Category Archives: Personal

OpenVPN re-visited

OpenVPN re-visited

It's been a very long time since I set up the VPN infrastructure at the office using OpenVPN. Today, I came across an interesting log entry that I would like to document quickly.

OpenVPN re-visited

At the time of writing I have OpenVPN 2.4.6 running on my Windows 10 machine. The existing infrastructure though is on a different version, and this morning I observed the following entries in the log file:

Tue Aug 28 08:50:09 2018 WARNING: INSECURE cipher with block size less than 128 bit (64 bit).  This allows attacks like SWEET32.  Mitigate by using a --cipher with a larger block size (e.g. AES-256-CBC).
Tue Aug 28 08:50:09 2018 WARNING: INSECURE cipher with block size less than 128 bit (64 bit).  This allows attacks like SWEET32.  Mitigate by using a --cipher with a larger block size (e.g. AES-256-CBC).
Tue Aug 28 08:50:09 2018 WARNING: cipher with small block size in use, reducing reneg-bytes to 64MB to mitigate SWEET32 attacks.

Curious about those entries I found Sweet32: Birthday attacks on 64-bit block ciphers in TLS and OpenVPN as an informative reference on the documented vulnerabilities CVE-2016-2183 and CVE-2016-6329. There I found the connection back to OpenVPN. Which is also described on the official wiki: OpenVPN and SWEET32

The default encryption for the transport protocol of OpenVPN is Blowfish – a 64-bit cipher – with the CBC mode.

Meaning, the default encryption of OpenVPN prior to version 2.4 is BF-CBC which doesn't provide enough security in recent times. Newer versions of OpenVPN though are using AES-256-CBC as default cipher.

Upgrade your cipher suite and block size

For your own sake and safety of your network(s) you should check and change your OpenVPN infrastructure right away, and if needed upgrade your defined cipher to a more secure encryption and larger block size.

OpenVPN users can change the cipher from the default Blowfish to AES

First, check which ciphers are available on your server and clients using the --show-ciphers option like so:

$ sudo openvpn --show-ciphers
The following ciphers and cipher modes are available
for use with OpenVPN.  Each cipher shown below may be
used as a parameter to the --cipher option.  The default
key size is shown as well as whether or not it can be
changed with the --keysize directive.  Using a CBC mode
is recommended.

DES-CBC 64 bit default key (fixed)
RC2-CBC 128 bit default key (variable)
DES-EDE-CBC 128 bit default key (fixed)
DES-EDE3-CBC 192 bit default key (fixed)
DESX-CBC 192 bit default key (fixed)
BF-CBC 128 bit default key (variable)
RC2-40-CBC 40 bit default key (variable)
CAST5-CBC 128 bit default key (variable)
RC2-64-CBC 64 bit default key (variable)
AES-128-CBC 128 bit default key (fixed)
AES-192-CBC 192 bit default key (fixed)
AES-256-CBC 256 bit default key (fixed)
CAMELLIA-128-CBC 128 bit default key (fixed)
CAMELLIA-192-CBC 192 bit default key (fixed)
CAMELLIA-256-CBC 256 bit default key (fixed)
SEED-CBC 128 bit default key (fixed)

Depending on your underlying Linux system the list might be more or less exhaustive. Have a look and then choose a key length of at least 128 bit.

OpenVPN currently recommends using AES-256-CBC or AES-128-CBC.

Following the article on OpenVPN and SWEET32 I chose to use AES-256-CBC cipher suite for my existing infrastructure. This seems to give me the largest compatibility between OpenVPN installations on various clients, including Raspberry Pi.

Change your OpenVPN configuration

Independent of the OpenVPN version installed, you can specify the cipher directive in your configuration files - server and client likewise. Usually that directive is either not present or commented, meaning it uses the compiled default value. Change it to your needs like so:

# Select a cryptographic cipher.
# This config item must be copied to
# the client config file as well.
;cipher BF-CBC        # Blowfish (default)
;cipher AES-128-CBC   # AES
cipher AES-256-CBC

This needs to be applied on the OpenVPN server first as well as on all OpenVPN clients. Save your configuration file and reload the new settings.

$ sudo service openvpn reload

Perhaps, you might like to publish your updated client configuration file(s) a bit earlier. With the newly set cipher any connecting client will be rejected now, if the cipher suites do not match. Monitor your syslog output on the OpenVPN server for that kind of entries:

Aug 28 07:33:26 smtp ovpn-server[18351]: WARNING: 'cipher' is used inconsistently, local='cipher AES-256-CBC', remote='cipher BF-CBC'
Aug 28 07:33:26 smtp ovpn-server[18351]: WARNING: 'keysize' is used inconsistently, local='keysize 256', remote='keysize 128'
Aug 28 07:34:08 smtp ovpn-server[18351]: client/ Authenticate/Decrypt packet error: cipher final failed

This way you are able to find out which clients are still running on the previous configuration and therefore would need a little bit of assistance.

Other hardware firewall based on OpenVPN

Thanks to some of the clients of my company IOS Indian Ocean Software Ltd. it happens that I have to connect to their networks via VPN from time to time. Given the changed cipher of my own OpenVPN infrastructure I wanted to see what others are using.

According to my own article Connecting Linux to WatchGuard Firebox SSL (OpenVPN client) one of the client configuration reads like this:

cipher AES-256-CBC

Whereas for another client who is using a firewall from Sophos the chosen cipher suite looks like this:

cipher AES-128-CBC

Well, looks like I'm in good company with my new option.

Security is a process, not a state

Again, lesson learned. Although running services on Linux is mainly about setting them up properly at the beginning, it surely doesn't mean to forget about them in the long run. Regular reviews and audits help to mitigate newer issues and threats to your network infrastructure.

If you are an active OpenVPN user please use the comment section to share other security related configuration settings and hardening tips on OpenVPN. That would be much appreciated by myself and other readers. Thanks!

Next stop: MCSA: Linux on Azure

Next stop: MCSA: Linux on Azure

Recently I decided to coordinate my work and learning activities a little bit. Turns out that while working with .NET Core, in particular developing an API project, I do quite some coding under Linux using Visual Studio Code.

During office hours I'm fully emerged into Visual Studio 2017 running on my Windows 10 machine but often I'm reviewing and tweaking some of my code during the evening hours on my secondary Xubuntu system.

Linux and me

The story about me using Linux goes back two decades. Actually, if I remember correctly it happened some time in summer of 1996 when I officially purchased a copy of S.u.S.E. Linux 4.2. At that time I was still studying Applied Chemistry at the University of Kaiserslautern and the Unix AG on the campus offered copies of Slackware among others for free; you only had to bring the empty CDs to get the software burned on.

Note: The Unix AG was founded and still is run by a group of students and assistants in the field of computer science. And at that time there was a nice fellow named Klaus Knopper, famously known for his Linux distribution Knoppix.

Apart from attending lectures and running experiments in the chemistry laboratory I spent a good amount of time in the university's computer labs, too. Over there you had access to graphical XTerminals running on AIX Unix compared to the regular ASCII terminals anywhere else on campus.

The two reasons I bought a copy of S.u.S.E were because I wanted to set up an internet gateway at home which I was not able to do so with Windows NT 4.0, and because the distribution was bundled with several books on installation, network configuration and Linux in general in German language. So, I started the initial installation on a Friday afternoon, worked through the whole night reading and configuring the system several times, and slept only a few hours over the whole weekend. Finally, on Monday morning after several attempts and lots of swearing/ranting over my own incapabilities I managed to run a working internet gateway. Dialup happened over ISDN on my freshly installed Linux computer while my parent's system running Windows 95 was attached to the 10base2 thin Ethernet network.

The rest is history...

Azure is running (on) Linux

Eventually you might be aware of the situation that Microsoft is actually using Linux technology to run its cloud solution named Azure.

Yes, they do... According to an article Whoa. Microsoft is using Linux to run its cloud published on Wired back in September 2015 it is referring to an official blog article by Microsoft. Get more details about the Azure Cloud Switch in Microsoft showcases the Azure Cloud Switch (ACS) by Kamala Subramaniam Principal Architect, Azure Networking.

It [note: The Azure Cloud Switch (ACS)] is a cross-platform modular operating system for data center networking built on Linux.

Nonetheless, I would assume that the main interest would be to run and operate Linux machines in Azure. According to Microsoft says 40 percent of all VMs in Azure now are running Linux we are in good company with like-minded system operators.

What better than combining two technology stacks? Although, I work on Windows systems during my day job, Linux plays a vital role. Our internet gateways are based on a designated Linux system which handles all internal traffic and provides access to the internet by providing essential services like DHCP, DNS, proxy and so forth. Services the standard router provided by a local ISP might not be capable of or with serious security concerns.

Using Azure to provision a Linux-based virtual machine takes less than 5 minutes and there are various options available.

Next stop: MCSA: Linux on Azure

I'm a big fan of Xubuntu but to prepare myself for MCSA: Linux on Azure I'm going to need a CentOS based system. So, instead of taking resources on my local machine using a virtualisation software like VirtualBox or VMware I'm going to entertain a Linux VM on Azure. It's more convenient after all.

MCSA: Linux on Azure

Combining both technology stacks into one sounds almost like a dream coming true for me. Using Linux has always been a passion and fun factor for me and being able to add it more and more to my professional services brought me to the decision to look into the benefits and requirements of Microsoft's MCSA: Linux on Azure certification.

Effectively, the exam requirements stipulate that one has to pass two independent certifications to achieve MCSA: Linux on Azure:

You might have noticed that it is not purely a Microsoft certification but integrates the work of the Linux Foundation. Interestingly Microsoft officially announced during the Connect(); 2016 that they joined the Linux Foundation as a Platinum Member. Which literally made the Linux on Azure certification possible.

Our membership to the Linux Foundation builds on our work with the foundation, including the creation of a Linux on Azure certification.

Exciting times, don't you think?

Exam formats

Both, Microsoft and the Linux Foundation, offer details about the skill sets being measured during the exams. The Microsoft exam 70-533 is based on the usual multiple choice format. Compared to that the LFCS is performance-based.

Candidates will need to perform tasks or solve problems using the command line interface in their chosen Linux distribution.

Meaning, you connect to an actual Linux system - running either CentOS 7 or Ubuntu 16 (as of writing) - and you have to get your "hands dirty" in order to qualify.

Learning resources

Check out the section Optional training and resources on the official LFCS website. The Linux Foundation provide free material like their Certification Candidate Handbook, their Certification Preparation Guide, and their LFSx01 courses online.

In similar fashion Microsoft lists multiple resources in the Preparation options of the exam 70-533. The online training is accessible for free through the edX platform and are part of the Microsoft Professional Program in Cloud Admin, too.
Using the same preparation material gives you the ability to achieve a second accreditation. Perhaps you are interested to read more about the Cloud Administration professional program.

Having an active, annual subscription with Pluralsight I browsed through their learning paths and discovered Pluralsight Path to MSCA: Linux on Azure. It's a combination of several courses provided by experts John Savill and Andrew Mallett.

More resources will be added regularly to my 100-days-of-exam repository on GitHub. You are hereby invited to fork it, to add more resources including other exam preparations, and to send me your pull requests (PRs).

Commitment to #100DaysOfExam

To keep myself accountable I am committed to the #100DaysOfExam challenge.

I will learn and prepare for an exam for at least an hour every day for the next 100 days.

Following the Rules section of #100DaysOfExam I will tweet about my progress using hashtag #100DaysOfExam and I will update my Log with the day's progress and provide a link every day, too.

Let's do it!

ICT skills at primary school

ICT skills at primary school

Our children have computer lectures at their primary school since this year. In general, it's a great idea that students are exposed to computer literacy at an early stage. But sometimes it comes with small hiccups. Like in our case...

Curriculum, literature and exercise book

Although our children have access to computers at home since a while already it is the curriculum of their primary school in regards to IT literacy that lead to this blog article.

The title "Let's Learn ICT Skills" by the Mauritius Institute of Education (MIE) introduces Computer Fundamentals and Operations to young learners at primary school level. The textbook is divided into six units and covers first steps into the world of ICT.

Starting with an orientation in Windows the title discusses the essential use of typical desktop applications to handle word processing, to introduce simple graphics and presentation skills, to cover basic functionality in spreadsheets and to venture into the unknown areas of the interweb.

Each chapter has different learning objectives and introduces elementary skills in various applications. To keep matters easy the textbook is focused on Windows operating system and the Microsoft Office suite. Which in general and most commonly okay for the majority of primary school students.
Not sure whether it classifies as a tutorial. You are most welcome to comment and assist. #BlogMore about modern parenting obstacles...
Well, most students... ;-)

Our start situation - Linux

As a parent it is not easy to trust a full-fledged computer into the hands of your youngster(s) without fearing the whole system might be infested by viruses, malware and ransomware in shortest time. Especially given recent reports on various problems.

Following my decision to provide our kids with family-friendly and security-enhanced tablets running on Amazon's Fire OS compared to regular Android, it was only right to provide them a similar experience on the desktop. At least in my point of view.

Personally, it was important for me to have peace of mind knowing our children are using Linux based system. Don't get me wrong Microsoft has done a tremendous job to improve security over the last decade. It's just that I didn't want to purchase a new laptop for them and Linux runs just fine on older hardware.

Instead of upgrading the available HP laptop from Windows Vista Business to latest Windows 10 I decided to install Xubuntu 17.04 originally. Some weeks back, I then upgraded their machine to Bionic Beaver (version 18.04) already, and they can "beta-test" the upcoming Ubuntu LTS version.

After all, as more and more software is moved towards web applications it really doesn't matter anymore whether Firefox is run under Windows or Linux, does it? Additionally, they have access to LibreOffice, GIMP and other educational software packages like GCompris, and so forth.

Well, the children's exercise book is explicitly covering Windows, some applications of the Microsoft Office suite as well as - software that isn't available on Linux out of the box.

Various approaches possible

Of course, there is no golden solution to this situation and multiple possibilities are given. All depending on circumstances, personal taste and eventual hardware constraints. Following, I would like to give you an overview of options - all of which I already used successfully in the past.


This might come first in someone's mind and I have to agree with that. Installing a virtualisation software like Oracle VirtualBox, VMware Workstation or even qemu can be done easily and the the actual experience can be seamless. In our situation though is the existing hardware with a previous generation CPU and 2 GB RAM only the limiting factor to this approach.

Using wine or CodeWeavers CrossOver

Emulation software like wine or CodeWeavers CrossOver eliminate the necessity to install and run a complete virtualisation solution. The software provides an abstraction layer of native Windows API functionality and allows to install and run Windows software like the Microsoft Office suite among others directly on a Linux machine. Luckily, the hardware wouldn't be the limiting factor but I have to confess that it is my laziness to opt-in for this viable approach. Also, the first chapter in the kids' literature - Getting familiar with Windows - wouldn't be possible for them using this approach.

Remote access

Last but not least, providing remote access to an existing instance of a Windows system seems to be one of the easiest options. Here, the kids get to experience Windows directly and it doesn't need any resources on their Linux system. Using a software package like rdesktop or remmina enables a Linux user to connect to a Windows system via Remote Desktop Protocol (RDP). So far so good, but I'm not interested to provision a dedicated machine for this purpose at home. The system would be idle most of the time and consume a good chunk of electricity instead.

As mentioned earlier I have used all those approaches successfully, and it is good fun to tinker around with them. But those are most likely options for an adult and not really suitable for a child attending primary school.

A solution - Cloud-based virtual machine

Taking the pro aspects of each of the approaches earlier I decided to provision a virtual machine running Windows 10 Professional in the cloud. Access to that machine is available using RDP and in regards to hardware constraints it requires an internet connection only.

Actually, this suits me very well as it gives me control on various levels:

  • Local network: I can control at any time whether the kids' laptop gets access to our WiFi network or the internet based on simple authentication and routing configuration.
  • Operating times: A virtual machine in Azure is fully controlled through the Azure portal. I can decide when the VM is running and when not.
  • Hardware on demand: Provisioning hardware to the VM on Azure is just a few clicks and a reboot away.
  • Data exchange: Synchronisation of files between the local Linux laptop and the Windows machine in Azure is based on cloud storage providers like OneDrive, Google Drive, Dropbox, etc. Meaning backup of files is integrated and additional devices like their tablets can be added easily.

Later on, if the VM isn't needed anymore or in case the children totally messed it up I don't have to worry about anything. The VM gets decommissioned and can be provisioned again within minutes if needed.

Azure configuration and fine-tuning

To start with this educational system for my children I went into the Azure Portal and created a new virtual machine using the Windows 10 Pro image. To keep nice and smooth I also created a new resource group to isolate it from other business-related activities.Given

Size of the VM

ICT skills at primary school
I chose a (hopefully) decent hardware setup running the virtual machine on a Standard B4MS (4 cores with 16 GB RAM) tier. This should be sufficient enough for Microsoft Office, and Firefox.


Also, I activated the Auto-shutdown feature which restricts the use of the system until a specified time, and helps me to save a heap of money, too.
ICT skills at primary school
The main purpose of that VM is to allow the children to follow the exercises and steps in their school book. At the given time the system simply shuts down, and it's dinner time in the off-line world.

Starting the VM

Now that we know how to stop the VM we should have a look about how to start it. There are multiple choices available. Most obvious you can launch the virtual machine via the Azure Portal itself. Nothing surprising here.

Next, Microsoft offers the free Azure mobile app for Android and iOS to stay connected to your Azure resources. This is quite neat to manage, monitor and operate Azure on the go.

And then there is azure-cli - the Command-line tools for Azure - which gives you the next generation multi-platform command line experience for Azure.

$ az 

    /  \    _____   _ _  ___ _
   / /\ \  |_  / | | | \'__/ _\
  / ____ \  / /| |_| | | |  __/
 /_/    \_\/___|\__,_|_|  \___|

Welcome to the cool new Azure CLI!

Usually, I have Visual Studio Code open almost the whole day and starting the kids' virtual machine is done using the Azure CLI Tools extension.
ICT skills at primary school

I'm currently using the following .azcli file to manage that VM:

# Logging into Azure
az login

# Starting kids' VM on Azure
az vm start -g Personal -n windows4kids

# Stopping kids' VM
az vm stop -g Personal -n windows4kids --no-wait

The az login triggers the device login on Azure and after entering a generated code to authenticate your machine you get access to your resources on Azure, like this:
ICT skills at primary school

Accessing the VM

Windows machines on Azure are accessed via RDP and Linux has a variety of client applications for that protocol. In the portal you should assign a static domain name to your VM as the public IP address is most likely to change between daily uses. The portal allows you to download the Connect parameters as a .rdp file that you can open in any text editor on Linux.

ICT skills at primary school
Using the details from the .rdp it is possible to set up a new connection in remmina for future use. I'm storing the password to keep it simple for the children to access their new Windows machine.

Now, remmina is configured to start automatically after they logged into their account and the Windows VM on Azure is easy accessible via shortcut from the system tray area.

Give it a try - Azure free credit

Microsoft gives new sign-ups on Azure an initial credit that allows you to explore the various options and get yourself familiar with the available resources. Why don't you give it a try?