Posts Tagged ‘Technology’

Check if port on remote server is open without using telnet

June 13, 2017

If you want to check if a given port is open on a given remote server, you can use telnet. More and more telnet disappears on server. So the question arises: What can we use instead of telnet? Short answer: netcat. For example:

nc -zv 192.168.1.15 22

Which checks if the port 22 (SSH) is open on the server 192.168.1.115

Debugging scalability issues in action: Twitch Plays Pokémon

December 28, 2016

On Infoq.com you can wath a presentation about that:

John Rizzo introduces Twitch’s chat’s architecture, telling how their engineers investigated and worked through the issues in what turned out to be a make-or-break situation for the company.

 

Where Has the Java PermGen in Java8 Gone?

November 30, 2016

Java 8 is released since 2014. If you are working in an enterprise oriented company, chances are you are only now getting experience with usage of Java 8. You may ask yourself what this warning is telling you.

VM warning: ignoring option MaxPermSize=128M; support was removed in 8.0

Why is PermGen gone? An article of infoq.com tells you something of the reasoning behind this change.

Howto access git via ssh behind a corporate proxy

November 22, 2016

If you are working behind a proxy and have to access git repositories on the internet via ssh (e.g. github), then you sooner or later (probably sooner) find out that this will not work out of the box. This is because you need to proxy your ssh connection. This summary on stackoverflow explains how to do this in a short but comprehensible manner.

 

Hardware based exploit: Rowhammer

November 10, 2016

The Rowhammer exploit is at least known since 2014 but only in the last months it seems that this exploit may be found out in the wild.

Row hammer (also written as rowhammer) is an unintended side effect in dynamic random-access memory (DRAM) that causes memory cells to leak their charges and interact electrically between themselves, possibly altering the contents of nearby memory rows that were not addressed in the original memory access. This circumvention of the isolation between DRAM memory cells results from the high cell density in modern DRAM, and can be triggered by specially crafted memory access patterns that rapidly activate the same memory rows numerous times.
[…]
Memory protection, as a way of preventing processes from accessing memory that has not been assigned to each of them, is one of the concepts behind most modern operating systems. By using memory protection in combination with other security-related mechanisms such as protection rings, it is possible to achieve privilege separation between processes, in which programs and computer systems in general are divided into parts limited to the specific privileges they require to perform a particular task. Using privilege separation can also reduce the extent of potential damage caused by computer security attacks by restricting their effects to specific parts of the system. [wikipedia]

In other words: Switching to another operating system or patching it may not solve the problem, because the root of the problem lies in the memory chips every which computer contain. An article on wired.com describes it like this

Both of those new attacks use a technique Google researchers first demonstrated last March called “Rowhammer.” The trick works by running a program on the target computer, which repeatedly overwrites a certain row of transistors in its DRAM flash memory, “hammering” it until a rare glitch occurs: Electric charge leaks from the hammered row of transistors into an adjacent row. The leaked charge then causes a certain bit in that adjacent row of the computer’s memory to flip from one to zero or vice versa. That bit flip gives you access to a privileged level of the computer’s operating system.

lwn.net is reporting that linux kernel developers are trying to mitigate the exploit.

An intriguing alternative turned up on the linux-kernel list, though its nature wasn’t immediately clear. Pavel Machek asked a question that raised some eyebrows: “I’d like to get an interrupt every million cache misses… to do a printk() or something like that.” Developers naturally wondered what he was up to. The answer turns out to be an in-kernel Rowhammer defense.

Intro to Reactive Programming in Java

September 27, 2016

Infoq.com has  a nice little intro and tutorial to reactive programming in Java with the RxJava library.

Key takeaways

  • Reactive programming is a specification for dealing with asynchronous streams of data
  • Reactive provides tools for transforming and combining streams and for managing flow-control
  • Marble diagrams provide an interactive canvas for visualizing reactive constructs
  • Resembles Java Streams API but the resemblance is purely superficial
  • Attach to hot streams to attenuate and process asynchronous data feeds

Also you should checkout the RxMarbles website which interactivly visualizes the reactive functions.

Mozilla Flyweb: How to get rid of custom apps to communicate with IOT devices…

September 8, 2016

Mozilla has an interesting project called flyweb. If you want to know what it is about and why you should have a look at it watch this video.

https://air.mozilla.org/friday-plenary-flyweb/video/

This specification aims to allow web applications to connect with and communicate to each other over local-area transport protocols. In particular, this specification aims to bring the web’s client/server application model to inter-device communication. The web’s application architecture enables an application running on a server to dynamically and incrementally send application state and logic to an intermittently connected client. This model enables a powerful multi-homed application architecture.

Git Staging: Video about what it is, how it is used and why we need it

August 29, 2016

Lenovo T460(s) suspend problem while on battery

March 26, 2016

In this blog post I want to summarize my investigations about problems concerning suspend with my lenovo t460 notebook. (Note: This problem seems to also exists with the lenovo T460s notebooks. More information about debian on the t460 can be found here.)

(The informations here are verified with a debian testing installation with kernel version 4.4. The problems is reported by people using other distributions too.)

What is the problem and how can you reproduce it?

  • If the device is on the AC, closing the lid causes the notebook to suspend. This works perfectly – in other words: The notebook wakes up after opening the lid and is usable.
  • If the notebook is on battery, closing the lid causes the notebook to freeze. In other words: If I open the lid again, everything is frozen. Not only the UI but I’m also not able to change to TTY1, 2, ..
  • Interestingly enough, manually suspending with systemctl suspend works without a glitch.

I reported this problem on the debian forums and the thinkpad subreddit.

There is an entry in the kernel bug tracker Bug 113551 – intel_pstate=no_hwp else Thinkpad T460s freezes on lid close on battery power. The bug entry indicates that the problem is also to be found kernel versions up to 4.5.0-rc6. It also looks like the source of the problem is found, patches are being tested. I also filed a debian bug entry for this problem as was suggested to my when asking in the debian IRC channel.

So it seems that newer kernel versions won’t have this problem. There is a workaround till this happens. You can add the intel_pstate=no_hwp parameter to the GRUB_CMDLINE_LINUX_DEFAULT section of the /etc/default/grub file. Attention: Don’t forget to run update-grub after every change to this file. Also: This may cause your notebook to use more power – the battery may empty itself faster.

I will try to update this blog post after more information, changes in the status of this problem occur. I maybe even try to patch the debian kernel with a patch and test if this helps to fix the suspend problem.

Update 2016-04-28: After upgrading to the Debian 4.5.1-1 (2016-04-14) x86_64 GNU/Linux kernel version, the suspend problem is gone. I can close the lid while being on batteries and the notebook is going into suspend and leaves it correctly.

Some interesting thoughts about Functional Programming

March 25, 2016

In this InfoQ.com video presentation called “Functional Programming You Already Know” Kevlin Henney is trying to reveal functional programming pattern where you would not expect it (i.e. in excel as the world most popular used function programming language). The presentation starts a little bit slow but it’s worth your time.