Unbrick a Netgear WNR3500L V2

It finally happened… After experimenting with alternative firmware for my WNR3500L router, I uploaded the wrong version and converted the device into a rather expensive paperweight. After some searching, I found this page, explaining how to revitalize the router by using a USB-TTL converter cable. You can get  (e.g. CP2102 Module Modul USB to TTL Converter) for a few bucks Ebay, but expect a few weeks for the delivery from China.  If you follow the tutorial on the OpenRouters page closely, it should work out quite nicely.

Update: 2018-01-21

It happened again. I flashed an unsupported version of LEDE on the device and it did not want to boot again. I followed the same procedure as described here, but the router was complaining about a version missmatch of the device ID and the image ID. Turns out, I had to explicitly use binary mode for FTP. Here are the steps again.

  1. Connect the serial cables as shown
  2. Make sure router is off
  3. Launch minicom in a terminal and make sure the settings are valid for ttyUSB0
  4. Boot the router and immediately press Ctrl+C in the terminal
  5. Type tftpd in the prompt, the router should start a FTP server on
  6. Open a second terminal, cd into the directory where the firmware is
  7. Make sure its the right version 🙂
  8. Start the FTP client: ftp
  9. Change to binary mode (this is very important): mode binary
  10. Move the file: put FIRMWARE.chr
  11. The router should detect the file and process it
  12. Reboot and enjoy

Connecting a Serial TTYUSB Device to the Router





Continue reading

Illegal mix of collations: IntelliJ and UTF8mb4

IntelliJ products do not yet support MySQL’s utf8mb4 character set encodings. The […]


When using variables inside SQL scripts within IntelliJ products (e.g. DataGrip), certain queries will not work because the encodings of the IntelliJ client and the server missmatch. This occurs for instance when you compare variables. A typical error message looks like this:

IntelliJ products do not yet support MySQL’s utf8mb4 character set encodings. The problem occurs when using variables in queries. Per default. IntelliJ uses a UTF-8 encoding for the connection. When you use utf8mb4 as the database default character set, then variables will be encoded in UTF-8 while the database content remailns in utf8mb4. It is not possible to provide the character set encodings to the IntelliJ settings, as it will refuse to connect.

Check your server settings using the MySQL client:

This seems correct, but when you connect with the IntelliJ client, you will get wrong results when you use variables. Until the products support utf8mb4, you would need to add the following settings to the script in order to force the right settings.

Continue reading

Fixing Random Freezes with Ubuntu 16.04 LTS, Intel Skylake and an Nvidia GPU

My Lenovo ThinkCentre m900 (10FHCTO1WW) with an Intel i7-6700 showed weird and random freezes from day 1 when trying to install Mint 18 / Ubuntu 16 with any kernel newer than 3x. After investigating for quite some hours, I gave up and installed an Ubuntu 14.04 LTS on it. The device is certified to it, but the old version did not support all features and even some basic things such as audio did not work. At lest the random freezes were gone and I could work with that machine. Now that the system will not receive updates soon, I gave it another try and setup Mint 18.2 (Sonya). Unfortunately, the Lenovo machine froze again after a few minutes, filling up the log again with the following error messages. 

I started the investigation again and found a different trace, which pointed to the graphics card. The important hint and solution came from SO. Following a few other forum posts, it became clear that the Nvidia drivers do not play nicely with recent kernels for some specific Nvidia cards ind combination with newer kernels. So I followed the proposed steps and disabled the card complete. Just removing the card in the BIOS and uninstalling the drivers was not enough. I also had to blacklist the modules for the nouveau kernel driver.

  1. Disable the Nvidia card in the BIOS and use the Intel onchip GPU
  2. Remove all Nvidia packages: 
    sudo apt-get remove nvidia* && sudo apt autoremove
  3. Blacklist the module:  
    sudo vim /etc/modprobe.d/blacklist.conf

  4. Reboot

The card is not used any more and the freezes stopped.

I hope I do not have to remove this article again and the system remains as stable as it is now for six hours.

Continue reading

Predicting Visitors with Facebook Prophet

Facebook open sourced its forecasting tool Prohpet for time series data. Although forecasting is not a trivial task, the libraries are very easy to use and produce nice results quickly.  In this basic blog post, I am going to forecast the visitor statistics based on the historical data I collected with Piwik.

Python Prerequisites

Install and initialize a new virtual Python environment

Install Prophet and its Dependencies

Within your new Python virtual environment, install the required dependencies first and then Prophet

Get the Data from your Piwik Database

We aggregate the data from the visitors table per day and store the result in a CSV file. In the case of this blog, I started collecting visitor traffic data from early 2013. Prophet allows displaying not only trends and seasonality, but also to forecast into the future.

Usually MySQL runs with a security setting that prevents writing files to the server’s disk (for a good reason). Check the variable secure-file-priv to find the path you can use for exporting.

The data now looks similar like this:

This is exactly the format which Prophet expects.

Forecasting with Prophet

The short but nice tutorial basically shows it all. Here is the script, it is basically the very same as from the tutorial:

The results are the forecast graph and the components as nice graphs. Facebook Prophet incorporates seasonal variations, holidays and trends derived from historical data.

As you can see, the weekend is rather low on visitors and that the beginning summer is also rather weak.



Continue reading

Validate Hibernate Search Input with an Analyzer

Stop Words

Hibernate Search lets you easily assign an @Analyzer on Fields, which are used to process terms before they are written to the index. An anlyzer can be used for instance for stemming and removing of words which are so frequent that they are insignificant for the results. These are examples for stop words:

It is a common technique, to split input search terms into single keywords and use these keywords for combining a complex queries over several fields. The problem with this approach is that if a user provides such stop words as input and you manually split the input string into a list of keywords, for instance with a split method, it can occur that a stop word becomes a single keyword for Hibernate Search to process.

Hibernate does not know what to search for an replies with this error message and also suggests a solution.

In orderb to validate the string with the same analyzer as hibernate does when building the index, we can retrieve the analyzer from the class we want to search on and parse the string by providing it as a stream to the analyzer. First we get the analyser like this:

And then we can write a simple method, which takes the whole string as input and chops the whole string into a list of keywords, which is sent through the analyzer. Thus if your input string contains a stop word and it would not be added to the index, it will also not be included in the list. Thus Hibernate won’t even try to search for it, as it never sees the stemmed word in the first place.


Continue reading