode at various entry-points 
(VLD, IDCT, motion compensation, deblocking) for the prevailing coding standards today (MPEG-2, MPEG-4 
ASP/H.263, MPEG-4 AVC/H.264, H.265/HEVC, and VC-1/WMV3). Extending XvMC was considered, but due to its 
original design for MPEG-2 MotionComp only, it made more sense to design an interface from scratch that 
can fully expose the video decode capabilities in today's GPUs.

This API is also usable directly with a DRM device, a DRI render node for example: very neat to offload the GPU decoding without a X server. You can also use it from Wayland of course.

The idea is to feed the GPU with a video stream (H264, VP9 ou MPEG) and the GPU will do the decoding and the rendering in a surface.

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Yes, another post on RDP ! I did some experiments with automatic reconnection, so this post talks about that project.


Automatic reconnection

Automatic reconnection allows a RDP client to automatically reconnect to the server without the need to re-authenticate. The typical example is when you close the lid of your laptop with an active RDP session, when you open it again, the client will reconnect automagically. This can also be the case with a network problem.


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Looking at my Piwik stats I've seen some interest with my previous DJI Phantom 3 posts. I've also seen a guy that has quite the same goal as me: writing a PC-based ground station software to drive a phantom 3.

So I guess it's time to speak of my discoveries regarding the protocol that is spoken between the remote controller, the camera, the mobile app and the drone.

Protocol basics

Header

First the packets are split in two parts: a header and a payload.

The header has the following format:

-------------------------------------------------------------------------
| 0 0 0 0 0 0 0 0 | 0 0 1 1 1 1 1 1 | 1 1 1 1 2 2 2 2 | 2 2 2 2 2 2 3 3 |
| 0 1 2 3 4 5 6 7 | 8 9 0 1 2 3 4 5 | 6 7 8 9 0 1 2 3 | 4 5 6 7 8 9 0 1 |
|-----------------------------------------------------------------------|
|  magic - 0x55   |    payload length    |  version   |     crc8        |
-------------------------------------------------------------------------

There's the 0x55 magic on 1 byte. Followed by a lenVer field on 2 bytes, it contains length of the payload and version of the protocol in the 6 upper bits. And then you have
a custom crc8 of the first 3 bytes.

The payload size is limited to 4096 bytes. As magic and protocol version never change, you can notice that only the size of the payload influence the crc8. So you can have a table that list some lengths and give the expected crc8 result.

The crc8 is there to be sure we have a header and that we may read the payload (well we can't be sure as it's just a crc8 but at least it give a good level of confidence).

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A post written after some recent inquiries with multi-monitor in firerds (so server side). It looked quite easy when I started working on this, but as usual with RDP I had lots of surprises (bad of course ;)

Testing multi-monitor

To begin, you need a test platform, the easiest way is to just plug 2 screens on your host and run xfreerdp:

# xfreerdp /v:myserver /multimon /f

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I had never looked at Android programming, in my mind it was smelling like desktop web apps. But when buying the drone and analyzing the network capture, it became obvious that I would have to look at how the pilot application was done. So this post tries to be an introduction to reverse engineering on Android.

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