CategorySysadmin

How To Connect Via SSH Using a Specified Cipher

When you use SSH, you normally just want to connect securely to a host, and you don’t really care about what cipher to use. In other cases, however, you might want to use a specific cipher, perhaps for compliance reasons, or perhaps because you are paranoid and want to make sure that you are connecting using your favorite cipher. Both are perfectly good reasons :P

SSH allows you to use a specific cipher when you connect to a host. First of all, you need to see what ciphers are available on your operating system. This is the command to list them:

ssh -Q cipher

At this point, you can connect to a host via SSH specifying one of these ciphers in the following way:

ssh user@host -c cipher

So, if you want to connect using AES256, you would type this:

ssh user@host -c aes256-cbc

How To Customize The Linux Kernel

The post title begs the question: Why should you customize the Linux kernel? There are three main reasons why you might want to do this:

  • Performance: by removing unnecessary features and by tuning the kernel to work on your particular machine, you could make your machine more performant;
  • Patching: rather than waiting for an official updated of the kernel, you can apply any patches you are interested in anytime;
  • Additional features: not everything is compiled into the kernel by default, mostly to keep it as lean as possible. If there is something you really use, however, you can add it to your kernel so the features are built in.

Perhaps surprisingly, building your own Linux kernel is far from a complicated thing. You just need a lot of patience as the build process can last quite a bit. Having said this, let’s take a look at how you can customize your Linux kernel.

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Virtual Machine Fails To Boot After Uploading to vSphere From VMware Fusion

As I wrote in my recent Projects for 2018 blog post, one of the things I wanted to do this year was to setup a VMware ESXi host once again. I have been using VMware Fusion and Workstation so much in the last few months that it would only make sense to also have a server to upload and download VMs to and from. So I did configure the host and I immediately started uploading all the VMs I had on my desktop machines.

All of them uploaded successfully, but one behaved strangely: in VMware Fusion, I would right-click on it, select Upload to Server…, choose my ESXi host and datastore and then select Upload, but the upload process completed immediately, in something like one second. This was very weird because this VM was 15 GBs in size so that was no way the upload had completed already.

However, what was even stranger was that the ESXi host showed the machine correctly uploaded to the host datastore, with configuration files and virtual hard drives. However, attempting to boot the VM would result in a network boot taking place, quickly followed by an Operating System not found error right afterwards, so the virtual machine would consistently fail to boot. This made me realize that there was definitely an issue with my virtual hard disk.

Operating System not found error after network boot

This is the Operating System not found error that would come up after uploading a virtual machine from VMware Fusion to a vSphere host

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My Projects For 2018

At the very beginning of last year I posted My Projects For 2017, in which I listed some of the technologies and projects I wanted to play with and study in the upcoming 12 months. I thought it could be a cool idea to do this regularly every year, so here is my projects for 2018 blog post, together with a short section looking back at what I wrote last year and what I ended up doing (and not doing).

My Projects From Last Year

These are the projects that, in January of last year, I said I wanted to work on during 2017, together with their status:

  • Learn and deploy Puppet [NOT STARTED]
  • Upgrade to XenServer 7 [NOT STARTED]
  • Work on a XenServer mobile app [NOT STARTED]
  • Find a good use for my new Raspberry Pi 2 [DONE]
  • Add some more web apps to my web server [DONE]
  • Test NextCloud and decide whether it’s going to be worth it migrating from ownCloud [DONE] (I migrated to Nextcloud after all)
  • Implement VLANs in my home network [DONE]
  • Consider switching to a hardware firewall [DONE]
  • Configure a new FreeNAS box to take the place of my Synology DS213J [NOT STARTED]

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How To Install Monica – Personal Relationship Manager – On Ubuntu 16.04

Monica is a web app for managing your personal connections. The official website for the project describes it this way:

Monica helps you have more meaningful relationships.

Monica is a simple, open source, personal CRM. For your personal life, not your business.

The project looks very interesting, and it sparked a ton of interest on is official GitHub page. That’s right, because Monica, besides offering a hosted version (both with a free and a paid tier) also decided to make the code completely open source so you can install it and download it on your own server (which is something that is definitely going to be appreciated by a lot of folks, looking at the intrinsically personal data that Monica will store).

Unfortunately, at the time of writing this article, the instructions to install the app on your own server are either outdated or incomplete, making the installation process not very simple (also considering all the tools necessary to get it up and running). This article seeks to solve this issue with some step-by-step instructions.

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How To Install tmux On Raspbian

One of my favourite utilities ever is tmux, no doubts about it. It allows you to create a session when you are connected to a machine via SSH and restore that session later even if your SSH connection drops for any reason. Once you reattach the session, you will be brought back exactly to the point where you were before the disconnection.

This is extremely useful in cases where the command you launched would take a long time to complete (for example, if you are burning in your hard drives for FreeNAS or if you are generating DH parameters when configuring OpenVPN).

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How To Enable File Uploads In MediaWiki (Tweeki Theme)

The MediaWiki official documentation for enabling file uploads is very thorough, but if this is a new installation (and it’s probably likely if you are enabling file uploads), it can be somehow stripped down to get the functionality simply up and running quickly. I still recommend reading the whole documentation to make sure you tweak the system in the best possible way, and so you don’t forget the necessary security precautions.

Also, if you are running the Tweeki theme for MediaWiki, there is one step that is not mentioned in the official documentation and that got me stuck for a while. So this article shows you how to enable file uploads in MediaWiki when you are using the Tweeki theme.

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How To Have The Ping Command Make a Sound

There might be cases where you are testing your network and it would be useful to know if your ping command is working without having to keep an eye on your screen. Imagine being in a server room behind a switch, while your computer is somewhere else in the room: it would be great if you could hear some sound while ping is working, and no sound when ping stops working (so you know when you have disconnected the correct cable).

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A Few Performance Tests Using Powerline Adapters

Powerline adapters are a pretty cool way to keep your network wired without having to run cables through the walls, particularly handy if you are renting your place (but even if you own the house and you don’t feel like doing this type of work).

Obviously, wiring the house would be the best choice, but when this is not possible, powerline adapters are a very good alternative, and probably still better than just connecting every device via Wi-Fi.

However, the performance of these adapters vary based on a lot of factors, from the distance between the adapters, the quality of your electrical system and, perhaps most importantly, where you decide to plug your adapters into.

Every powerline manufacturer recommends to plug them directly into a wall socket for maximum performance. You should avoid plugging them in anywhere else, including power strips or UPSs. If you do, they will likely still work, but you won’t get the maximum throughput supported.

I used to have a long ethernet cable running from my core switch in the office to the router in the other room, but since getting a dog who apparently loves to eat copper, I had to find an alternative. I removed the chewed ethernet cable and added two powerline adapters into my network, so it was a great opportunity to run some basic performance tests to see how positioning the adapters will affect the network speed. Here are the results.

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VirtualBox VBoxManage convertdd VS convertfromraw

If you need to convert a computer image (for example, taken with dd) to a VirtualBox VM, you might have seen a few tutorials online that show you how to convert the disk image to the VirtualBox own VDI format. Some of this guides mention either the convertdd or the convertfromraw command to achieve this, but are there any differences between the two?

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