Category: Sysadmin

  • How to make a better ARM virtual machine (armhf/aarch64) with UEFI

    Over a year ago, I wrote about making ARM virtual machines. Times have changed a lot since then — the release of Apple M1 has dramatically changed the perception of ARM. No longer is ARM a niche platform for low-power gadgets like phones or tablets, but a viable desktop computing platform. Similarly, in the server space, Amazon Graviton and Ampere Altra have gained traction. My old blog post presented only a quick hack to get the ARM virtual machine to boot — by copying the kernel image. This leaves a lot to be desired. Somehow, despite this, it quickly became one of the popular posts on my blog. Today, we shall rectify that flaw and present a way to boot the latest kernel installed in the virtual machine using the Unified Extensible Firmware Interface (UEFI).

    Again, like before, this tutorial will use Debian as an example, but the same methodology should work for other distributions. If you are looking for a simple chroot, you should instead follow the original post.

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  • Website Changes: New Domains, New Infrastructure

    I haven’t posted in a while now, but nevertheless, I have been working on this site. Perhaps I have been too bogged down in the minor details, but it was a relatively interesting experience nonetheless.

    You may have already noticed some differences:

    1. The most obvious change is probably the domain. Instead of using my old domain, I switched to the shiny new
    2. The second most obvious change is the short URL on every post.
    3. The last change is invisible: the backend is now distributed in three locations around the world. This is so that even if you are in faraway Australia, you can still load this website instantly, even if it’s not in the Cloudflare cache.

    Why did I make these changes? Well, this was because I became thoroughly nerd-sniped by some ideas…

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  • Serving Static Files from the Cloudflare Edge

    Three years ago, I wrote about a way to purge only changed static files when deploying a static site. It is very useful and I still use it for this website to this day. Its main advantage is that it only needs to be run on deploys. However, its main disadvantage is that it must be run on every deployment. Sometimes, this is not feasible.

    For example, I run a bunch of APT repositories on These repositories are constantly being updated by Jenkins and me personally, and using purge-static would require adding a purge-static command to every script that updates the repositories, which is clearly infeasible. Wouldn’t it be nice to just have a background daemon that purged the CDN cache automatically?

    As it turns out, I already wrote it back in 2015 before starting this blog. It was massively out-of-date (until very recently) and required you to use your all-powerful Cloudflare API key, providing a massive attack surface. However, I recently updated it, and hopefully, it will prove useful for you.

    Here’s a quick introduction to using it:

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  • On Backporting Kernel Modules with DKMS

    Recently, I bought a USB 3.0 2.5 Gbps Ethernet dongle for my Atomic Pi router. This dongle requires a version of the r8152 kernel driver with support for the RTL8156 chipset, which is only added in Linux 5.13. Now, I am running the Debian stable kernel and have no wish to backport the latest 5.13 kernel simply for that one driver. So of course, I came up with an approach to backport a driver from a newer version.

    In this blog post, I will walk you through the process of backporting a single kernel module, using the r8152 kernel driver as an example.

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  • On Building Custom Debian Kernels (and Backporting)

    It’s not often in 2021 that you find yourself building new kernels, but nevertheless, the occasion comes that you need to either enable a flag—or even worse—patch the kernel. This happened recently: on DMOJ, we recently run into a kernel issue that misreports the memory usage for processes as an “optimization.” For more information about this issue, see the excellent blog post by my friend Tudor. As a result of this, I was forced to build a patched kernel to work around this issue. Since the process was far from easy, I decided to write this blog post to help others in the future.

    Building a kernel is not too difficult, actually. The real challenge comes in the form of building the kernel in a maintainable way, which basically means that we should at least build the kernel into an easily installable package. For example, on DMOJ, we manage multiple judge virtual machines, and they all need to receive the same kernel. Furthermore, we want our custom build of the kernel to be distinct from the standard kernels that the operating system offers, as we don’t want a system upgrade to undo the patch that we applied.

    In this article, we will explore the process I used to build a custom kernel package on Debian for the scenario described above. This will involve both patching the kernel and subsequently changing a configuration option. Specifically, we will be applying this patch. These instructions should work with minor adaptations for other Debian-based distributions.

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  • Sharing Unix sockets between multiple users

    I am sure that if you managed a Linux system for a while, you probably have dealt with Unix sockets—special files that act like sockets. You probably also run into permission issues when dealing with these socket files.

    In this post, I’ll describe some methods of dealing with these permission issues, and a situation in which each might apply.

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  • How to make an ARM virtual machine (armhf/aarch64)

    Update (2022-03-19): I wrote about a new way to create an ARM virtual machine that’s simpler and handles kernel updates properly. I highly suggest you follow those instructions instead, unless you are building a chroot.

    I noticed that very few people seem to know how to create a full ARM virtual machine, so I decided to create a quick guide.

    This tutorial will use aarch64 and Debian as examples, but the same methodology should work for 32-bit ARM and other distributions. The instructions can also be adapted to create a simple chroot.

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  • Run a Linux Program on a Different Network Interface

    Sometimes, you have multiple Internet connections, whether physical or virtual, and you want a few programs to access the Internet through one connection without making it the default gateway. For example, if you want a program to connect to the Internet through a VPN, but without forcing the entire system’s traffic through the VPN as well.

    The traditional way to do this is with packet marking with iptables and an ip rule to force marked packets through a different routing table to send the traffic to the correct destination. However, as the source IP was selected before routing, an SNAT rule in iptables is required to change the source IP. This is ugly and clearly a hack.

    However, since around 2013, Linux has introduced networking namespaces, which can be managed via ip netns as part of the iproute2 package. We can easily exploit this feature to achieve the desired goal with minimal fuss.

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  • Install Debian on a VPS Provider without Debian Images

    Recently, I came across a VPS provider that does not provide Debian images. This is rather annoying since I much prefer a fresh minimal install of Debian over a “minimal” Ubuntu image that still has a lot of stuff that I don’t want.

    Naturally, I decided to install Debian anyways, and came up with an approach to do so.

    If you are feeling particularly bold, you can try running my pre-made scripts that would convert a fresh Ubuntu install to a fresh Debian install.

    To use the scripts, you should download either the UEFI version or the BIOS version, depending on whether your current OS is using BIOS or UEFI.

    At the top of the script, change the variables to match your system configuration. The most important one being BOOT_DRIVE so that grub would be installed on the correct device.

    The scripts will prompt you for a root password and SSH keys. Once the script finishes, the system will be rebooted and you should be able to SSH into the now-Debian machine as root via the SSH keys.

    If you don’t feel like using the script, I am also providing manual instructions. This also explains how the scripts work.

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  • Simple NDP Proxy to Route Your IPv6 VPN Addresses

    If you tried setting up an IPv6-capable VPN on a VPS provider that gave you an IP range to play with, perhaps a /64 or larger, you would want to assign some of the IPv6 addresses you have to your clients. In this post, we suppose that you have the range 2001:db8::/64.

    This should be a simple process: enable the sysctl option net.ipv6.conf.all.forwarding to 1 (or whatever the equivalent is on your system), use DHCPv6 or SLAAC to assign the addresses to the clients, and then your client should have working IPv6.

    The Problem

    Unfortunately, this is not so simple. Most VPS providers are not actually routing the entire subnet 2001:db8::/64 to you. Rather, they just connect a number of VPSes onto the same virtual Ethernet network and rely on the Neighbour Discovery Protocol (NDP) to find the router.

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  • On Invalidation of Aggressively Cached Static Sites

    I have always wanted to make this website load fast everywhere in the world, despite the server being in Montréal, Canada, without investing heavily. It shouldn’t be hard: after all, it is just a bunch of static files, generated with Jekyll.

    Cloudflare brings a free CDN. You can set a page rule to aggressively cache your website on their CDN edge nodes, allowing your site to load as if it is hosted locally, even if you are half a world away.

    There is just a little problem: how do you efficiently purge the cache when you update your site? It is quite easy to purge the entire cache on Cloudflare, but that is rather inefficient: most of your assets probably did not change, and now they will all have to be fetched again.

    Today I decided to tackle this problem by creating purge-static, a tool designed to purge your CDN cache. It can purge your Cloudflare cache for you. You can get started by running pip install purge-static.

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  • The fast way to install debs on Debian

    I personally prefer the packages for nginx over the ones that comes with Debian. They are usually newer and have a more sane amount of dependencies. I also prefer the conf.d system over the sites-available and sites-enabled system.

    The main challenge in installing these packages on Debian is the trouble you have to go through to get the PGP keys and sources.list set up. does not present a good setup script. This has become a repetitive and annoying experience, so I present a series of commands to set it up quickly.

    For stable:

    curl | sudo apt-key add -
    (codename="$(dpkg --status tzdata | grep Provides | cut -f2 -d'-')"; echo; for deb in deb deb-src; do echo $deb "$codename" nginx; done) | sudo tee -a /etc/apt/sources.list
    sudo apt update && sudo apt install nginx

    For mainline:

    curl | sudo apt-key add -
    (codename="$(dpkg --status tzdata | grep Provides | cut -f2 -d'-')"; echo; for deb in deb deb-src; do echo $deb "$codename" nginx; done) | sudo tee -a /etc/apt/sources.list
    sudo apt update && sudo apt install nginx
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  • Installing Debian ARM64 on Raspberry Pi 3 with WiFi

    Most users are probably using Raspbian on their Raspberry Pi 3. However, Raspbian is designed for all Raspberry Pi devices, back to the original Raspberry Pi, which is ARMv6 with an FPU. This does not take advantage of the 64-bit support on the ARMv8 CPU on the Raspberry Pi 3.

    Debian has offered ARM64 support for a while, and being the base distribution for Raspbian, is quite similar. Conveniently, there is a pre-built Debian image for Raspberry Pi 3. You can download it and copy it to a SD card, and it should work out of the box.

    On Linux, the simple dd command showed on the Debian Wiki works. On other platforms, notably Windows, Etcher is reputed to work well and has an easy interface.

    The one flaw with this image is that the WiFi does not work.

    Update: The 20180108 image now works with WiFi out of the box. The following instructions are no longer necessary.

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